MLT GlobeChemistry: Enzymes & cardiac markers
132 Chemistry MCQs on Enzymes & cardiac markers with answers and explanations.
A gout patient is started on a drug that lowers serum urate by blocking xanthine oxidase. The drug is:
Allopurinol (via oxypurinol) inhibits xanthine oxidase, reducing uric acid production. Probenecid increases urate excretion; colchicine only suppresses inflammation.
Glucocerebroside accumulates in macrophages in Gaucher disease. Which enzyme is deficient?
Glucocerebrosidase normally removes glucose from glucosylceramide. Hexosaminidase A deficiency causes Tay–Sachs; sphingomyelinase deficiency causes Niemann–Pick.
Lipase, amylase and proteases break bonds by adding water. They belong to which EC class?
Hydrolases (EC 3) cleave bonds using water. Ligases join molecules using ATP; oxidoreductases transfer electrons.
NSAIDs such as ibuprofen reduce prostaglandin synthesis by inhibiting:
NSAIDs block the cyclooxygenase activity of PGH synthase. Corticosteroids inhibit phospholipase A2 indirectly; zileuton targets 5-lipoxygenase.
Which of these proteins is a digestive enzyme released into pancreatic juice?
Pancreatic acinar cells secrete ribonuclease along with amylase, lipase and proteases. Myoglobin, cytochrome c and ferritin are intracellular oxygen-, electron- and iron-handling proteins, not secreted enzymes.
The Michaelis constant (Km) of an enzyme is best defined as the:
Km is the [S] at which v = ½Vmax; it has concentration units, not velocity units. It is independent of enzyme amount but changes with pH and temperature.
Plotting initial velocity against substrate concentration for an enzyme obeying Michaelis–Menten kinetics gives:
Velocity rises almost linearly at low [S] and levels off as the enzyme saturates. Sigmoidal curves indicate allosteric enzymes; bell shapes are typical of pH–activity plots.
A non-protein group that is permanently or very tightly bound to an enzyme, such as FAD in succinate dehydrogenase, is called a:
Prosthetic groups stay attached throughout catalysis. A cosubstrate such as NAD+ binds loosely and leaves after each reaction; the apoenzyme is the protein part alone.
Which peptidase is an exopeptidase that removes amino acids one at a time from the N-terminus?
Aminopeptidases act at the free amino end of peptides. Trypsin, chymotrypsin and pepsin are endopeptidases that cut bonds within the chain.
An inactive enzyme protein combines with its required coenzyme. The active complex formed is called the:
Apoenzyme plus cofactor equals holoenzyme. A zymogen is an inactive precursor activated by cleavage, isoenzymes are different forms of one enzyme, and ribozymes are catalytic RNA.
Allosteric enzymes, which do not follow Michaelis–Menten kinetics, give what shape of velocity versus substrate curve?
Cooperative binding between subunits produces an S-shaped curve. Hyperbolic curves are typical of Michaelis–Menten enzymes.
The enzyme complexes of the respiratory electron transport chain are located in the:
Complexes I–IV and ATP synthase are embedded in the folded inner membrane (cristae). The matrix holds Krebs cycle and β-oxidation enzymes; the outer membrane is porous.
Mature human red blood cells lack which of the following?
Having lost their mitochondria, mature red cells cannot run the Krebs cycle and rely on glycolysis for ATP and the HMP shunt for NADPH.
Which laboratory marker is now preferred for diagnosing acute myocardial infarction?
The universal definition of MI relies on a rise and/or fall of cardiac troponin, preferably high-sensitivity assays. CK-MB is less specific; AST and LDH are obsolete for this purpose.
Red cells in G6PD deficiency haemolyse under oxidant stress because they cannot produce enough:
The HMP shunt, starting with G6PD, is the red cell's only NADPH source; NADPH keeps glutathione reduced to neutralise oxidants. ATP and NADH come from glycolysis.
Mg2+, which kinases need for activity, is best described as a:
Inorganic ions required for enzyme activity are cofactors or activators. Coenzymes are organic molecules, often vitamin-derived; the apoenzyme is the protein alone.
Transamination reactions catalysed by ALT and AST require which coenzyme?
PLP (from vitamin B6) accepts the amino group, forming pyridoxamine phosphate, then passes it to the keto acid. Biotin serves carboxylases; CoA carries acyl groups.
Coenzyme A carries acetyl groups into the citric acid cycle. Which B vitamin forms part of its structure?
Coenzyme A is built from pantothenic acid, ATP and cysteamine. Biotin is the carboxylation cofactor, not part of CoA.
Sodium fluoride in grey-top tubes stops glucose falling in stored blood. Which glycolytic enzyme does fluoride block?
Fluoride, with phosphate and Mg2+, inhibits enolase and so halts glycolysis in the tube. The block is not immediate, so glucose may still fall in the first hour.
A serum amylase method measures breakdown of a polysaccharide substrate. Which natural substrate does amylase hydrolyse?
Alpha-amylase cleaves alpha-1,4 glycosidic bonds in starch and glycogen. Sucrose and lactose are disaccharides, and the beta bonds in cellulose are not attacked.