Loco – Biochemistry

• Vitamin D metabolism steps:

  1. Skin: 7-dehydrocholesterol → (UV light) → Cholecalciferol (Vitamin D3).

  2. Liver: Cholecalciferol → 25-hydroxyvitamin D3 (Calcidiol) — major storage form.

  3. Kidney (proximal tubule): 25-hydroxyvitamin D3 → 1,25-dihydroxyvitamin D3 (Calcitriol) via 1α-hydroxylase — active form.

• Calcitriol’s actions:

  • ↑ Calcium & phosphate absorption in intestine.

  • ↑ Bone mineralization (low dose) or resorption (high dose).

  • Acts like a hormone (nuclear receptor-mediated).

Why the others are wrong:

• 1-hydroxyvitamin D3 → Intermediate in synthetic analogs, not the active natural metabolite.

• 7-dehydrocholesterol → Precursor in the skin, not active.

• 25-hydroxyvitamin D3 → Storage form, not active.

• None of these → Incorrect, since calcitriol is in the options.Patients with chronic kidney disease cannot activate vitamin D (deficient 1α-hydroxylase), leading to renal osteodystrophy → they need active calcitriol supplementation.

• Type I collagen is the most abundant collagen in the human body (≈90% of total collagen).

• Location: Bone, skin, tendons, ligaments, dentin, fascia, cornea.

• Function: Provides tensile strength and structural support.

❌ Why the others are wrong

• Type II → Found mainly in cartilage, vitreous body, nucleus pulposus. Important for resistance to pressure, but not the most abundant.

• Type III → Found in reticular fibers of skin, blood vessels, granulation tissue. Associated with Ehlers-Danlos syndrome (vascular type).

• Type IV → Found in basement membranes, lens, glomeruli. Forms meshwork, not fibrils.

• Type V → Found in placenta, cornea, some basement membranes, but in very small amounts.

• Collagen has a unique triple-helical structure.

• Its basic repeating unit is Gly–X–Y, where:

  • Glycine (the smallest amino acid) is at every third position.

  • X is often proline.

  • Y is often hydroxyproline or hydroxylysine.

• The tiny size of glycine allows the three chains to pack tightly into the stable collagen triple helix.

❌ Why the others are wrong

• Lysine → present in collagen, some residues hydroxylated to hydroxylysine (important for cross-linking), but not every third residue.

• Hydroxyproline → stabilizes collagen helix by hydrogen bonding, but not regularly every third position.

• Glutamate → not a major collagen amino acid in the repeating structure.

• Proline → common at every third/fourth position but not in the invariant every third slot like glycine.

• Parathyroid hormone (PTH) is secreted by the parathyroid glands in response to low serum calcium.

• Its actions increase serum calcium and decrease serum phosphate by:

  1. Bone: Stimulates osteoclast activity indirectly (via osteoblasts) → ↑ bone resorption → release of calcium and phosphate into blood.

  2. Kidney: Increases calcium reabsorption in distal tubules, decreases phosphate reabsorption in proximal tubules.

  3. Vitamin D activation: Stimulates 1-α hydroxylase in the kidney → ↑ calcitriol (active vitamin D) → increases intestinal absorption of calcium.

❌ Why the others are incorrect:

• Calcitonin: Secreted by thyroid C-cells, does the opposite → inhibits osteoclasts, lowering calcium.

• Insulin: Anabolic hormone, no direct role in calcium homeostasis.

• Thyroxine (T4): Excess can cause bone loss, but not the main regulator.

• Testosterone: Supports bone density, not calcium regulation.

• Alkaline phosphatase (ALP) is present in many tissues, but especially high in the liver (bile canaliculi) and bone (osteoblasts).

• Elevated ALP levels are clinically significant in:

  • Liver disease → especially obstructive jaundice and cholestasis, where bile duct obstruction increases ALP release.

  • Bone disease → conditions with high osteoblastic activity, e.g., rickets, osteomalacia, Paget’s disease, bone metastases.

• To differentiate liver vs bone source of ALP, clinicians also check Gamma-glutamyl transferase (GGT) →

  • If both ALP and GGT are raised → source is liver.

  • If ALP is raised but GGT is normal → source is bone.

❌ Why the others are incorrect:

• AST (Aspartate aminotransferase): Primarily a marker of liver injury (also heart, muscle), but not bone disease. 

• ALT (Alanine aminotransferase): More specific to liver damage, not bone.

• GGT (Gamma glutamyl transferase): A marker of liver/biliary disease and alcoholism, but not bone.

• Osteocalcin: Specific marker of bone formation, but not used as a liver function test.

• The kidneys are the primary organs responsible for phosphate excretion.

• In renal failure, the glomerular filtration rate (GFR) decreases, meaning less phosphate is filtered out and excreted.

• This leads to hyperphosphatemia (elevated phosphate levels in the blood).

❌ Why the others are incorrect:

• Hyperparathyroidism → Causes hypophosphatemia, since PTH increases phosphate excretion in urine.

• Osteoporosis → Bone mineral density decreases, but phosphate levels in blood are usually not significantly elevated.

• Rickets → Caused by vitamin D deficiency; may have low or normal phosphate depending on type.

• Thymoma → Associated with myasthenia gravis, not phosphate imbalance.

• In the skin, 7-dehydrocholesterol (derived from cholesterol) absorbs UVB radiation and is converted into cholecalciferol (vitamin D₃).

• This cholecalciferol then enters the circulation and undergoes hydroxylation in the liver (25-hydroxylase → calcidiol) and later in the kidney (1-α hydroxylase → calcitriol, the active form).

❌ Why others are incorrect:

• Cholecalciferol → The product formed in skin after UV light acts, not the precursor.

• Cholesterol → The parent compound, but not the immediate skin precursor.

• Ergocalciferol → Plant-derived vitamin D₂, obtained from diet, not skin.

•  Ergosterol → Precursor for vitamin D₂ in plants, not in humans.

