Study guides
Biochemistry
4 guides for this subject — how to prepare, what to practise, and how to know it has gone in.
A practice question
Choose an answer and it is marked, with the explanation. From Biochemistry Practice Questions: 10 Worked MCQs.
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.
A spoken answer, marked
An examiner's question, answered out loud and marked criterion by criterion. From Biochemistry Viva Questions: A Practical Oral Method.
Examiner
Explain how you would distinguish competitive from non-competitive inhibition using an enzyme assay.
You correctly stated that competitive inhibition raises apparent Km while leaving Vmax unchanged in the ideal case.
ImproveState that pure non-competitive inhibition lowers Vmax without changing Km, then qualify that real mixed inhibition can change both parameters.
You proposed measuring initial rates over several substrate concentrations with and without inhibitor.
ImproveName technical replicates and explain why substrate, enzyme, pH and temperature must be controlled.
You recognised that the pattern should be judged from fitted kinetic data rather than one reaction tube.
ImproveExplain what result would support each model and avoid treating a Lineweaver–Burk plot as automatically definitive.
The answer followed a sensible definition, experiment and interpretation order.
ImproveGive the direct distinction in the first sentence before discussing the graph.
A strong answerI would measure initial reaction rates across a range of substrate concentrations, both without inhibitor and at a defined inhibitor concentration, while keeping enzyme, pH, temperature and ionic conditions constant. In the ideal competitive model, the apparent Km increases but Vmax is unchanged because sufficiently high substrate can overcome competition for the active site. In pure non-competitive inhibition, Vmax decreases while Km is unchanged, although real inhibitors may show mixed behaviour and alter both values. I would fit the data using an appropriate nonlinear model, include blanks and replicates, and use the residuals and confidence intervals rather than relying only on a linearised plot.
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Guides
- How to Study Biochemistry: A Practical GuideLearn how to study biochemistry with a practical method for pathways, enzymes, calculations, laboratory data and exam-style questions.
- Biochemistry Flashcards: 18 Core Facts to LearnStudy 18 high-yield biochemistry facts with a practical flashcard method for enzymes, metabolism, haemoglobin and oxidative phosphorylation.
- Biochemistry Practice Questions: 10 Worked MCQsAnswer 10 biochemistry practice questions on enzymes, metabolism, genetics and clinical chemistry, with explanations and a targeted revision method.
- Biochemistry Viva Questions: A Practical Oral MethodPractise biochemistry viva questions with a structured method for mechanisms, data interpretation, calculations and clear spoken answers.
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