MLT GlobeChemistry: Enzymes & cardiac markers – page 3
132 Chemistry MCQs on Enzymes & cardiac markers with answers and explanations.
Isoenzymes are best defined as:
Isoenzymes are structurally different forms of an enzyme (often from different tissues) that catalyse the same reaction, e.g. CK-MM, CK-MB and CK-BB.
Which creatine kinase isoenzyme is found mainly in the brain?
CK-BB is the main isoenzyme of brain and smooth muscle. CK-MM predominates in skeletal muscle, and CK-MB is found in highest proportion in heart muscle.
The CK-MB isoenzyme is found in its highest proportion in which tissue?
Cardiac muscle contains the highest proportion of CK-MB (roughly 20–30% of its CK), which is why CK-MB was used as a marker of myocardial infarction.
Serum gamma-glutamyltransferase (GGT) is mainly used as a marker of:
GGT is a sensitive marker of liver and biliary disease and is induced by alcohol. It is not raised in bone disease, which helps localise a raised ALP.
Many enzyme assays measure the change in NADH or NADPH. At which wavelength is this change usually read?
NADH and NADPH absorb strongly at 340 nm, whereas NAD+ and NADP+ do not, so the reaction is followed by absorbance change at 340 nm.
Alkaline phosphatase gets its name because:
ALP hydrolyses phosphate esters with a pH optimum around 9–10, so assays are run at alkaline pH.
Enzymes that transfer the amide nitrogen of glutamine to acceptors (e.g., in purine synthesis) belong to which class?
Amidotransferases hydrolyse glutamine's amide and pass the NH3 to substrates such as PRPP. Glutamine synthetase forms glutamine rather than using it.
Why does long-term aspirin use promote gastric hyperacidity and ulcers?
Prostaglandins E2 and I2 reduce acid secretion and boost mucus and bicarbonate. Aspirin irreversibly acetylates cyclooxygenase, removing this protection.
Which enzyme catalyses a reversible step used in both glycolysis and gluconeogenesis?
Phosphoglycerate kinase is near-equilibrium and works in both directions. Pyruvate kinase and hexokinase are irreversible glycolytic steps; fructose-1,6-bisphosphatase is used only in gluconeogenesis.
In Gq-coupled receptor signalling, PIP2 is split into inositol trisphosphate and diacylglycerol by:
Phospholipase C-β hydrolyses PIP2 to IP3 (releases Ca2+) and DAG. Protein kinase C is activated by DAG afterwards; phospholipase A2 releases arachidonic acid.
Aconitase, which converts citrate to isocitrate in the Krebs cycle, contains which prosthetic group?
Aconitase has a [4Fe–4S] cluster that binds citrate during dehydration–rehydration; fluoroacetate-derived fluorocitrate blocks it. It is not a zinc enzyme.
A competitive inhibitor is added to an enzyme assay. What happens to the kinetic constants?
The inhibitor competes for the active site, so more substrate is needed for half-maximal velocity, but high substrate still reaches Vmax. Lowered Vmax with unchanged Km is non-competitive inhibition.
Adding lactose to E. coli culture sharply increases synthesis of β-galactosidase. This regulation is called:
An inducer (allolactose) switches on new synthesis of the enzyme that acts on it. Allosteric activation increases activity of existing enzyme molecules without new synthesis.
Which enzyme incorporates free ammonium into an organic molecule, forming glutamine, and is important for ammonia detoxification in the brain?
Glutamine synthetase uses ATP to add NH4+ to glutamate. Glutaminase does the reverse, releasing ammonia; nitrogenase reduces N2 in bacteria but does not form glutamine.
IFCC reference methods specify which temperature for measuring serum enzyme activities?
IFCC primary reference procedures for enzymes such as ALT, AST and CK are performed at 37 °C, matching body temperature. 25 and 30 °C were older conventions; 56 °C denatures most enzymes.
An enzyme has a Km of 2 mmol/L. At a substrate concentration of 6 mmol/L, the velocity is what fraction of Vmax?
v/Vmax = [S]/(Km + [S]) = 6/8 = 0.75. Velocity equals 50% of Vmax only when [S] equals Km.
Dopamine β-hydroxylase, which converts dopamine to norepinephrine, requires which vitamin as a reductant?
This copper enzyme uses ascorbate to keep copper reduced during hydroxylation. Vitamin C also serves prolyl and lysyl hydroxylases in collagen synthesis.
Methylmalonyl-CoA mutase, whose failure raises urine methylmalonic acid, requires which coenzyme form of vitamin B12?
Deoxyadenosylcobalamin is the mutase cofactor; methylcobalamin serves methionine synthase. Cyano- and hydroxocobalamin are pharmaceutical forms that must be converted.
Which vitamin-derived coenzyme is NOT used by the α-ketoglutarate dehydrogenase complex?
The complex uses thiamine pyrophosphate, lipoate, CoA, FAD and NAD+, like pyruvate dehydrogenase. Tetrahydrofolate carries one-carbon units in other pathways.
Alpha-amylase needs a particular anion for full activity. Which ion is it?
Chloride is an allosteric activator of alpha-amylase, and amylase reagents often include it. Calcium is also needed, but as a structural cation.