Mbbs First year biochemistry protein metabolism short questions and answers test for revision class Maintain question level medium to hard according to pgims university Rohtak Maximum no . Of questions 40

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🧪 MBBS First Year Biochemistry

Protein Metabolism - Short Questions & Answers

Revision Test | PGIMS University Rohtak Standard

Level: Medium to Hard | Total: 40 Questions

SECTION A: Nitrogen Balance & General Protein Metabolism (Q1-8)

Q1. Define nitrogen balance. Name two conditions each of positive and negative nitrogen balance.

Answer: Nitrogen balance = Nitrogen intake minus nitrogen excreted in urine, feces, and sweat.

Positive N balance (intake > excretion): Growing children, pregnancy, recovery from illness, anabolic steroid use
Negative N balance (excretion > intake): Starvation, severe burns, uncontrolled diabetes mellitus, Cushing's syndrome, post-surgical catabolism

Q2. List all essential amino acids. Which two are conditionally essential?

Answer: Essential (10): Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine (Mnemonic: PVT TIM HArL)

Conditionally essential: Arginine (insufficient synthesis in neonates/stressed states) and Histidine (insufficient synthesis in infants)

Q3. What is the biological value (BV) of a protein? Which protein has the highest BV and why?

Answer: BV = Nitrogen retained / Nitrogen absorbed × 100

It measures the efficiency with which absorbed dietary nitrogen is incorporated into body proteins.

Highest BV: Egg white (albumin) = 100 (reference standard)
Egg contains all essential amino acids in optimal proportions with no limiting amino acid
Casein (milk) = 77; Beef = 74; Wheat gluten = 44

Q4. Differentiate between glucogenic and ketogenic amino acids. Give two examples of each.

Answer:

Feature	Glucogenic	Ketogenic
Degradation products	Pyruvate, OAA, α-KG, succinyl-CoA, fumarate	Acetyl-CoA, acetoacetyl-CoA
Can form glucose?	Yes	No
Examples	Alanine, Aspartate, Glutamate, Glycine	Leucine, Lysine (purely ketogenic)

Both glucogenic AND ketogenic: Isoleucine, Phenylalanine, Tyrosine, Tryptophan, Threonine
Purely ketogenic (only 2): Leucine and Lysine

Q5. What is the role of vitamin B6 in protein metabolism? Name the enzyme that depends on it.

Answer:

Vitamin B6 (pyridoxine) is converted to pyridoxal phosphate (PLP) - the active coenzyme
PLP acts as a prosthetic group for transaminases (aminotransferases) and also for:
- Amino acid decarboxylases
- Cystathionase (transsulfuration pathway)
- ALA synthase (heme synthesis)
- Glycogen phosphorylase
In transamination, PLP temporarily accepts the amino group, forming pyridoxamine phosphate

Q6. What is a limiting amino acid? Which is the limiting amino acid in wheat and rice?

Answer: A limiting amino acid is the essential amino acid present in the lowest amount relative to body requirements in a dietary protein. It limits protein synthesis even if all other amino acids are adequate.

Wheat: Lysine is the limiting amino acid
Rice: Lysine is the limiting amino acid (also low in threonine)
Corn/Maize: Tryptophan (also low in lysine)
Legumes/Pulses: Methionine

This is why combining cereals + pulses (dal-roti) improves protein quality - mutual supplementation.

Q7. What is the Kwashiorkor vs. Marasmus distinction in protein metabolism terms?

Answer:

Feature	Kwashiorkor	Marasmus
Deficiency	Protein deficiency (adequate calories)	Both protein + calories
Serum albumin	Markedly low	Relatively preserved
Edema	Present (hypoalbuminemia)	Absent
Fatty liver	Present	Absent
Muscle wasting	Moderate	Severe
Appearance	"Moon face," skin lesions, edematous	Wizened, "old man" look

Q8. What happens to amino acids during prolonged fasting (>7 days)?

Answer:

Muscle proteolysis is initiated by fall in insulin and sustained by rise in glucocorticoids
Alanine and glutamine are quantitatively the most important glucogenic amino acids released
They are produced by catabolism of branched-chain amino acids (BCAA)
Alanine travels to liver via glucose-alanine cycle for gluconeogenesis
Glutamine is used as fuel by enterocytes, which release alanine
After 2-3 weeks, ketone bodies replace glucose as brain fuel, reducing proteolysis

SECTION B: Transamination & Deamination (Q9-16)

Q9. What is transamination? Write the reaction for alanine transamination. Name the coenzyme.