• Vitamin D₃ (cholecalciferol) is first converted in the liver to 25-hydroxycholecalciferol (calcidiol) by the enzyme 25-hydroxylase.

• In the kidney (proximal tubule), 1-α hydroxylase converts 25-hydroxy D₃ into 1,25-dihydroxycholecalciferol (calcitriol), which is the active form of vitamin D.

• This enzyme is regulated by:

  • Parathyroid hormone (PTH) → stimulates it.

  • Phosphate levels → low phosphate stimulates activity.

  • Calcitriol itself → provides negative feedback.

❌ Why others are incorrect:

•  24 hydroxylase → Inactivates vitamin D by converting it to 24,25-dihydroxycholecalciferol.

• 25 hydroxylase → Acts in the liver, not the kidney.

•  7-dehydrocholesterol reductase → Enzyme in skin for cholesterol metabolism, unrelated to activation.

•  17-β hydroxysteroid dehydrogenase → Steroid hormone metabolism, not vitamin D.

Collagen synthesis involves several key steps, one of which is the post-translational hydroxylation of specific proline and lysine residues in the procollagen molecule. This hydroxylation is crucial for:

• Stabilizing the triple helical structure of collagen via hydrogen bonding.

• Allowing glycosylation and proper extracellular secretion of collagen.

Ascorbate (Vitamin C) acts as a cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase, donating electrons to maintain the iron atom in the enzyme in its active reduced state. Without vitamin C, these hydroxylation reactions fail, leading to unstable collagen. This underlies scurvy, characterized by fragile blood vessels, bleeding gums, poor wound healing, and defective connective tissue.

❌ Why other options are incorrect:

• Cholecalciferol (Vitamin D3) / Calcitriol: Regulate calcium and phosphate homeostasis, unrelated to collagen hydroxylation.

• Folate: Cofactor in one-carbon metabolism and DNA synthesis.

• Retinol (Vitamin A): Involved in vision, epithelial maintenance, and growth.

Chondroitin sulfate is a type of glycosaminoglycan (GAG), which are long, unbranched polysaccharides composed of repeating disaccharide units.

• Each disaccharide unit of chondroitin sulfate consists of:

  • N-acetylgalactosamine (GalNAc) – an amino sugar

  • Glucuronic acid (GlcA) – a uronic acid

• These units are often sulfated at various positions, giving chondroitin sulfate its negative charge and contributing to the resilience of cartilage and connective tissues.

• Functions:

  • Provides structural support in cartilage

  • Contributes to resistance against compression

  • Plays a role in extracellular matrix organization

❌ Why other options are wrong:

• Glucuronic acid + iduronic acid: This is found in dermatan sulfate and heparin, not chondroitin sulfate.

• N-acetylglucosamine + iduronic acid or glucuronic acid: These combinations are characteristic of heparan sulfate or keratan sulfate, not chondroitin sulfate.

• N-acetylgalactosamine + iduronic acid: This is seen in dermatan sulfate, not chondroitin sulfate.

Vitamin D exists in several forms, and understanding their sources and activity is key:

• Cholecalciferol (Vitamin D3):

  • Made in the skin when 7-dehydrocholesterol is exposed to UVB rays from sunlight.

  • Also found in dietary sources such as fatty fish, egg yolk, and liver.

  • Inactive in this form; it requires hydroxylation in the liver (to 25-hydroxyvitamin D) and then in the kidney (to 1,25-dihydroxyvitamin D, calcitriol) to become active.

• Ergocalciferol (Vitamin D2):

  • Derived from plants and fungi; can be consumed in diet or supplements.

  • Less potent than cholecalciferol in raising serum 25(OH)D levels.

• Calcitriol (1,25-dihydroxyvitamin D):

  • The active hormonal form of vitamin D.

  • Not made in skin or consumed directly; produced in the kidney from calcidiol.

• Calciferol:

  • General term sometimes used for vitamin D compounds, not specific to the form made in skin.

• Cholesterol:

  • Precursor for vitamin D synthesis but not itself vitamin D.

❌ Why other options are wrong:

• Calcitriol: Active form, not made directly in skin or diet.

• Ergocalciferol: Plant/fungal source only; skin does not produce it.

• Calciferol: Non-specific term; does not indicate the physiologically relevant form.

• Cholesterol: Precursor molecule, not the vitamin form.

During collagen synthesis, hydroxylation of proline and lysine residues occurs in the rough endoplasmic reticulum.

• The enzyme prolyl hydroxylase (and lysyl hydroxylase) requires:

  • Ascorbate (Vitamin C) → cofactor

  • Fe²⁺

  • O₂

  • α-ketoglutarate

👉 Hydroxylation stabilizes collagen by forming hydrogen bonds between chains.

🔴 Deficiency of ascorbate → defective hydroxylation → unstable collagen → Scurvy (bleeding gums, poor wound healing, fragile capillaries).

• Calcitriol (active Vit D) → calcium & phosphate metabolism.

• Cholecalciferol (Vit D3 precursor) → inactive form.

• Retinol (Vit A) → vision, epithelial growth.

• Folate (B9) → one-carbon metabolism, nucleotide synthesis.

Vitamin D synthesis in humans begins in the skin, where 7-dehydrocholesterol—a derivative of cholesterol—is converted to cholecalciferol (vitamin D₃) upon exposure to UVB radiation (wavelength ~290–315 nm).

Here’s the stepwise process:

1. UVB light acts on 7-dehydrocholesterol in the epidermis.

2. This results in the formation of previtamin D₃.

3. Thermal isomerization converts it to cholecalciferol (vitamin D₃).

4. Cholecalciferol is then hydroxylated in the liver and kidneys to form the active hormone calcitriol (1,25-dihydroxyvitamin D₃).

Since 7-dehydrocholesterol is synthesized from cholesterol, it is the ultimate precursor for vitamin D production.

❌ Why the Other Options Are Incorrect:

• HDL (High-Density Lipoprotein):
  Transports cholesterol in the blood, but is not involved in skin-based vitamin D synthesis.

