Page 8 of 8
BI5.1-9 | Chemistry & Metabolism of Proteins and Immunology — Gate Quiz
Click any question card to reveal the correct answer.
Which amino acid contains an imidazole side chain that acts as a proton shuttle in enzyme catalysis and has a pKa near physiological pH (6.0)?
Correct! Histidine has an imidazole side chain (pKa approximately 6.0), making it uniquely suited as a proton acceptor/donor at physiological pH. It is the only amino acid that functions as a proton shuttle near pH 7.4.
Histidine is crucial in: haemoglobin (Bohr effect), enzyme active sites (serine proteases, carbonic anhydrase), and haem binding. Its pKa is closest to blood pH, allowing it to buffer effectively.
Incorrect. Histidine imidazole (pKa approximately 6.0) is the proton shuttle near physiological pH.
Click to reveal answer
Which of the following amino acids is essential (cannot be synthesized by humans) and is important for serotonin and melatonin synthesis?
Correct! Tryptophan is an essential amino acid and is the precursor for serotonin (5-HT) and melatonin synthesis.
Essential amino acids (PVT TIM HaLL): Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Arginine (conditionally), Lysine, Leucine. Tryptophan is also precursor for nicotinamide (NAD+).
Incorrect. Tryptophan is essential and is the precursor for serotonin and melatonin.
Click to reveal answer
The alpha-helix and beta-pleated sheet are examples of which level of protein structure?
Correct! Secondary structure refers to local folding patterns (alpha-helix, beta-sheet) stabilized by hydrogen bonds between backbone amide groups.
Protein structure levels: Primary = amino acid sequence (peptide bonds). Secondary = local folding (alpha-helix, beta-sheet, beta-turn) via backbone H-bonds. Tertiary = overall 3D shape via side chain interactions. Quaternary = multi-subunit assembly (e.g., haemoglobin = 2 alpha + 2 beta).
Incorrect. Alpha-helices and beta-sheets are secondary structural elements.
Click to reveal answer
A patient with hyperammonaemia due to a urea cycle defect presents with encephalopathy. Ammonia is toxic to the brain primarily because it:
Correct! Excess ammonia reacts with alpha-ketoglutarate (via glutamate dehydrogenase in reverse) to form glutamate/glutamine, depleting alpha-KG from the TCA cycle, impairing ATP production in neurons.
Hyperammonaemia treatment rationale: Lactulose (lowers gut ammonia), sodium benzoate/phenylbutyrate (alternative nitrogen excretion pathways), arginine supplementation (for some cycle defects), protein restriction, dialysis in acute crises.
Incorrect. Ammonia toxicity is primarily due to depletion of alpha-ketoglutarate from the TCA cycle, reducing neuronal ATP.
Click to reveal answer
In transamination reactions, pyridoxal phosphate (PLP) acts as a coenzyme. PLP is the active form of which vitamin?
Correct! Pyridoxal phosphate (PLP) is the active form of vitamin B6 (pyridoxine). It is the coenzyme for all aminotransferases (transaminases) and many other reactions of amino acid metabolism.
B6 (PLP) is required for: all transamination reactions (ALT, AST), decarboxylation of amino acids (dopa decarboxylase for dopamine), glycogen phosphorylase, and cystathionine beta-synthase. B6 deficiency causes peripheral neuropathy, dermatitis, and microcytic anaemia.
Incorrect. PLP is the active form of vitamin B6 (pyridoxine).
Click to reveal answer
The Bohr effect describes which phenomenon related to haemoglobin oxygen binding?
Correct! The Bohr effect: increased CO2 and decreased pH (as in exercising tissues) shift the oxygen dissociation curve to the right, decreasing Hb-O2 affinity and promoting O2 release to tissues.
Factors shifting O2-dissociation curve RIGHT (decreased affinity, more O2 delivery): increased CO2, decreased pH (Bohr effect), increased temperature, increased 2,3-BPG. LEFT shift (increased affinity, less O2 delivery): fetal Hb (HbF), decreased CO2, increased pH, decreased temperature, decreased 2,3-BPG, CO poisoning.
Incorrect. The Bohr effect is the rightward shift of the O2 dissociation curve with increased CO2 and decreased pH.
Click to reveal answer
Sickle cell anaemia is caused by a single amino acid substitution in the beta-globin chain. The substitution is:
Correct! In HbS, glutamate (hydrophilic, negatively charged) at position 6 of the beta-globin chain is replaced by valine (hydrophobic). This causes HbS to polymerize under hypoxic conditions.
The Glu6Val substitution changes a hydrophilic surface residue to hydrophobic. Under low O2, HbS polymerizes into fibres distorting RBCs into sickle shape. This causes vaso-occlusion, haemolysis, and multiple organ damage. Single point mutation = pleiotropic effects.
Incorrect. In sickle cell anaemia, glutamate is replaced by valine at position 6 of beta-globin.
Click to reveal answer
An IgG molecule consists of which combination of polypeptide chains?
Correct! IgG (and all immunoglobulins) consist of two identical heavy chains and two identical light chains connected by disulfide bonds, forming a Y-shaped structure.
Immunoglobulin structure: Variable regions (VH + VL) form the antigen-binding site (Fab). Constant region of heavy chains forms Fc (complement activation, receptor binding). IgG: most abundant, crosses placenta, secondary immune response. IgM: primary response, pentamer. IgA: secretory, mucosal immunity. IgE: allergy/parasites.
Incorrect. Immunoglobulins have two heavy chains + two light chains in a Y-shaped structure.
Click to reveal answer
The classical pathway of complement activation is initiated by:
Correct! The classical pathway is initiated when C1q binds to antigen-antibody complexes (IgG or IgM bound to antigen). This triggers C1r and C1s activation.
Three complement pathways: Classical (antibody-antigen complex + C1q), Lectin (MBL + mannose), Alternative (C3 spontaneous hydrolysis on pathogen surfaces). All converge at C3 convertase. Terminal pathway: C5b-9 = membrane attack complex (MAC).
Incorrect. The classical pathway is initiated by C1q binding to antigen-antibody complexes.
Click to reveal answer
A patient with bacterial pneumonia has markedly elevated C-reactive protein (CRP). CRP is synthesized primarily by the liver in response to which cytokine?
Correct! IL-6 is the primary stimulus for hepatic synthesis of acute phase proteins including CRP, fibrinogen, serum amyloid A, and haptoglobin.
Acute phase proteins (IL-6 driven): CRP (opsonin, complement activator), fibrinogen (coagulation), serum amyloid A, haptoglobin (binds free Hb), ferritin (sequesters iron from bacteria). Negative acute phase proteins decrease: albumin, transferrin. CRP is used clinically to monitor infection and inflammation.
Incorrect. IL-6 is the primary cytokine stimulating hepatic production of acute phase proteins like CRP.
Click to reveal answer