Biochemistry - MCQ Practice Questions
Biochemistry sits at the point where chemistry stops being abstract and starts describing living systems. Practice covers carbohydrates, proteins and amino acids, lipids, nucleic acids, enzymes and enzyme kinetics, metabolic pathways, and vitamins and coenzymes. Pathway questions include the regulation step in the explanation, because that is usually what the question is really testing rather than the sequence itself.
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Which vitamin is essential for the carboxylation of glutamate residues in clotting factors II, VII, IX, and X?
Understanding:
We need to identify which vitamin is required for the post-translational modification (carboxylation) of specific glutamate residues in coagulation factors.
Step 1: Identifying the biochemical reaction
Clotting factors II (prothrombin), VII, IX, and X require gamma-carboxylation of glutamate (Glu) residues to form gamma-carboxyglutamate (Gla) residues. This reaction is catalysed by gamma-glutamyl carboxylase, which requires reduced Vitamin K (hydroquinone form) as a cofactor.
Step 2: Mechanism
Vitamin K acts as an essential cofactor in the carboxylation reaction. During this process, Vitamin K is oxidised to its epoxide form and must be recycled by Vitamin K epoxide reductase. Warfarin inhibits this recycling step, thereby acting as an anticoagulant.
Step 3: Ruling out other options
Vitamin E is an antioxidant with no direct role in coagulation factor carboxylation. Vitamin D is involved in calcium homeostasis and gene regulation. Vitamin A is involved in vision and epithelial differentiation.
Answer:
Vitamin K is the essential cofactor for gamma-carboxylation of glutamate residues in coagulation factors II, VII, IX, and X.
Quick Tip:
Vitamin K-dependent clotting factors can be remembered as "1972" — factors I (fibrinogen, though not truly K-dependent), II, VII, IX, X — along with Protein C and Protein S.
Which of the following hormones uses cyclic AMP (cAMP) as its second messenger?
Understanding:
We need to identify which listed hormone signals through the adenylyl cyclase–cAMP pathway.
Step 1: Classifying hormones by signalling mechanism
Hormones can be broadly classified by the receptors and second messengers they use:
Step 2: Identifying glucagon's pathway
Glucagon binds to a Gs-protein-coupled receptor on hepatocytes and adipocytes. Gs protein activates adenylyl cyclase, which converts ATP to cAMP. cAMP then activates Protein Kinase A (PKA), leading to glycogenolysis and gluconeogenesis.
Step 3: Ruling out other options
Aldosterone, cortisol, and testosterone are all steroid hormones derived from cholesterol. They use nuclear receptor-mediated gene regulation, not cAMP.
Answer:
Glucagon signals through the cAMP second messenger pathway via Gs-protein-coupled receptors.
Quick Tip:
Other hormones using cAMP include PTH, ADH (V2 receptor), TSH, LH, FSH, ACTH, and adrenaline (beta receptors). A common exam trap is confusing ADH — its V1 receptor uses IP3/DAG, but its V2 receptor uses cAMP.
Biotin serves as a coenzyme in which of the following reactions?
Understanding:
We need to identify which metabolic reaction requires biotin (Vitamin B7) as a cofactor.
Step 1: Role of biotin
Biotin is covalently attached to carboxylase enzymes and functions as a carrier of activated carbon dioxide (CO2) in carboxylation reactions. It is covalently bound to the epsilon-amino group of a lysine residue in the enzyme (forming biocytin).
Step 2: Key biotin-dependent enzymes
The major biotin-dependent carboxylases in humans are:
Step 3: Ruling out other options
Oxidative decarboxylation of pyruvate uses the pyruvate dehydrogenase complex (requiring TPP, lipoic acid, FAD, NAD+, and CoA — not biotin). Transamination requires pyridoxal phosphate (Vitamin B6). Hydroxylation of proline requires Vitamin C (ascorbic acid) and requires Fe2+ as a cofactor.
Answer:
Biotin is the coenzyme required for carboxylation of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase.
Quick Tip:
A helpful mnemonic for biotin-dependent enzymes is "ACC PP" — Acetyl-CoA Carboxylase, Pyruvate Carboxylase, Propionyl-CoA Carboxylase. All are carboxylases; biotin never participates in decarboxylation or transamination.
A deficiency of which vitamin leads to pellagra, characterized by the classic triad of dermatitis, diarrhea, and dementia?
Understanding:
We need to identify the vitamin whose deficiency causes pellagra with the classic triad of dermatitis, diarrhea, and dementia (the "3 Ds").
Step 1: Identifying Pellagra
Pellagra is caused by deficiency of Vitamin B3 (Niacin/Nicotinic acid) or its precursor, the amino acid tryptophan. Niacin is a precursor for NAD+ and NADP+, which are essential coenzymes in numerous oxidation-reduction reactions.
Step 2: Clinical features
The classic triad of pellagra is:
Some sources add a 4th D — Death — if untreated.
Step 3: Ruling out other options
Thiamine (B1) deficiency causes beriberi and Wernicke-Korsakoff syndrome. Riboflavin (B2) deficiency causes angular stomatitis, glossitis, and corneal vascularisation. Pyridoxine (B6) deficiency causes peripheral neuropathy, sideroblastic anemia, and glossitis.