Answer: Transamination is the reversible transfer of an α-amino group from an amino acid to an α-keto acid, forming a new amino acid and a new keto acid.

Reaction:

L-Alanine + α-Ketoglutarate ⇌ Pyruvate + L-Glutamate (Enzyme: Alanine aminotransferase/ALT; Coenzyme: Pyridoxal phosphate/PLP)

Key points:

Glutamate is the "collection point" for amino groups
No net deamination - amino group is merely transferred
SGPT (ALT) is elevated in liver damage (most specific for hepatocellular injury)

Q10. What is oxidative deamination? Which enzyme catalyzes it and where does it occur?

Answer: Oxidative deamination is the removal of an amino group from an amino acid with simultaneous oxidation, releasing free ammonia (NH₃).

Reaction:

L-Glutamate + NAD⁺ → α-Ketoglutarate + NH₄⁺ + NADH + H⁺ (Enzyme: Glutamate dehydrogenase/GDH)

Located in: Mitochondrial matrix of liver (primarily)
GDH uses NAD⁺ or NADP⁺ as coenzyme
Allosteric regulation: Activated by ADP, GDP; Inhibited by ATP, GTP
This is the primary route by which amino groups enter as free NH₃ before urea synthesis

Q11. Distinguish between SGOT (AST) and SGPT (ALT) in clinical significance.

Answer:

Feature	SGOT (AST)	SGPT (ALT)
Full name	Aspartate aminotransferase	Alanine aminotransferase
Main location	Heart, liver, muscle, RBC	Liver (most specific)
Elevation in	MI, hepatitis, muscle disease	Viral hepatitis, drug-induced liver injury
Specificity for liver	Lower	Higher (liver-specific)
De Ritis ratio (AST/ALT)	>2 suggests alcoholic hepatitis	<1 suggests viral hepatitis
Coenzyme	PLP	PLP

Q12. What is the glucose-alanine cycle? State its significance.

Answer: The glucose-alanine cycle is an interorgan cycle between muscle and liver:

Muscle catabolizes BCAA → produces pyruvate (from glycolysis) + amino groups
Pyruvate + amino groups → Alanine (via ALT)
Alanine travels via blood to liver
Liver: Alanine → Pyruvate + amino group (via ALT)
Pyruvate → Glucose via gluconeogenesis
Glucose returns to muscle

Significance:

Transports amino groups from muscle to liver as non-toxic alanine
Maintains blood glucose during fasting/exercise
Analogous to the Cori cycle (but carries amino groups, not lactate)

Q13. What is the purine nucleotide cycle? Where does it function?

Answer: The purine nucleotide cycle operates primarily in skeletal muscle (also heart and brain) where GDH activity is low.

Steps:

AMP + H₂O → IMP + NH₃ (AMP deaminase)
IMP + Aspartate + GTP → Adenylosuccinate (adenylosuccinate synthetase)
Adenylosuccinate → AMP + Fumarate (adenylosuccinate lyase)

Net reaction: Aspartate → Fumarate + NH₃

Significance:

Allows deamination in muscle (where GDH is low)
Fumarate replenishes TCA cycle (anaplerosis)
Generates NH₃ for removal

Q14. What is transsulfuration? Name the enzymes and the product formed.

Answer: Transsulfuration is the pathway by which homocysteine is converted to cysteine using the sulfur from methionine.

Pathway:

Methionine → SAM → SAH → Homocysteine → Cystathionine → Cysteine + α-ketobutyrate

Key enzymes (both require PLP):

Cystathionine β-synthase (CBS): Homocysteine + Serine → Cystathionine
Cystathionase (γ-lyase): Cystathionine → Cysteine + α-ketobutyrate + NH₃

Clinical note: CBS deficiency causes homocystinuria - with lens dislocation (downward), mental retardation, thromboembolism, Marfanoid features.

Q15. What is meant by "transamination always precedes oxidative deamination"? Explain.

Answer: Most amino acids cannot be directly deaminated. The process is a two-step sequence:

Step 1 - Transamination: Amino acid transfers its -NH₂ to α-ketoglutarate, forming glutamate (via specific aminotransferases + PLP).

Step 2 - Oxidative deamination: Glutamate undergoes oxidative deamination by GDH in mitochondria, releasing free NH₄⁺ and regenerating α-ketoglutarate.

