MLT GlobeChemistry: Enzymes & cardiac markers – page 2
132 Chemistry MCQs on Enzymes & cardiac markers with answers and explanations.
A patient has severe epigastric pain radiating to the back. Which serum enzyme is the most specific test for the likely diagnosis?
Lipase rises in acute pancreatitis and is more specific than amylase. ALP points to bone or biliary disease. Glucocerebrosidase is the enzyme deficient in Gaucher disease.
Which coenzyme carries an acyl group from one molecule to another, rather than carrying hydrogen or electrons?
CoA forms thioester bonds with acyl groups and transfers them. NAD+, NADP+ and FAD carry reducing equivalents in oxidation–reduction reactions.
An enzyme is produced at a steady level whatever the growth conditions, and no inducer is needed. What is it called?
Constitutive enzymes are expressed continuously, usually for housekeeping. Inducible enzymes such as beta-galactosidase need an inducer.
Which enzyme belongs to the oxidative phase of the pentose phosphate pathway and produces NADPH?
The oxidative phase has two NADPH-producing steps: G6PD and 6-phosphogluconate dehydrogenase. Pyruvate kinase and aldolase are glycolytic enzymes.
Trypsinogen is secreted inactive and later cleaved to trypsin. What is this kind of precursor called?
A zymogen is an inactive enzyme precursor activated by limited proteolysis. An apoenzyme is the protein part that lacks its cofactor.
Which cofactor is NOT used in mitochondrial beta-oxidation of fatty acids?
Beta-oxidation uses FAD (acyl-CoA dehydrogenase), NAD+ (hydroxyacyl-CoA dehydrogenase) and CoA (thiolase). NADPH is used in fatty acid synthesis, not breakdown.
Tetrahydrofolate works as a carrier of which chemical group?
THF carries one-carbon units for purine and thymidylate synthesis, so deficiency causes megaloblastic anaemia. Amino groups are carried by PLP and acyl groups by CoA.
Compared with amylase, serum lipase in acute pancreatitis is:
Lipase comes mainly from the pancreas, whereas amylase is also made by salivary glands. Lipase rises in 4-8 hours and stays high for 8-14 days, longer than amylase, which normalises within 3-5 days.
One international unit (U) of enzyme activity is the amount of enzyme that converts:
The international unit is 1 µmol/min under defined conditions. The SI unit, the katal, is 1 mol/s, so 1 U = 16.67 nkat.
Which serum enzyme is most falsely increased by in-vitro hemolysis?
Red cells contain LD at many times the plasma level, so even mild hemolysis raises LD markedly. GGT, lipase and amylase are not significant red-cell enzymes.
Which creatine kinase isoenzyme makes up almost all of the CK activity in normal skeletal muscle?
Skeletal muscle CK is nearly all CK-MM, which is also the main form in normal serum. CK-MB is enriched in myocardium, and CK-BB in brain, bowel and prostate.
Which aminotransferase is more specific for liver injury?
ALT is found mainly in hepatocyte cytoplasm, while AST is also present in heart, skeletal muscle and red cells. Therefore ALT is more liver-specific.
A healthy 14-year-old boy has ALP twice the adult upper reference limit, with normal GGT, ALT and bilirubin. The most likely reason is:
Osteoblast activity during growth raises bone ALP in children and adolescents, so pediatric reference intervals are higher. Normal GGT and other liver tests argue against a liver cause.
Enzymes speed up chemical reactions by:
Enzymes are catalysts: they lower activation energy so equilibrium is reached faster, but they do not change the equilibrium position and are not used up.
In the IUBMB enzyme classification, ALT and AST belong to which class?
Aminotransferases move an amino group from an amino acid to a keto acid, so they are transferases (EC 2). Dehydrogenases such as LD are oxidoreductases (EC 1).
In an enzyme rate assay, the first seconds after mixing show a slow, non-linear change before a steady rate is reached. This period is called the:
During the lag phase, coupled reactions and temperature are still reaching steady state, so readings are not used. Activity is calculated only from the linear phase that follows.
B-type natriuretic peptide (BNP) is mainly secreted by:
Despite its name (first found in brain), BNP comes mainly from the ventricles when they are stretched by volume or pressure overload. Atrial natriuretic peptide comes from the atria.
Besides CK, which serum enzyme is used as a marker of skeletal muscle disease?
Aldolase, a glycolytic enzyme, is high in skeletal muscle and rises in muscular dystrophies and myositis. GGT and 5′-nucleotidase are markers of hepatobiliary disease.
A patient's total CK is 600 U/L the morning after an intramuscular injection; troponin is normal. The raised CK most likely comes from:
Intramuscular injections damage muscle fibers and release CK-MM, raising total CK for a few days. Normal troponin excludes myocardial injury.
Enzymes are biological catalysts made mainly of:
Almost all enzymes are proteins. They speed reactions without being consumed. (A few RNA catalysts exist, but they are exceptions.)