Step 4: Additional association
Carcinoid syndrome and Hartnup disease can also cause pellagra due to impaired tryptophan availability for niacin biosynthesis.
Answer:
Vitamin B3 (Niacin) deficiency causes pellagra, presenting with dermatitis, diarrhea, and dementia.
Quick Tip:
Remember: in Hartnup disease, a defect in neutral amino acid transport impairs tryptophan absorption, leading to secondary niacin deficiency and pellagra-like symptoms despite an adequate diet.
Calcitriol (1,25-dihydroxycholecalciferol), the active form of Vitamin D, exerts its primary genomic effects by binding to which receptor type?
Understanding:
We need to identify the receptor through which calcitriol (active Vitamin D) mediates its primary genomic actions.
Step 1: Nature of Vitamin D
Vitamin D3 (cholecalciferol) is a fat-soluble, steroid-derived hormone. It is hydroxylated in the liver to 25-hydroxycholecalciferol and then in the kidney (by 1-alpha-hydroxylase) to 1,25-dihydroxycholecalciferol (calcitriol), its biologically active form.
Step 2: Receptor mechanism
Because calcitriol is lipid-soluble, it freely crosses the plasma membrane and binds to the Vitamin D Receptor (VDR), a member of the nuclear receptor superfamily. The calcitriol-VDR complex heterodimerises with the Retinoid X Receptor (RXR) and binds to Vitamin D Response Elements (VDREs) in the promoter regions of target genes, regulating their transcription.
Step 3: Genomic effects
Target genes include those encoding calcium-binding proteins (e.g., calbindin), TRPV6 calcium channels, and RANKL, which collectively mediate increased intestinal calcium absorption, renal calcium reabsorption, and bone mineralisation.
Step 4: Ruling out other options
G-protein-coupled receptors, receptor tyrosine kinases, and ligand-gated ion channels are used by water-soluble messengers that cannot cross the lipid bilayer. Calcitriol is lipid-soluble and uses a nuclear receptor.
Answer:
Calcitriol binds to the nuclear Vitamin D Receptor (VDR), which acts as a transcription factor to regulate gene expression.
Quick Tip:
All steroid hormones and thyroid hormone use nuclear receptors — this is a consistent exam theme. Remember: lipid-soluble = nuclear receptor; water-soluble = cell-surface receptor.
Thyroid hormone synthesis requires adequate dietary iodine. The step in which iodide is oxidised and incorporated into thyroglobulin tyrosine residues is catalysed by which enzyme?
Understanding:
We need to identify the enzyme that catalyses iodide oxidation and its organification onto thyroglobulin tyrosine residues.
Step 1: Thyroid hormone biosynthesis steps
The key steps in thyroid hormone synthesis are:
1. Iodide trapping: I− is actively transported into follicular cells via the Na+/I− symporter (NIS).
2. Oxidation and organification: Iodide is oxidised by thyroid peroxidase (TPO) using H2O2, and the reactive iodine is incorporated onto tyrosine residues of thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
3. Coupling: TPO also couples MIT and DIT to form T3 (MIT + DIT) and T4 (DIT + DIT).
4. Secretion: Thyroglobulin is retrieved by endocytosis, proteolysed in lysosomes, releasing T3 and T4.
Step 2: Role of thyroid peroxidase
Thyroid peroxidase (TPO) is the key enzyme responsible for both organification and coupling reactions. Antithyroid drugs such as propylthiouracil (PTU) and methimazole act by inhibiting TPO.
Step 3: Ruling out other options
Deiodinase enzymes convert T4 to the active T3 peripherally. Adenylyl cyclase is activated downstream of TSH receptor signalling (Gs pathway) but does not directly catalyse iodination. "Thyroglobulin synthase" is not a recognised enzyme.
Answer:
Thyroid peroxidase catalyses the oxidation and organification of iodide onto thyroglobulin tyrosine residues.
Quick Tip:
Propylthiouracil (PTU) has a dual advantage in thyrotoxicosis — it inhibits TPO AND blocks peripheral conversion of T4 to T3 by inhibiting Type 1 deiodinase, making it the preferred agent in thyroid storm.
Vitamin B12 (cobalamin) deficiency leads to megaloblastic anaemia partly because it impairs the conversion of which metabolite, thereby trapping folate in an unusable form?
Understanding:
We need to identify the metabolic reaction impaired in B12 deficiency and explain how it leads to folate trapping.
Step 1: The methylfolate trap
Vitamin B12 is required as a coenzyme for methionine synthase, which catalyses the transfer of a methyl group from N5-methyltetrahydrofolate (N5-methyl THF) to homocysteine, generating methionine and regenerating tetrahydrofolate (THF).
Step 2: Consequence of B12 deficiency
When B12 is deficient, methionine synthase cannot function. N5-methyl THF accumulates and cannot be converted back to THF. Since N5-methyl THF is the principal circulating form of folate, this traps the folate pool in the form of N5-methyl THF — a form that cannot participate in nucleotide synthesis. The consequence is functional folate deficiency and impaired DNA synthesis, leading to megaloblastic anaemia.
