Enzyme Kinetics and Inhibition

This lesson covers the effect of substrate concentration on enzyme activity, the concept of Vmax and Km, and the mechanisms of competitive and non-competitive inhibition, as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to interpret rate–concentration graphs and explain how inhibitors affect enzyme kinetics.

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Effect of Substrate Concentration

When enzyme concentration is held constant, increasing the substrate concentration increases the rate of reaction — but only up to a point.

The Relationship

Substrate Concentration	Explanation	Rate
Low [S]	Many active sites are unoccupied — there are more enzyme molecules than substrate molecules. Increasing [S] increases the chance of enzyme–substrate collisions.	Rate increases proportionally with [S]
Moderate [S]	More active sites are occupied. The rate of increase begins to slow.	Rate increases but the curve begins to plateau
High [S] (saturating)	All active sites are occupied — the enzyme is working at maximum capacity. Adding more substrate has no effect because there are no free active sites.	Rate reaches a plateau — this is Vmax