MLT GlobeChemistry: Enzymes & cardiac markers – page 4
132 Chemistry MCQs on Enzymes & cardiac markers with answers and explanations.
When a cell's energy falls and AMP rises, which enzyme is activated to speed up the committed step of glycolysis?
PFK-1 is inhibited by ATP and activated by AMP, so it responds to energy status. Hexokinase is regulated mainly by glucose-6-phosphate.
Serine dehydratase deaminates serine to pyruvate without oxidation. Which cofactor does it need?
Non-oxidative deamination by dehydratases uses pyridoxal phosphate (B6), as transamination does. Oxidative deamination by glutamate dehydrogenase uses NAD(P)+ instead.
Which enzyme is a ligase, using ATP to join two molecules together?
Glutamine synthetase joins glutamate and NH3 using ATP (class 6, ligase). Aldolase is a lyase, and arginase and guanine deaminase are hydrolases.
Which mechanism changes enzyme activity within seconds to minutes without changing the amount of enzyme?
Covalent modification, like allosteric control, alters existing enzyme molecules quickly. Induction, repression and degradation change enzyme quantity over hours.
Which human enzyme needs a vitamin B12 coenzyme?
Humans have two B12 enzymes: methylmalonyl-CoA mutase (adenosylcobalamin) and methionine synthase (methylcobalamin). Human ribonucleotide reductase uses iron and thioredoxin, not B12.
Gluconeogenesis must bypass how many irreversible steps of glycolysis?
The hexokinase, PFK-1 and pyruvate kinase steps are bypassed by glucose-6-phosphatase, fructose-1,6-bisphosphatase, and pyruvate carboxylase plus PEPCK.
Lingual lipase, which starts fat digestion in the mouth, is secreted from where?
Lingual lipase comes from serous (von Ebner) glands on the dorsal tongue. Gastric lipase, a separate enzyme, is secreted by gastric chief cells.
In B12 deficiency, folate gets stuck as N5-methyl-THF (the 'methyl-folate trap'). Which cobalamin form is missing to cause this?
Methionine synthase moves the methyl group from N5-methyl-THF to homocysteine using methylcobalamin. Without it, folate cannot be recycled.
Which enzyme links amino acid nitrogen to carbon metabolism by reversibly converting glutamate to alpha-ketoglutarate and NH4+?
Glutamate dehydrogenase uses NAD(P)+ to release free ammonia and form a TCA-cycle intermediate. Aminotransferases move amino groups but release no ammonia.
Which enzyme needs a riboflavin-derived coenzyme?
Acyl-CoA dehydrogenase uses FAD in the first step of beta-oxidation. Lactate, malate and glutamate dehydrogenases use NAD(P)+, which comes from niacin.
Which cardiac marker rises earliest after myocardial injury but has poor cardiac specificity?
Myoglobin is a small protein that appears in blood within 1-3 hours of muscle injury. Because skeletal muscle also contains it, a raised level is not specific for the heart; its main value is a negative result early on.
A jaundiced patient has markedly raised ALP and GGT with only mildly raised ALT and AST. This pattern suggests:
ALP and GGT are made by bile duct cells and rise most in cholestasis. In hepatitis the transaminases rise much more than ALP; in bone disease ALP rises but GGT stays normal.
An AST/ALT ratio greater than 2 with raised GGT most suggests:
Alcohol damages mitochondria (rich in AST) and lowers ALT through pyridoxine deficiency, so AST exceeds ALT, often by more than 2:1. Viral hepatitis and fatty liver usually show ALT greater than AST.
Enzyme activity assays are designed so that substrate is in large excess. Why?
With substrate in excess, the reaction follows zero-order kinetics, so the rate reflects enzyme concentration only. In first-order conditions the rate would vary with substrate concentration.
An ALT result of 60 U/L must be reported in SI units. What is the result in µkat/L? (1 U = 16.67 nkat)
60 U/L × 16.67 nkat/U = 1000 nkat/L = 1.0 µkat/L. Multiplying by 0.06 confuses minutes and seconds.
In the IFCC-type ALT method, pyruvate formed is reduced by lactate dehydrogenase. What is measured?
LD converts pyruvate to lactate while oxidizing NADH to NAD+; NADH absorbs at 340 nm, so absorbance falls at a rate proportional to ALT activity. The 405 nm reading applies to ALP methods.
A laboratory changes its enzyme assay temperature from 30 °C to 37 °C without changing reference intervals. What is the likely effect?
Within the physiological range, enzyme reaction rates rise with temperature (roughly doubling per 10 °C), so activity results rise. Reference intervals must match the assay temperature; IFCC methods use 37 °C.
A kinetic LD assay flags 'substrate depletion'; the absorbance change stops after the first readings. What should the technologist do?
Very high enzyme activity uses up the substrate early, so the rate is no longer linear and the result is falsely low. Diluting the sample and rerunning restores zero-order kinetics.
A patient with pernicious anemia has very high serum LD with LD-1 greater than LD-2. This pattern is explained by:
Megaloblastic anemia causes ineffective erythropoiesis; destroyed red cell precursors release LD-1, giving a 'flipped' LD-1 > LD-2 pattern and very high total LD. Liver and muscle mainly release LD-5.
Predominance of LD-5 in serum is most consistent with damage to:
LD-5 (M4) is the main isoenzyme in liver and skeletal muscle. Heart and red cells are rich in LD-1 and LD-2.