# Enzyme Kinetics

> Enzyme kinetics for MBBS Biochemistry — Km, Vmax, Michaelis-Menten equation, inhibition types and Lineweaver-Burk plots for exam revision.

- Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/enzyme-kinetics
- Exam / course: MBBS · Subject: Biochemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Enzyme Kinetics", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/enzyme-kinetics

## Direct answer

Enzyme kinetics describes how the velocity of an enzyme-catalysed reaction changes with substrate concentration, summarised by the Michaelis-Menten equation. Two constants define an enzyme's behaviour: Km, the substrate concentration at half-maximal velocity and an inverse index of affinity, and Vmax, the maximal velocity reached when the enzyme is saturated. Plotting velocity against substrate gives a rectangular hyperbola for most enzymes and a sigmoid curve for allosteric enzymes.

## What you must remember

- The Michaelis-Menten equation is v = Vmax[S] / (Km + [S]); Km has units of millimoles per litre, and a low Km signals high apparent affinity between enzyme and substrate.
- Vmax is directly proportional to enzyme concentration; the turnover number kcat equals Vmax divided by total enzyme concentration.
- The Lineweaver-Burk double-reciprocal plot gives a straight line: the y-intercept is 1/Vmax, the x-intercept is −1/Km, and it cleanly separates the three inhibition patterns.
- Competitive inhibition: the inhibitor resembles the substrate and occupies the active site; apparent Km rises, Vmax is unchanged, and excess substrate overcomes it — examples include methotrexate on dihydrofolate reductase and statins on HMG-CoA reductase.
- Non-competitive inhibition: the inhibitor binds a separate site on both enzyme and enzyme-substrate complex; Vmax falls while Km stays the same.
- Uncompetitive inhibition: the inhibitor binds only the enzyme-substrate complex; both Km and Vmax fall, producing parallel Lineweaver-Burk lines.
- Clinically, saturable (zero-order) kinetics matter: ethanol, phenytoin and high-dose salicylates are eliminated at a constant amount per unit time, so small dose increases cause disproportionate plasma rises.

## Common confusion

Competitive versus non-competitive inhibition is the perennial trap. On a Lineweaver-Burk plot, competitive inhibitors pivot the line on the same y-intercept (Vmax unchanged, Km increased), while non-competitive inhibitors pivot it on the same x-intercept (Km unchanged, Vmax decreased). Students also invert the Km-affinity relationship — remember that a lower Km means a higher affinity, not the reverse.

## Exam-focused takeaway

For theory, state the Michaelis-Menten equation, define Km and Vmax, sketch the hyperbola and the Lineweaver-Burk plot, and contrast the three inhibition types with one drug example each. Viva examiners probe definitions — "what does Km represent" and "which inhibition is overcome by excess substrate". MCQs ask which inhibition changes which parameter, which plot lines are parallel, and which clinically used drugs show zero-order elimination.

## Frequently asked questions

### What does Km represent?

The substrate concentration at which velocity is half of Vmax; it is inversely related to the apparent affinity of the enzyme for its substrate.

### Which inhibition raises Km without changing Vmax?

Competitive inhibition, because the inhibitor competes with substrate at the active site and can be out-competed by raising substrate concentration.

### What is the Lineweaver-Burk plot?

A double-reciprocal graph of 1/v against 1/[S] that yields a straight line with x-intercept −1/Km and y-intercept 1/Vmax, useful for identifying inhibition type.

### What is kcat?

The turnover number — the number of substrate molecules each enzyme molecule converts to product per second at saturating substrate.

### Why is phenytoin difficult to dose?

It exhibits saturable zero-order kinetics at therapeutic concentrations, so small increases in dose can produce large, potentially toxic jumps in plasma levels.