How to use these biochemistry practice questions
These questions cover enzyme kinetics, metabolism, molecular biology, clinical biochemistry and acid–base chemistry. They are written in a single-best-answer style, but the method is useful for university courses and biomedical science assessments internationally.
Answer each question as it appears, then read the explanation immediately. For every wrong answer, identify whether the problem was a missing fact, a misread value or a failure to connect two steps in a pathway. That distinction matters: rereading will not fix all three problems in the same way.
In MySummaries, a question set built from a biochemistry board can be marked one question at a time, so the explanation is attached to the decision you actually made.
1. Competitive enzyme inhibition
Start with the effect of an inhibitor on apparent Km and Vmax. This tests whether you can distinguish competitive inhibition from changes that reduce the amount of active enzyme.
An enzyme is studied with and without an inhibitor. In the presence of the inhibitor, the apparent Km increases but Vmax is unchanged. Which type of inhibition is most consistent with these findings?
Competitive inhibition increases the apparent Km because more substrate is needed to reach half Vmax, but sufficiently high substrate concentration can overcome the inhibition, so Vmax remains unchanged. The strongest distractor is uncompetitive inhibition: it lowers both apparent Km and Vmax because the inhibitor binds the enzyme–substrate complex.
The key phrase is “Vmax is unchanged”. Do not choose non-competitive inhibition merely because an inhibitor is present; in the pure form, non-competitive inhibition lowers Vmax without changing Km.
2. Glycolysis
A pathway question often gives you a rate-limiting enzyme rather than asking for every intermediate. Remember the difference between an irreversible control step and a reversible reaction close to equilibrium.
Which enzyme catalyses the major regulated, rate-limiting step of glycolysis in most tissues?
Phosphofructokinase-1 converts fructose 6-phosphate to fructose 1,6-bisphosphate and is the major committed control point of glycolysis. The strongest distractor is pyruvate kinase, which catalyses the final substrate-level phosphorylation step but is not the principal rate-limiting enzyme of the pathway.
When explaining this aloud, include the reaction rather than naming the enzyme alone. ATP and citrate inhibit phosphofructokinase-1, whereas AMP and fructose 2,6-bisphosphate promote glycolytic flux.
3. Oxidative phosphorylation
This question links electron transport to the proton gradient. A molecule that carries electrons into the respiratory chain is not necessarily the molecule that directly powers ATP synthase.
What directly drives ATP synthesis by mitochondrial ATP synthase?
The proton-motive force, generated by proton pumping through complexes I, III and IV, drives protons through ATP synthase and supports phosphorylation of ADP. The strongest distractor is electron transfer from NADH: it helps create the gradient, but ATP synthase is directly driven by proton movement down that gradient.
A useful chain is: electron transfer → proton pumping → electrochemical gradient → proton flow through ATP synthase → ATP formation. If one link is missing, your explanation is incomplete.
4. Fatty-acid oxidation
The location of a process can separate closely related answers. Long-chain fatty acids require transport into the mitochondrial matrix before beta-oxidation can proceed.
A defect in carnitine palmitoyltransferase I most directly impairs which process?
Carnitine palmitoyltransferase I transfers the fatty acyl group to carnitine at the outer mitochondrial membrane, allowing transport through the carnitine shuttle. The strongest distractor is ketone-body synthesis: it may fall secondarily because less acetyl-CoA is generated from fatty acids, but it is not the direct defect.
Use “directly” carefully. Many clinical effects follow from the primary block, but the question is asking for the first biochemical step affected.
5. Amino-acid metabolism
Ammonia handling is a frequent source of confusion because different tissues use different transport forms. The liver converts ammonia to urea, while peripheral tissues need safer ways to transport nitrogen.
Which amino acid commonly transports amino nitrogen from peripheral tissues to the liver for urea synthesis?
Alanine carries amino nitrogen from muscle to the liver as part of the glucose–alanine cycle. In the liver, its carbon skeleton can contribute to gluconeogenesis and its nitrogen can enter the urea cycle. The strongest distractor is glutamine, which is also an important nitrogen transporter, but alanine is the classic muscle-to-liver carrier in this cycle.
Do not treat “nitrogen transport” and “urea synthesis” as identical ideas. Glutamine is especially important for transporting ammonia from many tissues and for renal acid–base handling; alanine is central to the glucose–alanine cycle.
6. DNA replication
The enzyme names in replication become easier when you assign each one a physical job: opening the helix, laying down a primer, extending DNA or joining fragments.