This coupling allows:

Efficient funneling of nitrogen from all amino acids into glutamate
Controlled release of NH₃ in liver mitochondria
α-Ketoglutarate to serve as a recycling "carrier"

Q16. What is reductive amination? Give an example.

Answer: Reductive amination is the reverse of oxidative deamination - the addition of NH₃ to an α-keto acid to form an amino acid, using NADH or NADPH.

Example:

α-Ketoglutarate + NH₄⁺ + NADPH + H⁺ → L-Glutamate + NADP⁺ + H₂O (Enzyme: Glutamate dehydrogenase, running in reverse)

Significance:

Primary route for NH₃ fixation into organic form
Glutamate then donates its amino group via transamination to other keto acids
Occurs in liver, kidneys, and brain

SECTION C: Urea Cycle (Q17-24)

Q17. Write the steps of the urea cycle. Indicate which steps occur in mitochondria vs. cytoplasm.

Answer:

In MITOCHONDRIA:

Carbamoyl phosphate synthesis: NH₄⁺ + CO₂ + 2ATP → Carbamoyl phosphate (Enzyme: CPS-I; Activator: N-acetylglutamate)
Citrulline formation: Carbamoyl phosphate + Ornithine → Citrulline (Enzyme: Ornithine transcarbamylase)

In CYTOPLASM (after citrulline exits mitochondria): 3. Argininosuccinate synthesis: Citrulline + Aspartate + ATP → Argininosuccinate (Enzyme: Argininosuccinate synthetase) 4. Argininosuccinate cleavage: Argininosuccinate → Arginine + Fumarate (Enzyme: Argininosuccinase) 5. Urea release: Arginine + H₂O → Urea + Ornithine (Enzyme: Arginase)

Ornithine re-enters mitochondria to restart the cycle

Q18. What is the energy cost of the urea cycle?

Answer: Synthesis of one mole of urea requires 4 moles of ATP (3 ATP equivalents expended as 4 high-energy phosphate bonds):

Step 1 (CPS-I): 2 ATP → 2 ADP + 2 Pi
Step 3 (Argininosuccinate synthetase): 1 ATP → AMP + PPi (equivalent to 2 ATP)

Total: 4 high-energy phosphate bonds per urea molecule

The two nitrogens in urea come from:

Free ammonia (from GDH in mitochondria) - 1st nitrogen
Aspartate (via transamination from OAA) - 2nd nitrogen

Q19. What is N-acetylglutamate? What is its role in the urea cycle?

Answer: N-acetylglutamate (NAG) is formed from glutamate + acetyl-CoA by N-acetylglutamate synthase (NAGS).

Role: NAG is the obligatory allosteric activator of CPS-I (carbamoyl phosphate synthetase I), the rate-limiting enzyme of the urea cycle. Without NAG, CPS-I is inactive.

Regulation:

Arginine (product of urea cycle) activates NAGS → more NAG → more urea synthesis (feed-forward activation)
High protein intake → high arginine → high NAG → increased urea production
NAGS deficiency causes hyperammonemia (treated with N-carbamylglutamate, a NAG analogue)

Q20. Compare CPS-I and CPS-II.

Answer:

Feature	CPS-I	CPS-II
Location	Mitochondrial matrix	Cytosol
Pathway	Urea cycle	Pyrimidine synthesis
Nitrogen source	Free NH₄⁺	Glutamine
Activator	N-acetylglutamate (essential)	PRPP, ATP
Inhibitor	None known	UTP (feedback)
ATP used	2 ATP	2 ATP

Q21. Name the inherited disorders of the urea cycle and the enzyme deficient in each.

Answer:

Disorder	Deficient Enzyme	Accumulating Metabolite
Hyperammonemia type I	CPS-I	NH₄⁺
Hyperammonemia type II	OTC (ornithine transcarbamylase)	NH₄⁺, orotic acid (most common; X-linked)
Citrullinemia	Argininosuccinate synthetase	Citrulline
Argininosuccinic aciduria	Argininosuccinase	Argininosuccinate
Hyperargininemia	Arginase	Arginine

OTC deficiency is the most common urea cycle disorder and the only X-linked one. It presents with elevated orotic acid (because excess carbamoyl phosphate spills into pyrimidine synthesis).

Q22. What is the link between the urea cycle and the TCA cycle (Krebs-Henseleit bicycle)?

