Enzymes and clinical enzymology

49 Kerala MSc MLT Entrance practice questions on Enzymes and clinical enzymology, part of Biochemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   Aspartate transcarbamoylase gives a sigmoid, not hyperbolic, curve of velocity against substrate concentration. What does this show?

   1. AIt is allosteric, with cooperative subunits
   2. BIt is inhibited competitively by its product
   3. CIt follows Michaelis-Menten kinetics
   4. DIt has lost activity from substrate depletion

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   Answer: A · It is allosteric, with cooperative subunits

   Allosteric enzymes have several subunits, and binding of substrate to one makes binding to the others easier. This cooperativity gives a sigmoid curve, so a small change in substrate near the steep part changes velocity greatly. Aspartate transcarbamoylase is inhibited by CTP and activated by ATP at separate allosteric sites. Michaelis-Menten enzymes give a hyperbola.

   • B: Product inhibition does not turn a hyperbola into a sigmoid curve.
   • C: Michaelis-Menten kinetics give a hyperbolic curve.
   • D: Substrate depletion makes a progress curve level off; it does not make the velocity curve sigmoid.

2. Question 2 · Medium

   In the GGT assay with L-gamma-glutamyl-3-carboxy-4-nitroanilide as substrate, glycylglycine is added to act as:

   1. AThe acceptor of the gamma-glutamyl group
   2. BAn inhibitor of interfering peptidases
   3. CAn activator replacing magnesium ions
   4. DThe chromogen read at 405 nm

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   Answer: A · The acceptor of the gamma-glutamyl group

   GGT transfers the gamma-glutamyl group from the substrate to glycylglycine. The coloured product, 5-amino-2-nitrobenzoate, is released and read at about 405 nm as the rate of increase in absorbance. A good acceptor speeds the transfer, which is why glycylglycine is in excess. GGT is found mainly in liver, kidney and pancreas, and serum GGT comes mainly from the liver.

   • B: Glycylglycine is a reactant, not a peptidase inhibitor.
   • C: GGT does not need magnesium, and glycylglycine is not an activator ion.
   • D: The coloured product is released from the substrate, not from glycylglycine.

3. Question 3 · Medium

   Reagents for creatine kinase activity contain N-acetylcysteine. Its role is to:

   1. ASupply the phosphate donor for the reaction
   2. BInhibit the adenylate kinase released from red cells
   3. CChelate calcium ions that inhibit the enzyme
   4. DReactivate CK by reducing its oxidised thiol groups

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   Answer: D · Reactivate CK by reducing its oxidised thiol groups

   CK has sulfhydryl groups at its active site that oxidise during storage, which lowers its activity. N-acetylcysteine reduces them and restores full activity, so results reflect the true amount of enzyme. Adenylate kinase is inhibited separately by AMP and diadenosine pentaphosphate. Creatine phosphate is the phosphate donor in the usual reverse reaction.

   • A: Creatine phosphate is the phosphate donor.
   • B: Adenylate kinase is inhibited by AMP and diadenosine pentaphosphate.
   • C: EDTA, not N-acetylcysteine, is added to bind calcium.

4. Question 4 · Hard

   A 30-year-old has 3 hours of apnoea after suxamethonium. Two weeks later, plasma cholinesterase activity is low and the dibucaine number is 20 (usual 75-85; carriers about 40-70). Liver tests are normal and there is no organophosphate exposure. The most likely explanation is:

   1. AHeterozygous carrier of the atypical gene
   2. BHomozygous atypical cholinesterase
   3. CUsual enzyme inhibited by organophosphate
   4. DUsual enzyme reduced by liver disease

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   Answer: B · Homozygous atypical cholinesterase

   Dibucaine inhibits the usual cholinesterase by about 80% but the atypical variant by only about 20%. A dibucaine number of 20 therefore means nearly all the enzyme is the atypical form, so the patient is homozygous. Carriers have intermediate numbers and usually only a slightly prolonged block. Liver disease or organophosphates lower the amount of usual enzyme but leave the dibucaine number normal.

   • A: Carriers have dibucaine numbers of about 40-70.
   • C: Organophosphates lower activity but do not change the dibucaine number.
   • D: Liver disease lowers activity but leaves the dibucaine number normal.