• LDL (Low-Density Lipoprotein):
  Also transports cholesterol but is unrelated to the UV-initiated synthesis of vitamin D.

• Triglycerides:
  Serve as energy stores and are not precursors for vitamin D.

• Fatty acids:
  While essential for many biological functions, they do not contribute to vitamin D formation via UV radiation.

Ascorbic acid (Vitamin C) acts as a cofactor for lysyl hydroxylase and prolyl hydroxylase, two key enzymes responsible for the post-translational hydroxylation of lysine and proline residues during collagen synthesis.

This hydroxylation is vital because:

• It allows for cross-linking of collagen fibrils.

• It stabilizes the triple-helical structure of collagen.

• Without it, collagen is weak and poorly functional, leading to symptoms of scurvy.

❌ Why the Other Options Are Incorrect:

• Pantothenate (Vitamin B5):
  Precursor of Coenzyme A, involved in fatty acid metabolism, not collagen formation.

• Riboflavin (Vitamin B2):
  Component of FAD and FMN coenzymes, involved in redox reactions, not hydroxylation in collagen synthesis.

• Thiamine (Vitamin B1):
  Coenzyme for dehydrogenase complexes (e.g., pyruvate dehydrogenase) — important in carbohydrate metabolism, not collagen.

• Vitamin B6 (Pyridoxine):
  Cofactor in transamination, decarboxylation, and glycogen phosphorylase reactions — not hydroxylation.

Vitamin D metabolism occurs in steps:

1. Cholecalciferol (Vitamin D₃) – synthesized in skin from 7-dehydrocholesterol (or obtained from diet).

2. 25-hydroxycholecalciferol (calcidiol) – formed in the liver by hydroxylation.

   • This is the major circulating form of vitamin D.

   • It has a long half-life (2–3 weeks), making it the primary storage form.

3. 1,25-dihydroxycholecalciferol (calcitriol) – formed in the kidney.

   • This is the active hormonal form of vitamin D.

   • Regulates calcium and phosphate metabolism.

❌ Why the other options are incorrect:

• Cholecalciferol (Vitamin D₃): Precursor from skin or diet, but not the main storage form.

• 7-dehydrocholesterol: Pro-vitamin in the skin, not a storage form.

• 1,25-dihydroxycholecalciferol: Active form, but short half-life; not used for storage.

• Ergocalciferol (Vitamin D₂): Plant-derived form, can be ingested, but not the main storage form.

The strength and stability of collagen fibers depend on covalent cross-linking between collagen molecules. This cross-linking is mediated by lysyl oxidase, a copper-dependent enzyme that oxidatively deaminates lysine and hydroxylysine residues, creating aldehyde groups that form covalent bonds with adjacent residues. This process is essential for collagen tensile strength and proper connective tissue integrity.

❌ Why the other options are incorrect:

• Hydroxy collagenase: Not an enzyme involved in collagen maturation; sounds like a distractor term.

• Prolyl oxidase: Doesn’t exist; possibly a confusion with prolyl hydroxylase, which hydroxylates proline residues (important for collagen triple helix stability, but not for cross-linking).

• Elastase: Breaks down elastin, not involved in collagen cross-linking.

• Hyaluronidase: Degrades hyaluronic acid in ground substance, not collagen fibers.

Hurler syndrome (Mucopolysaccharidosis type I, MPS I) is an autosomal recessive lysosomal storage disorder.

• The hallmark features include corneal clouding, developmental delay/mental retardation, coarse facial features (“gargoylism”), hepatosplenomegaly, dwarfism, and cardiac issues.

• The underlying problem is deficiency of α-L-iduronidase, an enzyme responsible for the degradation of glycosaminoglycans (GAGs) such as dermatan sulfate and heparan sulfate.

• As a result, these GAGs accumulate in tissues, causing the systemic symptoms.

❌ Why the other options are incorrect:

• Heparan sulfamidase → deficient in Sanfilippo syndrome.
• β-Glucuronidase → deficient in Sly syndrome (MPS VII).
• N-Acetylglucosamine → seen in Sanfilippo syndrome type IIIb.
• Iduronate sulfate → not the specific enzyme involved in Hurler’s

Hurler syndrome (MPS I) is caused by a deficiency of alpha-L-iduronidase, a lysosomal enzyme needed for the degradation of dermatan sulfate and heparan sulfate. Without this enzyme, these glycosaminoglycans accumulate in tissues, leading to developmental delay, coarse facial features, hepatosplenomegaly, corneal clouding, and skeletal abnormalities.

❌ Why the other options are incorrect:

• Iduronate-2-sulfatase: Deficient in Hunter syndrome (MPS II).

• Heparin sulfamidase: Deficient in Sanfilippo syndrome (MPS IIIA).

• N-acetyl-glucosaminidase: Deficient in Sanfilippo syndrome (MPS IIIB).

• Beta-glucuronidase: Deficient in Sly syndrome (MPS VII).

During collagen synthesis, hydroxylation of proline and lysine residues is essential for the stability of the collagen triple helix. This reaction requires ascorbic acid (Vitamin C) as a cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase.
In the absence of Vitamin C, defective hydroxylation leads to weak collagen, resulting in scurvy (bleeding gums, poor wound healing, fragile blood vessels).

❌ Why the other options are incorrect:

• Vitamin B: General term; not specifically involved in hydroxylation of collagen.

• Vitamin E: Functions mainly as an antioxidant; not required for collagen synthesis.

• Vitamin B12: Important for DNA synthesis and red blood cell maturation; no role in collagen hydroxylation.

• Vitamin A: Involved in vision, epithelial differentiation, and bone growth; not directly linked to hydroxylation in collagen synthesis.

Chondroitin sulfate is a glycosaminoglycan (GAG) found in cartilage, tendons, ligaments, and aorta. Its repeating disaccharide unit is composed of:

• N-acetylgalactosamine (amino sugar)

• Glucuronic acid (uronic acid)

These disaccharides are sulfated at various positions, giving chondroitin sulfate its negative charge and ability to attract water, which provides compressive strength and elasticity to cartilage.