Step 3: Ruling out other options
The conversion of methylmalonyl-CoA to succinyl-CoA is also B12-dependent (adenosylcobalamin form), but this causes neurological disease (subacute combined degeneration), not folate trapping. Dihydrofolate reductase converts DHF to THF and is inhibited by methotrexate, not B12 deficiency. The serine hydroxymethyltransferase reaction uses N5,N10-methylene THF but is not impaired by B12 deficiency.
Answer:
B12 deficiency impairs the conversion of homocysteine to methionine, trapping folate as N5-methyl THF and causing functional folate deficiency.
Quick Tip:
This is why giving folate alone to a B12-deficient patient corrects the anaemia but DOES NOT prevent neurological damage — the methylfolate trap is bypassed, but the adenosylcobalamin-dependent myelin synthesis pathway remains impaired.
Insulin promotes glucose uptake in muscle and adipose tissue primarily by stimulating the translocation of which glucose transporter to the plasma membrane?
Understanding:
We need to identify the specific glucose transporter isoform whose plasma membrane expression is acutely regulated by insulin in muscle and adipose tissue.
Step 1: Glucose transporter isoforms and their tissue distribution
Step 2: Mechanism of insulin action on GLUT4
In the basal (fasting) state, GLUT4 is sequestered in intracellular vesicles. When insulin binds its receptor tyrosine kinase, autophosphorylation occurs, activating the PI3K-Akt signalling cascade. Akt phosphorylates AS160 (TBC1D4), which releases the inhibitory brake on GLUT4 vesicle fusion, causing GLUT4 translocation to the plasma membrane and increased glucose uptake.
Step 3: Clinical relevance
In Type 2 diabetes, insulin resistance impairs this GLUT4 translocation, leading to hyperglycaemia despite normal or elevated insulin levels.
Answer:
Insulin stimulates translocation of GLUT4 to the plasma membrane in muscle and adipose tissue to promote glucose uptake.
Quick Tip:
Exercise also stimulates GLUT4 translocation via an AMP-activated protein kinase (AMPK) pathway, independently of insulin. This is why exercise improves glycaemic control even in insulin-resistant states.
Which of the following correctly pairs a hormone with its site of synthesis and its primary chemical nature?
Understanding:
We need to identify which pairing correctly matches a hormone, its site of synthesis, and its chemical class.
Step 1: Evaluating each option
Step 2: Confirming Option C
Aldosterone is the primary mineralocorticoid. It is produced in the adrenal cortex (zona glomerulosa), regulated by the renin-angiotensin-aldosterone system (RAAS), and acts on the distal nephron to promote Na+ reabsorption and K+ excretion.
Answer:
Aldosterone is correctly paired with the adrenal cortex as its site of synthesis and is a steroid hormone.
Quick Tip:
Remember the adrenal cortex layers from outside in: Glomerulosa (mineralocorticoids — aldosterone), Fasciculata (glucocorticoids — cortisol), Reticularis (androgens). Mnemonic: "GFR" — same as Glomerular Filtration Rate.
Vitamin C (ascorbic acid) is essential for the activity of prolyl hydroxylase in collagen synthesis. Which of the following correctly describes its biochemical role in this reaction?
Understanding:
We need to identify the precise biochemical role of Vitamin C in the prolyl hydroxylase reaction during collagen synthesis.
Step 1: The prolyl hydroxylase reaction
Prolyl hydroxylase converts proline residues in collagen to 4-hydroxyproline, which is essential for the stability of the collagen triple helix through hydrogen bonding. The enzyme belongs to the family of iron- and 2-oxoglutarate-dependent dioxygenases, requiring:
Step 2: Role of Vitamin C
During the catalytic cycle, the Fe2+ cofactor at the active site becomes oxidised to Fe3+ (ferric state). Vitamin C (ascorbic acid) is required to reduce Fe3+ back to Fe2+, thereby regenerating the active form of the enzyme. Without adequate Vitamin C, prolyl hydroxylase becomes inactive (Fe3+ cannot be recycled), hydroxyproline cannot be formed, and the collagen triple helix is destabilised.
Step 3: Clinical consequence
Scurvy results from Vitamin C deficiency. Defective collagen causes perifollicular haemorrhages, gum disease, poor wound healing, and corkscrew hairs. The symptoms reflect the widespread requirement for stable collagen.
Step 4: Ruling out other options
Vitamin C does not act as a classical coenzyme, does not donate the hydroxyl group itself (O2 is the source via the dioxygenase mechanism), and does not activate the enzyme by phosphorylation.
Answer:
Vitamin C maintains the iron cofactor of prolyl hydroxylase in the active Fe2+ state by reducing Fe3+ back to Fe2+.
Quick Tip:
Lysyl hydroxylase, which hydroxylates lysine residues in collagen (essential for cross-linking), also requires Fe2+ and Vitamin C by the same mechanism. So Vitamin C deficiency impairs both proline and lysine hydroxylation.