5. Question 5 · Medium

   On a Lineweaver-Burk plot, the lines with and without an inhibitor meet on the x-axis. The inhibitor therefore:

   1. ARaises Km but leaves Vmax unchanged
   2. BLowers Vmax but leaves Km unchanged
   3. CLowers Km and Vmax by the same factor
   4. DRaises both Km and Vmax

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   Answer: B · Lowers Vmax but leaves Km unchanged

   The x-intercept of a Lineweaver-Burk plot is -1/Km, so lines meeting on the x-axis share the same Km. The inhibited line is steeper with a higher y-intercept (1/Vmax), so Vmax is lower. This is pure non-competitive inhibition: the inhibitor binds away from the active site and adding substrate does not overcome it. Competitive inhibitors give lines meeting on the y-axis.

   • A: Raised Km with unchanged Vmax is competitive inhibition; lines meet on the y-axis.
   • C: Lowering both by the same factor is uncompetitive; the lines are parallel.
   • D: No common type of inhibition raises Vmax.

6. Question 6 · Medium

   Which agent inhibits prostatic acid phosphatase but not red cell acid phosphatase, so that the two can be told apart?

   1. AEthanol
   2. BCupric ions
   3. CFormaldehyde
   4. DL-tartrate

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7. Question 7 · Easy

   Enzymes that join two molecules using energy from ATP hydrolysis belong to which IUBMB class?

   1. ALigases (EC 6)
   2. BIsomerases (EC 5)
   3. CLyases (EC 4)
   4. DTransferases (EC 2)

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8. Question 8 · Easy

   Isoenzymes are best defined as enzyme forms that:

   1. AAre inactive until activated by cleavage
   2. BAct on one substrate in different reactions
   3. CCatalyse the same reaction but differ in structure
   4. DAre bound to their coenzymes as holoenzymes

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9. Question 9 · Hard

   A Lineweaver-Burk plot for an enzyme has a y-intercept of 0.02 L/U and crosses the x-axis at -0.25 L/mmol. What are its Vmax and Km?

   1. AVmax 50 U/L; Km 4 mmol/L
   2. BVmax 50 U/L; Km 0.25 mmol/L
   3. CVmax 0.02 U/L; Km 4 mmol/L
   4. DVmax 0.02 U/L; Km 0.25 mmol/L

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10. Question 10 · Hard

    Several men are admitted after drinking illicit liquor containing methanol. Ethanol is infused while dialysis is arranged. Ethanol protects them because, at alcohol dehydrogenase, it acts as a:

    1. AAllosteric activator that speeds removal of formate
    2. BNon-competitive inhibitor that lowers the enzyme's Vmax
    3. CCompeting substrate that raises the apparent Km for methanol
    4. DIrreversible inhibitor that destroys the enzyme

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11. Question 11 · Hard

    An enzyme has Km 2.0 mmol/L and Vmax 100 µmol/min. A competitive inhibitor with Ki 0.5 mmol/L is added at 1.0 mmol/L. What are the apparent kinetic constants?

    1. AKm 6.0 mmol/L, Vmax 100 µmol/min
    2. BKm 4.0 mmol/L, Vmax 100 µmol/min
    3. CKm 6.0 mmol/L, Vmax 33 µmol/min
    4. DKm 2.0 mmol/L, Vmax 33 µmol/min

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12. Question 12 · Medium

    Disulfiram, used to help patients stop drinking, causes flushing, nausea and palpitations if alcohol is taken, because it inhibits:

    1. ACytochrome P450 2E1
    2. BAldehyde dehydrogenase
    3. CCatalase
    4. DAlcohol dehydrogenase

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13. Question 13 · Hard

    A 19-year-old student swallowed about 30 paracetamol tablets 3 days ago and kept it secret; her blood pressure has been normal throughout. ALT 6,200 U/L, AST 5,400 U/L (both up to 40), ALP 160 U/L (40-130), bilirubin 3.4 mg/dL, INR 3.2. HBsAg and anti-HAV IgM are negative. What is the most likely cause of these results?

    1. AExtrahepatic biliary obstruction
    2. BAcute viral hepatitis B
    3. CIschaemic hepatitis after shock
    4. DDrug-induced hepatocellular necrosis

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14. Question 14 · Hard

    A laboratory designing a new assay wants its enzyme to work at nine-tenths of maximal velocity. If the Km is 0.5 mmol/L, how much substrate must the reagent provide?