❌ Why the other options are incorrect:

• N-acetylglucosamine + iduronic acid: Forms part of heparan sulfate, not chondroitin sulfate.

• Glucuronic acid + iduronic acid: Found in dermatan sulfate, but lacks an amino sugar.

• N-acetylglucosamine + glucuronic acid: Forms part of hyaluronic acid, not sulfated.

• N-acetylgalactosamine + iduronic acid: Found in dermatan sulfate, not chondroitin sulfate.

Glycine is the most abundant amino acid in collagen, making up about one-third of its amino acid content. In the triple-helix structure of collagen, every third residue is glycine. This regular spacing is essential because glycine is the smallest amino acid, with a single hydrogen atom as its side chain. Its small size allows the three collagen alpha chains to pack tightly together in the triple helix without steric hindrance.

This triple-helix is stabilized by interchain hydrogen bonds, and the repeating sequence is typically Gly-X-Y, where X and Y are often proline and hydroxyproline, respectively.

Why the other options are incorrect:

• Histidine ❌
  Not particularly abundant in collagen and does not significantly contribute to the triple-helix structure.

• Proline ❌
  While proline and hydroxyproline are important for collagen stability and give rigidity to the helix, they are not the most abundant amino acids. Proline is often found in the X or Y positions of the Gly-X-Y repeat.

• Arginine ❌
  Not a major component of collagen and plays no significant structural role in the helix.

• Lysine ❌
  Lysine and its hydroxylated form hydroxylysine are involved in cross-linking collagen fibrils, which strengthens the matrix, but they are not the most abundant and are not central to the triple helix formation.

Parathyroid hormone (PTH) plays a central role in calcium homeostasis. It is secreted in response to low serum calcium levels and acts on multiple organs to raise calcium levels in the blood — but directly on only bones and kidneys.

Here’s how it works:

• Bone:
  PTH stimulates osteoclast activity indirectly via osteoblasts, increasing bone resorption. This releases calcium and phosphate into the bloodstream.

• Kidneys:
  PTH:

  • Increases calcium reabsorption in the distal tubules, preventing its excretion.

  • Decreases phosphate reabsorption in the proximal tubules, helping prevent calcium-phosphate precipitation in blood.

  • Stimulates production of active vitamin D (calcitriol) via 1-alpha-hydroxylase, which acts on the intestine indirectly.

Let’s rule out the other options:

• Kidney ❌
  It is one of the correct sites, but not the only one. Bone is equally important.

• Bone ❌
  Also only partially correct. PTH acts directly on both bone and kidney.

• Intestine ❌
  PTH does not act directly on the intestine. The intestinal effect (increased calcium absorption) is indirectly mediated by calcitriol (activated vitamin D), which is synthesized in the kidney under PTH stimulation.

• Skeletal muscle ❌
  PTH has no direct action on skeletal muscle related to calcium homeostasis.

Type I collagen is the most abundant collagen in the human body. It is found in bones, tendons, skin, cornea, and scar tissue.

Structure:

• Composed of three polypeptide alpha chains (usually two α1 and one α2 chains)

• These chains wind around each other to form a right-handed triple helix

• Each alpha chain itself is a left-handed helix, but the three combine into a right-handed triple helix

This triple helical structure gives collagen high tensile strength, which is crucial for supporting connective tissues.

❌ Why the Other Options Are Incorrect

• Coiled structure:
  ❌ Non-specific and doesn’t describe the correct architecture of collagen.

• None of these:
  ❌ Incorrect, as the triple helix structure is well-established in collagen biology.

• Double helix structure:
  ❌ Refers to DNA, not collagen.

• Tertiary structure:
  ❌ Collagen is considered to be organized at a quaternary structural level, since it involves the assembly of multiple chains.

Vitamin D plays a central role in maintaining calcium and phosphate homeostasis, which is crucial for:

• Bone mineralization

• Neuromuscular function

• Cell signaling

How Vitamin D Regulates Calcium and Phosphorus:

1. Increases intestinal absorption of calcium and phosphate by upregulating calcium-binding proteins.

2. Stimulates bone resorption in conjunction with parathyroid hormone (PTH) when calcium is low, releasing both calcium and phosphate.

3. Promotes renal reabsorption of calcium and excretion of phosphate, balancing serum levels.

The active form of Vitamin D is calcitriol (1,25-dihydroxycholecalciferol), synthesized in the kidneys under PTH influence.

❌ Why the Other Options Are Incorrect

• Vitamin B:
  ❌ Involved in energy metabolism and nervous system function, not mineral regulation.

• Glycogen:
  ❌ A storage form of glucose, not related to calcium or phosphate homeostasis.

• Vitamin C:
  ❌ Important for collagen synthesis and immune function, but not directly involved in calcium/phosphorus regulation.

• Insulin:
  ❌ Regulates glucose metabolism, not calcium or phosphate levels directly.

The extracellular matrix (ECM) is a complex, dynamic network composed of fibrous proteins (like collagen and elastin) and ground substance, which includes glycosaminoglycans (GAGs) and proteoglycans.

Proteoglycans are proteins that have one or more glycosaminoglycan chains covalently attached to a core protein. These GAGs are long, linear, and negatively charged polysaccharides, and they give proteoglycans their ability to retain water, resist compressive forces, and mediate cell signaling.

Structure:

• Core protein

• Covalently attached GAGs (like chondroitin sulfate, keratan sulfate, heparan sulfate, etc.)

• Often linked to hyaluronic acid as a backbone (non-covalently), especially in large ECM aggregates

❌ Why the Other Options Are Incorrect:

• Chondroitin sulphate → This is a type of GAG, not a protein. It can be part of a proteoglycan but is not a proteoglycan itself.