Answer: The two cycles are interconnected at fumarate:

Urea cycle: Argininosuccinate → Fumarate + Arginine (by argininosuccinase)
Fumarate enters TCA cycle → Malate → OAA
OAA + Glutamate → Aspartate + α-KG (by AST)
Aspartate re-enters urea cycle to donate its nitrogen to citrulline

This linkage is called the "Krebs bicycle" or hepatic bicycle. It ensures:

Efficient nitrogen disposal
Anaplerosis of TCA cycle
No net carbon loss

Q23. How is ammonia transported from peripheral tissues to the liver?

Answer: Two main transport forms:

1. As Glutamine (major transport form - from most tissues including brain):

Glutamate + NH₃ → Glutamine (Enzyme: Glutamine synthetase; requires ATP)
Non-toxic, neutral amino acid
In liver: Glutamine → Glutamate + NH₃ (by glutaminase)

2. As Alanine (from muscle):

Via glucose-alanine cycle (described above)
In liver: Alanine → Pyruvate + NH₃ → enters urea cycle

In kidney:

NH₃ from glutamine is excreted directly as NH₄⁺ into urine (important in acid-base balance)
During acidosis, renal glutaminase activity increases markedly

Q24. What are the clinical features of hyperammonemia? Explain the mechanism of CNS toxicity.

Answer: Clinical features: Vomiting, irritability, lethargy, asterixis, ataxia, seizures, cerebral edema, coma, death.

Mechanism of CNS toxicity:

Glutamine hypothesis (primary):
- NH₃ + Glutamate → Glutamine (consumes glutamate, depletes TCA α-KG)
- Glutamine accumulates in astrocytes → osmotic swelling → cerebral edema
- Depletion of glutamate → impaired excitatory neurotransmission
Energy depletion: α-KG depletion impairs TCA cycle → reduced ATP in brain
Neurotransmitter imbalance: NH₃ interferes with GABA and glutamate signaling
Alkalosis: NH₃ is a base; causes cerebral alkalosis

Treatment: Protein restriction, lactulose, sodium benzoate (traps glycine), sodium phenylbutyrate (traps glutamine).

SECTION D: Amino Acid Catabolism & Special Pathways (Q25-32)

Q25. Describe the catabolism of phenylalanine and tyrosine. Name the enzyme deficient in PKU.

Answer: Normal pathway: Phenylalanine → Tyrosine (Phenylalanine hydroxylase + BH4) → DOPA → Dopamine → Norepinephrine/Epinephrine → Homogentisate → Fumarylacetoacetate → Fumarate + Acetoacetate

Phenylketonuria (PKU):

Deficiency: Phenylalanine hydroxylase (PAH) (or BH4 deficiency)
Accumulation: Phenylalanine → phenylpyruvate, phenyllactate, phenylacetate (musty odor)
Features: Intellectual disability, fair skin/hair (low melanin), seizures, "mousy" odor
Treatment: Low-phenylalanine diet + tyrosine supplementation; Sapropterin (BH4) for responsive patients
Screening: Guthrie test (neonatal heel prick)

Q26. What is alkaptonuria? What accumulates and why does urine turn black?

Answer:

Deficient enzyme: Homogentisate oxidase (homogentisic acid oxidase)
Accumulated metabolite: Homogentisic acid (HGA) in blood and urine

Mechanism of black urine:

HGA undergoes auto-oxidation in alkaline conditions → forms a dark alkapton pigment (benzoquinone acetic acid)
Urine darkens on standing (or immediately if alkaline)

Clinical triad:

Black urine on standing
Ochronosis - bluish-black pigment deposition in connective tissue (ears, sclera, cartilage)
Arthritis - ochronotic arthropathy (hip and spine)

Inheritance: Autosomal recessive. Benign in childhood; arthritis develops in adulthood.

Q27. Name inborn errors of amino acid metabolism and the associated enzyme defects (tabular form).

Answer:

Disease	Amino Acid Affected	Enzyme Deficient	Key Feature
PKU	Phenylalanine	Phenylalanine hydroxylase	Intellectual disability, mousy odor
Alkaptonuria	Tyrosine	Homogentisate oxidase	Black urine, ochronosis
Albinism	Tyrosine	Tyrosinase	Lack of melanin
Tyrosinemia type I	Tyrosine	Fumarylacetoacetase	Liver failure, "cabbage" odor
Homocystinuria	Methionine	Cystathionine β-synthase	Downward lens dislocation
Maple syrup urine disease	BCAA (Leu, Ile, Val)	BCAA dehydrogenase	Sweet urine, cerebral edema
Hartnup disease	Tryptophan	Neutral amino acid transporter	Pellagra-like rash, ataxia

Q28. What is maple syrup urine disease (MSUD)? Which amino acids accumulate?