During eukaryotic DNA replication, which enzyme activity removes RNA primers from newly synthesised DNA fragments?
RNA primers are removed through RNase H activity and flap endonuclease processing, after which the resulting gaps are filled with DNA and sealed by ligase. The strongest distractor is helicase: helicase unwinds the parental DNA but does not remove primers.
The wording matters because different textbooks describe the eukaryotic enzymes in slightly different detail. The accepted biochemical function is primer removal followed by gap filling and ligation.
7. Acid–base chemistry
For a buffer, use the Henderson–Hasselbalch relationship: pH = pKa + log([A−]/[HA]). A tenfold increase in the base-to-acid ratio raises pH by one unit.
A weak acid has a pKa of 6.1. If the concentration of its conjugate base is ten times the concentration of the weak acid, what is the approximate pH?
pH = 6.1 + log(10), and log(10) equals 1, giving a pH of approximately 7.1. The strongest distractor is 6.1, which would apply when the acid and conjugate-base concentrations are equal.
Write the ratio explicitly before calculating. This prevents the common error of adding the pKa to the concentration ratio rather than to its logarithm.
8. Clinical enzymes
A raised enzyme result is meaningful only when you connect it to tissue distribution and the pattern over time. One enzyme rarely identifies an organ without supporting evidence.
Which pattern most strongly suggests hepatocellular injury rather than a predominantly cholestatic process?
A prominent ALT and AST rise indicates leakage from injured hepatocytes, although interpretation requires the clinical context and other liver tests. The strongest distractor is marked alkaline phosphatase with minor aminotransferase elevation, a pattern more suggestive of cholestatic or biliary injury.
Avoid saying that ALT is found only in liver or that AST is liver-specific. Both have extrahepatic sources, so the result must be interpreted with the full pattern and clinical presentation.
9. Protein structure
Amino-acid substitutions can affect a protein at several structural levels. A substitution that changes a charged residue to a hydrophobic residue is especially likely to alter folding or interactions.
A mutation replaces a surface-exposed glutamate with valine. Which change is most likely?
Glutamate is negatively charged at physiological pH, whereas valine is hydrophobic and uncharged. The substitution can alter local electrostatic interactions, solubility or binding. The strongest distractor is loss of a peptide bond: a missense substitution changes a residue but does not automatically break the protein backbone.
The word “possible” is important. A substitution may have little functional effect, or it may be severe, depending on location, conservation and the role of the residue.
10. Quantitative clinical biochemistry
The final question uses dilution. Keep the units consistent and remember that the measured concentration in the diluted tube is lower than the original concentration.
A serum sample is diluted by mixing 1 part serum with 4 parts diluent. The diluted sample measures 18 mmol/L glucose. What was the approximate glucose concentration in the original sample?
One part sample plus four parts diluent gives a fivefold dilution. The original concentration is 18 × 5 = 90 mmol/L. The strongest distractor is 72 mmol/L, which results from multiplying by four rather than by the total dilution factor of five.
For laboratory calculations, state the dilution factor in words first: total volume divided by sample volume. That single step prevents many avoidable arithmetic errors.
What to do with your results
A score is useful only if it changes what you study next. Record the topic, the error type and the exact fact or relationship that would have earned the mark. A question missed because you confused CPT I with ketone-body synthesis needs a different correction from a question missed because you inverted a dilution factor.
A practical review table might contain:
- Topic: pathway, enzyme, molecular process or calculation.
- Error: knowledge gap, interpretation error or arithmetic error.
- Repair: one flashcard or one worked example.
- Recheck date: when you will answer a similar question without notes.
A board that turns these results into follow-up work could look like this:
Prioritise a section with both a low percentage and repeated appearances. It is usually a better use of time than rereading a topic you already answer accurately.
If the same fact is missed twice, reduce it to a precise prompt rather than copying the whole explanation. For example:
You lost this mark twice: which enzyme transfers a long-chain fatty acyl group to carnitine before mitochondrial entry?
Answer the prompt from memory, then check that you included carnitine palmitoyltransferase I and its location at the outer mitochondrial membrane. Revisit the question later in a different context so you test transfer, not recognition.
How MySummaries helps
MySummaries can turn your own biochemistry notes into a revision board, generate question sets from that material, mark your responses and send repeated errors to a remediation tray. Use it to keep the facts, equations and pathway explanations tied to the notes for your course: open MySummaries.