• Hyaluronic acid → A non-sulfated GAG that is unique because it is not covalently linked to a protein core. It serves as a backbone for large proteoglycan aggregates but is not a proteoglycan itself.

• Heparin sulphate → Another individual GAG, found in basement membranes and involved in cell signaling, but again, not a protein-GAG complex by itself.

• Keratan sulphate → A type of GAG, found in the cornea and cartilage. Like the others, it may be a part of a proteoglycan, but it is not the full complex.

Vitamin D is not active in the form we ingest it. It must undergo two hydroxylation steps—first in the liver, then in the kidney—to become biologically active.

🧬 Step-by-step Activation of Vitamin D:

• Cholecalciferol (Vitamin D₃)
  → This is the inactive form obtained from the skin (via UV light) or diet.
• 25-hydroxycholecalciferol (Calcidiol)
  → Formed in the liver through hydroxylation of cholecalciferol.
  → This is the major circulating form, used to assess vitamin D levels in blood.
  → Still inactive.
• 1,25-dihydroxycholecalciferol (Calcitriol)
  → Formed in the kidney by another hydroxylation.
  → This is the biologically active form of vitamin D.
  → Regulates calcium and phosphate absorption in the gut, bone remodeling, and kidney reabsorption of calcium.

❌ Why the Other Options Are Incorrect:

• 25-hydroxycholecalciferol: Intermediate form; not the biologically active hormone.
• Riboflavin: Vitamin B2, unrelated to vitamin D metabolism.
• Cholecalciferol: Inactive precursor form of vitamin D₃.
• Niacin: Vitamin B3, used in NAD⁺/NADP⁺ metabolism; unrelated to vitamin D activation.

The most important repeating unit in the collagen alpha chain is the Gly–X–Y motif, where:

• Glycine is always at every third position,

• X is often proline, and

• Y is often hydroxyproline.

Why glycine?
Because it’s the smallest amino acid, with only a hydrogen atom as its side chain. This minimal steric hindrance allows the three chains to pack tightly together. In the central axis of the triple helix, no other amino acid is small enough to fit without disrupting the structure.

If glycine is replaced by a bulkier residue (as in some mutations), the tight packing is disturbed, leading to improper folding and structurally weak collagen — which is the root cause of osteogenesis imperfecta.

❌ Why the Other Options Are Incorrect:

Confers rigidity to the collagen molecule
→ That function is primarily due to proline and hydroxyproline, which impose conformational constraints. Glycine, being flexible and small, doesn’t contribute rigidity.

Exists in D and L forms
→ Although amino acids can exist in D and L forms, only L-amino acids are used in mammalian proteins. This has no relevance to glycine’s specific role in collagen structure.

Is hydroxylated easily
→ It’s proline and lysine, not glycine, that undergo hydroxylation to form hydroxyproline and hydroxylysine — important for collagen crosslinking and stability, but unrelated to glycine’s positional significance.

Is a polar amino acid
→ Glycine is technically non-polar, although it’s often referred to as ambivalent. Regardless, its polarity is not the reason it appears every third residue in collagen. The key reason is its small size.

✅ Aggrecan – Correct.
Aggrecan is a major proteoglycan in the extracellular matrix (ECM) of cartilage. It contains glycosaminoglycan (GAG) side chains, like chondroitin sulfate and keratan sulfate, which are highly negatively charged and attract water. This gives cartilage its ability to resist compressive forces, maintain elasticity, and function as a smooth, lubricated surface for joint articulation.

Now, why are the others incorrect?

❌ Desmin – This is an intermediate filament protein found in muscle cells, where it helps maintain the structural integrity of sarcomeres. It is not found in cartilage ECM.

❌ Laminin – A glycoprotein that is a key component of the basement membrane, important in epithelial and endothelial tissue, not cartilage.

❌ Dystrophin – A cytoskeletal protein found in muscle fibers, linking the cytoskeleton to the extracellular matrix. Its absence leads to muscular dystrophies. It is not part of cartilage.

❌ Fibrillin – An ECM protein important for the formation of elastic fibers, especially in connective tissues like ligaments and skin. While it supports elasticity, it’s not a key ECM component of cartilage.

The parathyroid hormone (PTH) is a key regulator of serum calcium levels. It increases calcium levels by acting on bones, kidneys, and indirectly on the intestines via vitamin D activation. When calcium levels are low, PTH is secreted to restore balance.

✅ Softening of bones – Correct.
As PTH stimulates osteoclast activity, it increases bone resorption, releasing calcium into the bloodstream. Over time, excessive PTH or chronic stimulation of bone resorption can lead to decreased bone mineral density, resulting in bone softening (osteomalacia in adults, rickets in children). This is a compensatory mechanism to preserve serum calcium levels but at the expense of bone strength.

Let’s clarify why the other options are incorrect:

❌ Decline in parathyroid hormone levels – PTH levels increase, not decline, when calcium is low. A decline would worsen hypocalcemia.

❌ Inhibition of vitamin D synthesis – PTH actually stimulates the conversion of vitamin D to its active form (calcitriol) in the kidney, which then enhances intestinal calcium absorption.

❌ Excretion of calcium from kidneys – PTH promotes reabsorption of calcium in the distal tubules, reducing urinary calcium loss. It actually promotes phosphate excretion, not calcium.

❌ Hypertrophy of chondrocytes – This is more relevant to growth plate activity during development, not directly tied to serum calcium regulation or PTH action.

Calcitriol is the active hormonal form of vitamin D3. Its chemical name is:

1,25-dihydroxycholecalciferol

It is formed through a two-step hydroxylation process:

1. First hydroxylation in the liver → converts cholecalciferol (vitamin D3) to 25-hydroxycholecalciferol

2. Second hydroxylation in the kidney → converts it to 1,25-dihydroxycholecalciferol (calcitriol)

❌ Why the Other Options Are Incorrect:

• Cholecalciferol:
  → This is vitamin D3, the precursor, not the active form.

• 25-hydroxycholecalciferol:
  → This is calcidiol, the storage form found in blood, but not the biologically active form.