Answer: MSUD is an autosomal recessive disorder of branched-chain amino acid (BCAA) catabolism.

Deficient enzyme: BCAA α-ketoacid dehydrogenase (branched-chain keto acid dehydrogenase complex - BCKAD complex; requires TPP, lipoic acid, CoA, FAD, NAD⁺)

Accumulated amino acids and keto acids:

Leucine, Isoleucine, Valine (and their α-keto acids)

Features:

Sweet/maple syrup odor in urine (from sotolone)
Neonatal onset: poor feeding, vomiting, lethargy, seizures, cerebral edema
Leucine toxicity is most neurotoxic
Treatment: Dietary restriction of BCAA; thiamine supplementation (thiamine-responsive variant); BCAA-free formula

Q29. What is the one-carbon metabolism? Name the reactions that require tetrahydrofolate (THF).

Answer: One-carbon metabolism involves the transfer of single-carbon units (from methyl to formyl) carried by tetrahydrofolate (THF) as a coenzyme.

Sources of one-carbon units (loaded onto THF):

Serine → glycine + methylene-THF (main source)
Histidine catabolism → formimino-THF
Formate → formyl-THF

Reactions requiring THF:

Reaction	Form of THF	Product
dTMP synthesis	5,10-methylene-THF	dTMP (DNA synthesis)
Purine ring synthesis	10-formyl-THF	C2 and C8 of purine ring
Methionine regeneration	5-methyl-THF	Methionine (via B12-dependent MTR)
Serine ↔ Glycine	5,10-methylene-THF	Interconversion

Folate/B12 deficiency: Impairs dTMP synthesis → megaloblastic anemia. Folate "trap" occurs in B12 deficiency (5-methyl-THF cannot donate methyl without B12).

Q30. Describe the metabolism of tryptophan. What is its relationship to niacin (vitamin B3)?

Answer: Tryptophan has three major fates:

Protein synthesis
Serotonin synthesis: Tryptophan → 5-Hydroxytryptophan → Serotonin (5-HT) → Melatonin (requires B6, B12)
Kynurenine pathway (major, ~95%): Tryptophan → Kynurenine → 3-Hydroxykynurenine → Quinolinate → NAD⁺ (Niacin equivalent)

Tryptophan-Niacin relationship:

60 mg tryptophan = 1 mg niacin equivalent
This conversion requires B2 (FAD), B6 (PLP), and B3 itself
Deficiency states:
- Hartnup disease (impaired tryptophan absorption) → pellagra-like features
- Carcinoid syndrome (excess tryptophan → serotonin) → niacin deficiency → pellagra
- Isoniazid (INH) treatment → B6 antagonism → pellagra

Q31. What are branched-chain amino acids? Why are they metabolized differently from other amino acids?

Answer: BCAAs: Leucine, Isoleucine, Valine (all essential)

Key differences:

Site of catabolism: Primarily in peripheral tissues (muscle, adipose, heart, brain) - NOT in liver
- Liver lacks BCAA transaminase (branched-chain aminotransferase 2, BCAT2)
- Muscle is the major site
First step: Transamination by BCAT (not by ALT/AST)
Second step: Oxidative decarboxylation by BCKAD complex (requires TPP, lipoate, CoA, FAD, NAD⁺) - deficient in MSUD

Significance:

Important fuel during exercise and fasting
Leucine is unique: purely ketogenic AND the only amino acid that stimulates insulin secretion directly
Amino groups from BCAA catabolism donate to glutamate → alanine → glucose-alanine cycle

Q32. What is the role of glutamine in acid-base balance?

Answer: Glutamine plays a key role in renal acid-base regulation:

During acidosis: Renal glutaminase activity markedly increases
Glutamine → Glutamate + NH₃ (glutaminase)
Glutamate → α-KG + NH₃ (GDH)
NH₃ is secreted into renal tubular lumen → combines with H⁺ → NH₄⁺ (excreted in urine)
This buffers urinary pH and allows H⁺ excretion without lowering urine pH too drastically

Net effect of acidosis: Increased urinary NH₄⁺ excretion, increased renal glutaminase

Also: Glutamine → HCO₃⁻ generated during metabolism enters blood → helps correct acidosis

In alkalosis, the opposite occurs - reduced ammoniagenesis.