    1. A0.45 mmol/L
    2. B1.0 mmol/L
    3. C5.0 mmol/L
    4. D4.5 mmol/L

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15. Question 15 · Medium

    Most enzyme reactions have a temperature coefficient (Q10) of about 2 over the physiological range. This means that a rise of 10 °C:

    1. ARoughly doubles the rate, until the enzyme denatures
    2. BDoubles the Km of the enzyme for its substrate
    3. CLeaves the rate unchanged once the optimum is passed
    4. DRoughly halves the reaction rate by denaturing the enzyme

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16. Question 16 · Medium

    In organophosphate poisoning, which measurement best reflects inhibition of the enzyme at nerve synapses?

    1. APlasma lipase activity
    2. BRed cell acetylcholinesterase
    3. CPlasma pseudocholinesterase
    4. DSerum creatine kinase activity

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17. Question 17 · Medium

    On a Lineweaver-Burk plot, the line with inhibitor runs parallel to the line without inhibitor. This indicates:

    1. AUncompetitive inhibition, with Km and Vmax both lowered
    2. BNon-competitive inhibition, with Vmax lowered alone
    3. CIrreversible inhibition, with Km raised alone
    4. DCompetitive inhibition, with Km raised and Vmax unchanged

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18. Question 18 · Easy

    The Regan isoenzyme, found in the serum of some patients with lung or ovarian cancer, is a form of:

    1. AAcid phosphatase
    2. BCreatine kinase
    3. CLactate dehydrogenase
    4. DAlkaline phosphatase

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19. Question 19 · Easy

    Sulphonamides stop bacterial folate synthesis because they compete at the enzyme's active site with:

    1. ADihydrofolic acid
    2. BPara-aminobenzoic acid
    3. CFolic acid
    4. DGlutamic acid

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20. Question 20 · Hard

    A 58-year-old man had an anterior MI 5 days ago. CK-MB peaked at 140 U/L on day 1 and was normal (12 U/L) by day 3; troponin I has stayed raised. Today he has new chest pain: CK-MB 6 hours later is 64 U/L (reference below 25 U/L) and troponin I has risen by 60%. How are these results best interpreted?

    1. AResidual enzyme from the first infarct, with no new event
    2. BReinfarction, shown by a fresh CK-MB rise after it had returned to normal
    3. CAssay error, because CK-MB cannot rise twice in one week
    4. DSkeletal muscle injury from injections, as CK-MB is not cardiac-specific

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21. Question 21 · Easy

    Lactate dehydrogenase is a tetramer built from two kinds of subunit, H and M. How many LDH isoenzymes does this give?

    1. AThree
    2. BFour
    3. CFive
    4. DTwo

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22. Question 22 · Hard

    On the morning run, every serum amylase and both QC levels read high. A water blank tested as a sample shows 90 U/L of activity, though the same lot was fine yesterday. A new bottle of reagent was reconstituted this morning. What is the most likely cause?

    1. AAmylase contamination of the reagent, for example by saliva
    2. BA calibrator value entered as lower than its true value
    3. CA calcium-binding contaminant in the reconstitution water
    4. DA wrong wavelength filter selected on the analyser

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23. Question 23 · Medium

    Liver glucokinase has a much higher Km for glucose than the hexokinase of other tissues. The practical result is that glucokinase:

    1. AIs saturated with glucose even during fasting
    2. BPhosphorylates glucose faster than hexokinase in hypoglycaemia
    3. CActs mainly when blood glucose is high, after a meal
    4. DIs strongly inhibited by its product glucose-6-phosphate

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24. Question 24 · Hard

    A 66-year-old man has back pain and widespread punched-out lytic lesions on X-ray. Calcium 12.0 mg/dL (8.5-10.5), total protein 9.6 g/dL with an M band, ALP 95 U/L (40-130), GGT normal. Why is ALP normal despite extensive bone disease?

    1. AThe paraprotein binds ALP and inhibits it in the assay
    2. BMyeloma suppresses osteoblasts, so lesions are lytic without new bone formation
    3. CHypercalcaemia inhibits ALP activity in the reaction cuvette
    4. DALP is cleared quickly by the kidneys damaged by light chains

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25. Question 25 · Easy

    Acid phosphatase from the prostate shows its highest activity at a pH of about:

    1. A5
    2. B10
    3. C7.4
    4. D2

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