• 7-hydroxycholesterol:
  → Not directly related to vitamin D metabolism; it’s involved in cholesterol synthesis.

• 1,25-dihydroxycholesterol:
  → Incorrect name; the correct molecule is cholecalciferol, not cholesterol.

This question relates to the biochemistry of connective tissue, specifically the extracellular matrix (ECM). To answer it accurately, we need to understand what glycosaminoglycans (GAGs) are and which types of macromolecules contain them.

🔷 What Are Glycosaminoglycans (GAGs)?

• GAGs are long, unbranched polysaccharides composed of repeating disaccharide units.
• These disaccharides usually include a sugar acid (like glucuronic acid) and an amino sugar (like N-acetylglucosamine).
• GAGs are negatively charged, highly hydrophilic, and bind water, giving tissues the ability to resist compression and retain shape.

🔷 Proteoglycans: The Only Option That Contains GAGs

• Proteoglycans are proteins that consist of a core protein with one or more GAG chains covalently attached.
• Found abundantly in cartilage, tendons, and other connective tissues, they play critical roles in:
  • Providing mechanical strength
  • Allowing cell signaling
  • Maintaining hydration of tissues
• Examples include:

• Aggrecan (found in cartilage)
• Decorin
• Versican
• Perlecan

❌ Why the Other Options Are Incorrect

Carbonic anhydrase

• A zinc-containing enzyme involved in acid-base balance by catalyzing the reversible hydration of CO₂.
• Found in red blood cells and renal tubules.
• Does not contain GAGs.

Hemoglobin

• A globular protein responsible for oxygen transport in red blood cells.
• Composed of globin chains and heme groups.
• No structural or functional connection to GAGs.

Collagens

• Structural proteins in connective tissue.
• While they interact with proteoglycans and GAGs in the ECM, collagen molecules themselves do not contain GAG chains.

Albumin

• The most abundant plasma protein.
• Responsible for maintaining oncotic pressure and transporting substances.
• A simple protein with no GAG content.

Hurler syndrome (also known as Mucopolysaccharidosis type I, or MPS I) is a lysosomal storage disorder caused by a deficiency in the enzyme α-L-iduronidase.

🔹 Normal Function of α-L-iduronidase:

• Breaks down heparan sulfate and dermatan sulfate, both glycosaminoglycans (GAGs), in lysosomes.

• Without this enzyme, GAGs accumulate in various tissues and organs.

❌ Why the Other Options Are Incorrect:

• Beta-glucuronidase
  → Deficient in Sly syndrome (MPS VII), not Hurler syndrome.

• Heparin sulfamidase
  → Deficient in Sanfilippo syndrome type A (MPS IIIA).

• Iduronate-2-sulfatase
  → Deficient in Hunter syndrome (MPS II), which is X-linked and milder than Hurler’s; no corneal clouding.

• N-acetyl-glucosaminidase
  → Deficient in Sanfilippo syndrome type B (MPS IIIB).

During collagen synthesis, hydroxylation of lysine and proline residues is a critical post-translational modification that occurs in the rough endoplasmic reticulum. This hydroxylation allows for proper cross-linking of collagen fibers, which gives collagen its strength and stability.

🔹 Role of Vitamin C (Ascorbic Acid):

• Serves as a cofactor for:

  • Prolyl hydroxylase → hydroxylates proline

  • Lysyl hydroxylase → hydroxylates lysine

• These enzymes require Fe²⁺ (iron) to function. Vitamin C helps maintain iron in its reduced (active) form, preventing enzyme inactivation.

• Without sufficient vitamin C, these hydroxylation steps fail, leading to weak collagen fibers.

❌ Why the Other Options Are Incorrect:

• Vitamin A – Involved in epithelial maintenance and vision; not a cofactor in collagen synthesis.

• Vitamin E – Antioxidant; protects membranes, but not involved in hydroxylation.

• Vitamin B12 – Important in DNA synthesis and nervous system function; not involved here.

• Vitamin B (non-specific) – Several B vitamins exist (B1–B12), none are directly involved in collagen hydroxylation.

Parathyroid hormone (PTH) is secreted by the parathyroid glands in response to low serum calcium levels. It acts through multiple mechanisms to increase serum calcium, but not all actions are direct.

Let’s break down how PTH works:

1. Direct Action on Bone:
   PTH stimulates osteoblasts to increase the expression of RANKL, which in turn activates osteoclasts — the cells that resorb bone, releasing calcium into the blood.

2. Direct Action on Kidney:
   PTH increases calcium reabsorption in the distal tubules of the nephron and decreases phosphate reabsorption, helping to retain calcium in the body.

3. Indirect Action on Intestine:
   PTH does not act directly on the intestine. Instead, it stimulates the kidney to produce active vitamin D (calcitriol), which then promotes calcium absorption from the gut.

✅ Bones and kidneys both — Correct. PTH acts directly on bone and kidney to increase calcium levels.

❌ Intestine — Incorrect. PTH does not act directly on the intestine.

❌ Kidney — Partially correct, but not complete; bone is also a target.

❌ Skeletal muscle — Incorrect. PTH has no direct role in calcium handling in skeletal muscle.

❌ Bone — Also incomplete. Kidneys are also a direct target of PTH.

Heparan sulfate and dermatan sulfate are glycosaminoglycans (GAGs) — long unbranched polysaccharides found in the extracellular matrix and on cell surfaces. These GAGs are normally degraded in lysosomes through a series of enzyme-mediated steps.

One key enzyme in this pathway is iduronate sulfatase.

🔹 Iduronate sulfatase deficiency causes:

• Hunter syndrome (Mucopolysaccharidosis type II)

• It is an X-linked recessive lysosomal storage disorder.

• Leads to accumulation of heparan sulfate and dermatan sulfate in tissues.

❌ Why the Other Options Are Incorrect:

• Xylulose reductase
  → Involved in pentose metabolism, not GAG degradation.