SECTION E: Protein Structure, Synthesis & Turnover (Q33-40)

Q33. What is the half-life concept in protein turnover? Which proteins have the shortest and longest half-lives?

Answer: Half-life of a protein is the time required for degradation and replacement of 50% of the protein pool in the body.

Shortest half-lives (most rapidly turned over):

Ornithine decarboxylase: ~11 minutes
HMG-CoA reductase: ~2-4 hours
Proto-oncoproteins (c-fos, c-myc): 30 min - 2 hours

Longest half-lives:

Collagen (skin/tendon): >300 days
Lens crystallins: Virtually the lifetime of the individual
DNA methyltransferases: Months

Determinants of protein half-life (PEST rule): Proteins rich in Proline, Eglutamate, Serine, Threonine sequences are rapidly degraded.

Q34. What is the ubiquitin-proteasome pathway? How does it mark proteins for degradation?

Answer: The ubiquitin-proteasome system (UPS) is the major intracellular protein degradation pathway.

Steps:

Activation: Ubiquitin (76 amino acid protein) is activated by E1 (ubiquitin-activating enzyme) using ATP
Conjugation: Ubiquitin transferred to E2 (ubiquitin-conjugating enzyme)
Ligation: E3 (ubiquitin ligase) transfers ubiquitin to target protein lysine residue
Polyubiquitination: Multiple ubiquitin molecules added (chain of ≥4 ubiquitins)
Degradation: Polyubiquitinated protein is recognized and degraded by the 26S proteasome (barrel-shaped)
Recycling: Ubiquitin is released and recycled by deubiquitinating enzymes

What triggers degradation?

N-end rule: Proteins with destabilizing N-terminal residues (Arg, Lys, Leu, Phe) have short half-lives
PEST sequences
Abnormal/misfolded proteins
Phosphorylation marks

Q35. What is the post-translational modification (PTM)? Give 4 biochemically significant examples.

Answer: PTMs are covalent modifications of polypeptide chains after ribosomal synthesis that alter function, localization, or stability.

Key examples:

Modification	Example	Significance
Phosphorylation	Glycogen phosphorylase (Ser)	Enzyme activation/inactivation
Glycosylation	N-glycosylation of Asn in IgG	Protein folding, cell recognition
Hydroxylation	Proline → Hydroxyproline in collagen (requires Vit C)	Triple helix stability
Carboxylation	Glu → γ-carboxyGlu in clotting factors II, VII, IX, X (Vit K)	Ca²⁺ binding for coagulation
Acetylation	N-terminal methionine removal + Ala acetylation	Protein stability (N-end rule)
Ubiquitination	Cyclin-CDK degradation	Cell cycle control

Q36. What is the signal hypothesis in protein targeting? Name the components.

Answer: The signal hypothesis (Blobel, 1975 - Nobel Prize 1999) explains how proteins destined for the ER, Golgi, lysosomes, or secretion are targeted.

Components:

Signal sequence (signal peptide): N-terminal hydrophobic sequence of 15-30 amino acids on the nascent polypeptide
Signal recognition particle (SRP): Ribonucleoprotein that recognizes and binds the signal peptide
SRP receptor (docking protein): On ER membrane; binds SRP
Translocon (Sec61 complex): Protein-conducting channel in ER membrane

Process: Signal peptide emerges → SRP binds → translation pauses → ribosome-SRP docks on ER → translation resumes → protein threaded into ER lumen → signal peptide cleaved by signal peptidase

Clinical relevance: Mutations in signal peptides cause misrouting → disease (e.g., I-cell disease - defective mannose-6-phosphate targeting to lysosomes)

Q37. Explain the structural organization of collagen. What is the role of vitamin C?

Answer: Collagen is the most abundant protein in the body (~30% of total protein).

Structural hierarchy:

Primary: Repeating tripeptide sequence (Gly-X-Y)n (Gly at every 3rd position; X = often Pro; Y = often Hydroxyproline)
Secondary/Tertiary: Left-handed polyproline II helix for each α-chain
Quaternary: Three α-chains wound into right-handed triple helix (tropocollagen) - stabilized by H-bonds via hydroxyproline and hydroxylysine
Extracellular: Tropocollagen → collagen fibrils → collagen fibers (cross-linked by allysine)

Role of Vitamin C (Ascorbic acid):

Required for prolyl hydroxylase and lysyl hydroxylase
Hydroxylates proline → hydroxyproline; lysine → hydroxylysine
Hydroxyproline stabilizes triple helix (H-bonding)
Hydroxylysine is glycosylated and cross-linked

Deficiency (Scurvy): Unstable collagen → bleeding gums, perifollicular hemorrhage, poor wound healing, corkscrew hair.

Q38. What is I-cell disease (Mucolipidosis II)? Explain the biochemical defect.

Answer: I-cell disease (Mucolipidosis type II) is a lysosomal storage disorder caused by defective lysosomal enzyme targeting.

Normal targeting: Lysosomal enzymes synthesized in ER → Golgi → mannose-6-phosphate (M6P) added → M6P receptor in TGN → directed to lysosomes

Biochemical defect:

Deficiency of GlcNAc-phosphotransferase (in cis-Golgi)
This enzyme adds phosphate to mannose residues
Without M6P signal, lysosomal enzymes cannot bind M6P receptor
Enzymes are secreted into extracellular space instead of reaching lysosomes
Lysosomes lack digestive enzymes → accumulation of undegraded material (inclusion bodies = "I cells")

Features: Coarse facial features, gingival hyperplasia, corneal clouding, skeletal abnormalities, intellectual disability; very high plasma lysosomal enzymes.

Q39. What is the SGOT/SGPT ratio (De Ritis ratio) and its clinical significance?

Answer: De Ritis ratio = AST(SGOT)/ALT(SGPT)

Ratio	Interpretation
>2 (typically 2-4)	Alcoholic hepatitis (mitochondrial AST released; also low ALT due to B6 deficiency from alcohol)
<1	Viral hepatitis (predominantly ALT rise; liver parenchymal damage)
>3	Severe alcoholic liver disease
Elevated AST >> ALT	Myocardial infarction (AST also elevated), hemolysis, muscle disease

Normal range:

ALT: 7-56 U/L
AST: 10-40 U/L

Why ALT is more liver-specific: ALT is found predominantly in liver cytoplasm; AST is found in liver, heart, muscle, kidney, and RBCs.

Q40. What is the biochemical basis of ochronosis in alkaptonuria? How does it differ from melanin?

Answer: Ochronosis in Alkaptonuria:

Homogentisic acid (HGA) accumulates due to homogentisate oxidase deficiency
HGA undergoes oxidative polymerization (auto-oxidation) catalyzed by peroxidases
Forms alkapton/ochronotic pigment (benzoquinone acetic acid polymer)
Pigment deposits in collagen-rich tissues: cartilage, tendons, sclera, skin
Macroscopically: blue-black pigmentation despite "ochre" (yellow) histological appearance
Weakens connective tissue → joint damage (ochronotic arthropathy)

Differences from Melanin:

Feature	Ochronotic pigment	Melanin
Precursor	Homogentisic acid	DOPA (from tyrosine)
Enzyme	Auto-oxidation	Tyrosinase
Location	Connective tissue, cartilage	Skin, hair, retina
Color	Blue-black (macroscopic); ochre (micro)	Brown-black
Disease association	Alkaptonuria	Albinism (deficiency)

📊 Quick Revision Summary Table

Topic	High-Yield Points
Urea cycle	5 steps; CPS-I in mitochondria; 4 ATP cost; NAG activator
Essential AA	10 essential; Leu + Lys purely ketogenic
PKU	PAH deficiency; BH4 cofactor; Guthrie test
MSUD	BCKAD deficiency; TPP cofactor; Leu most toxic
Transamination	PLP coenzyme; glutamate is central collector
OTC deficiency	Most common urea cycle disorder; X-linked; orotic acid elevated
Homocystinuria	CBS deficiency; downward lens; thrombosis
Collagen	Gly-X-Y; Vit C for hydroxylation; scurvy
One-carbon	THF + B12; folate trap; megaloblastic anemia
De Ritis ratio	>2 = alcoholic; <1 = viral hepatitis

Sources: Lippincott's Illustrated Biochemistry 8th Ed. (Harvey & Ferrier), Harper's Illustrated Biochemistry 32nd Ed., Guyton & Hall Physiology Standard: PGIMS Rohtak MBBS First Year - Medium to Hard level examination questions

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