• Glucose dehydrogenase
  → Involved in carbohydrate metabolism, not lysosomal function.

• Gulonolactone oxidase
  → Enzyme for vitamin C synthesis in animals (not functional in humans), unrelated to GAG breakdown.

• Amidotransferase
  → Generic name for a class of enzymes involved in nucleotide biosynthesis, not relevant here.

Vitamin D metabolism primarily involves two hydroxylation steps:

1. Liver:

• Enzyme: 25-hydroxylase
• Converts cholecalciferol (vitamin D₃) into 25-hydroxyvitamin D₃ [25(OH)D₃], the major circulating storage form.

2. Kidney:

• Enzyme: 1α-hydroxylase (CYP27B1)
• Converts 25(OH)D₃ into 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], the biologically active form (calcitriol).
• This active form increases calcium and phosphate absorption in the intestines and promotes bone health.

🔽

How the Body Lowers Active Vitamin D Levels:

• Induction of 24-hydroxylase (CYP24A1) — This enzyme deactivates both 25(OH)D₃ and 1,25(OH)₂D₃ by converting them to 24,25-dihydroxyvitamin D₃, an inactive form that is excreted.
• Reduction of 1α-hydroxylase activity — Leads to less synthesis of active 1,25(OH)₂D₃.

Together, these two mechanisms — less activation (via 1α-hydroxylase inhibition) and increased degradation (via 24-hydroxylase induction) — result in a significant reduction in vitamin D levels.

🔧 Factors That Trigger This:

• High calcium and phosphate levels
• FGF-23 (Fibroblast Growth Factor 23)
• Suppresses 1α-hydroxylase and stimulates 24-hydroxylase to maintain mineral balance.

❌ Why the Other Options Are Incorrect:

• Induction of 25-hydroxylase in kidney – Incorrect because 25-hydroxylase primarily acts in the liver, not kidney, and increasing it would raise the storage form rather than reduce vitamin D.
• Induction of 24-hydroxylase in kidney (alone) – This would reduce vitamin D partially, but the question asks for the most complete answer. Without reducing 1α-hydroxylase, some active vitamin D would still be produced.
• Reduction of 12-hydroxylase in kidney – There is no enzyme called 12-hydroxylase involved in vitamin D metabolism. This is a distractor.
• None of these – Incorrect because option 4 correctly describes the physiologic mechanism for decreasing vitamin D.

Vitamin D undergoes a two-step hydroxylation process to become biologically active. The process begins with 7-dehydrocholesterol, a precursor located in the skin. Upon exposure to ultraviolet B (UVB) radiation from sunlight, this precursor is converted to cholecalciferol (vitamin D3).

Once formed, cholecalciferol travels through the bloodstream to the liver, where it undergoes hydroxylation at the 25th carbon position by the enzyme 25-hydroxylase. This converts it into 25-hydroxyvitamin D3 (calcidiol). Calcidiol is the major circulating form of vitamin D in the body and serves as the best indicator of vitamin D status. However, while it has some biological activity, it is not the most potent form.

The next critical transformation occurs in the kidney, where the enzyme 1-alpha-hydroxylase hydroxylates calcidiol at the 1-alpha position, producing 1,25-dihydroxyvitamin D3 (calcitriol). This molecule is the biologically active form of vitamin D, functioning as a hormone.

Calcitriol binds to vitamin D receptors (VDRs) in target tissues, exerting key physiological effects. It plays a crucial role in:

• Enhancing intestinal absorption of calcium and phosphate
• Promoting bone mineralization and remodeling
• Regulating renal reabsorption of calcium
• Maintaining overall calcium and phosphate balance

Without this active metabolite, the body would struggle to maintain proper bone health and mineral homeostasis.

❌ Why the Other Options Are Incorrect:

None of these is incorrect because the correct active metabolite is clearly listed among the options.

25-hydroxyvitamin D3 is the storage and circulating form, not the fully active hormonal form. Its primary role is as a reservoir that can be converted when needed.

7-dehydrocholesterol is the precursor molecule in the skin that starts the pathway upon UVB exposure. It is not biologically active.

1-hydroxyvitamin D3 is an incorrect or incomplete term. The physiologically relevant molecule is 1,25-dihydroxyvitamin D3, not a single hydroxylated form at position 1 alone.

Calcitonin is a hormone secreted by the parafollicular (C cells) of the thyroid gland. It functions to lower elevated plasma calcium levels by:

• Inhibiting osteoclast activity, thereby reducing bone resorption

• Promoting calcium excretion by the kidneys

Its role is more prominent in protecting against hypercalcemia rather than maintaining day-to-day calcium balance.

✅ Decreases plasma calcium concentration — Correct. Calcitonin lowers blood calcium levels.

❌ Increases plasma calcitriol concentration — Calcitriol (active vitamin D) is regulated by parathyroid hormone and kidney function, not directly by calcitonin.

❌ Increases plasma calcium concentration — Opposite of calcitonin’s function; this is the role of parathyroid hormone and calcitriol.

❌ Increases plasma potassium concentration — Unrelated to calcitonin; potassium is regulated primarily by aldosterone.

❌ Decreases plasma magnesium concentration — Calcitonin does not regulate magnesium levels.

Plasma calcium levels are tightly regulated by three key hormones:

1. Parathyroid Hormone (PTH)
   – Secreted in response to low calcium
   – Increases calcium levels by:

   • Stimulating bone resorption (osteoclast activity)

   • Increasing renal calcium reabsorption

   • Stimulating production of calcitriol (active Vitamin D)

2. Calcitriol (Vitamin D₃)
   – Enhances intestinal absorption of calcium
   – Works synergistically with PTH

3. Calcitonin
   – Secreted by parafollicular (C) cells of the thyroid gland
   – Released when plasma calcium levels rise
   – Lowers blood calcium by:

   • Inhibiting osteoclast activity (reducing bone resorption)

   • Promoting calcium deposition into bone

Thus, calcitonin serves as a protective hormone to prevent hypercalcemia.

❌ Why the Other Options Are Incorrect:

• Aldosterone
  → Regulates sodium and potassium balance, not calcium

• Parathyroid hormone (PTH)
  → Increases when calcium is low, not high

• Calcitriol
  → Activated in response to low calcium or PTH stimulation, not directly secreted due to high calcium

• Vasopressin (ADH)
  → Regulates water retention and plasma osmolality, not calcium levels

Vitamin C (ascorbic acid) is essential for collagen synthesis, which is critical for wound healing. It acts as a cofactor for prolyl and lysyl hydroxylase enzymes that hydroxylate proline and lysine residues in collagen. Without sufficient collagen, the body cannot properly repair connective tissue, resulting in poor wound healing.

✅ Vitamin C — Correct. Deficiency impairs collagen synthesis, leading to delayed wound healing.

❌ Vitamin D — Important for calcium metabolism and bone health, but not primarily involved in tissue healing.

❌ Vitamin E — Functions as an antioxidant; does not directly affect collagen formation or wound repair.

❌ Vitamin K — Important for blood clotting; affects hemostasis more than tissue healing.

❌ Vitamin A — Important for epithelial regeneration, but collagen synthesis relies more directly on Vitamin C.

Collagen has a unique triple-helical structure formed by three polypeptide chains. To allow tight packing of these chains, every third amino acid in the sequence must be very small — glycine is the only amino acid small enough to fit into this restricted space. The characteristic repeating sequence of collagen is Gly-X-Y, where X and Y are often proline or hydroxyproline, but glycine is always at every third position.

✅ Glycine — Correct. Glycine is present at every third position in collagen, allowing the triple helix to form.

❌ Glutamate — Occasionally present but not in a fixed position or pattern.

❌ Hydroxyproline — Important for stability of the collagen helix but does not occur at every third position.

❌ Proline — Common in the X or Y position, but not at every third position.

❌ Lysine — Involved in cross-linking of collagen fibers but not regularly in every third position.

During collagen synthesis, the hydroxylation of proline and lysine residues in the collagen polypeptide chains is an essential step. This hydroxylation is catalyzed by prolyl hydroxylase and lysyl hydroxylase enzymes. These enzymes require ascorbate (vitamin C) as a cofactor to function properly. Without ascorbate, hydroxylation is impaired, leading to unstable collagen — this is the biochemical basis of scurvy.

✅ Ascorbate — Correct. It is the necessary cofactor for proline and lysine hydroxylation in collagen synthesis.

❌ Calcitriol — The active form of vitamin D; regulates calcium metabolism but is not involved in collagen hydroxylation.

❌ Retinol — Vitamin A; involved in epithelial differentiation and vision, not collagen hydroxylation.

❌ Folate — Functions in nucleotide synthesis and methylation reactions, unrelated to collagen hydroxylation.

❌ Cholecalciferol — Inactive precursor of vitamin D3; not related to collagen synthesis.

When prescribing calcium supplements or analyzing dietary intake, it’s crucial to consider the % of elemental calcium in a given compound — this determines how much usable calcium the body can absorb from the dose.

Let’s compare the elemental calcium content:

🔹. Calcium Carbonate (CaCO₃)

• Elemental calcium content: ~40%

• Most concentrated form among common supplements

• Requires an acidic environment (i.e., must be taken with food)

• ⚠️ Less effective in people with achlorhydria (low stomach acid)

🔹. Calcium Citrate

• Elemental calcium content: ~21%

• Better bioavailability than carbonate in low acid environments

• Can be taken without food

• ⚠️ Needs larger doses to deliver equivalent calcium

❌ Why the Other Options Are Incorrect:

. Calcitriol

• This is not a calcium salt but the active form of vitamin D3 (1,25-dihydroxycholecalciferol).

• It contains no elemental calcium — it enhances calcium absorption, but doesn’t provide calcium itself.

• ❌ Not a calcium source

. Phytic acid

• A plant-based compound that actually binds calcium and inhibits its absorption.

• It may contain phosphorus, but not calcium in a usable form.

• ❌ Calcium blocker, not a calcium source

. None of these

• ❌ Incorrect — we clearly have valid calcium-containing compounds listed.

🔬 Summary of Elemental Calcium % in Common Forms:

📌 Let’s first understand how calcium absorption works:

• Calcium is absorbed in the small intestine, both actively (vitamin D–dependent) and passively (paracellular diffusion).

• Several dietary and physiological factors affect its bioavailability.

🔹 Phytic Acid (also called phytate):

• Found in whole grains, seeds, legumes, and fiber-rich foods.

• It chelates (binds) minerals like calcium, iron, and zinc, forming insoluble complexes that cannot be absorbed.

• Hence, phytic acid inhibits calcium absorption in the gut.

This is why diets high in unrefined cereals (without fermentation or soaking) may lead to mineral deficiencies.

❌ Why the Other Options Are Incorrect:

. Dihydrate

• This is not a specific compound unless a context is given.

• “Dihydrate” just means a substance containing two molecules of water, e.g., calcium oxalate dihydrate — but as written, it’s ambiguous and not a direct disruptor of calcium absorption.

• ❌ Not meaningful without a proper chemical name

. Calcium citrate

• A soluble form of calcium that’s easily absorbed, especially in people with low stomach acid (e.g., elderly or on PPIs).

• It does not inhibit calcium absorption — in fact, it’s better absorbed than calcium carbonate in some situations.

• ❌ Enhances absorption

. Calcitriol

• The active form of vitamin D (1,25-dihydroxycholecalciferol).

• It stimulates active transport of calcium in the small intestine by increasing expression of calcium-binding proteins (like calbindin).

• ❌ Promotes calcium absorption

Calcium carbonate

• A common calcium supplement.

• Requires an acidic environment for absorption — best taken with food.

• May be less bioavailable in people with low gastric acid, but it does not disrupt absorption.

• ❌ Provides calcium, doesn’t inhibit it