Enzyme Kinetics and Inhibition

Why the rate curve flattens

Raise the substrate concentration and the reaction rate rises — then stops rising. At saturation every active site is already occupied, so adding substrate changes nothing. The plateau is set by the amount of enzyme, not by the amount of substrate. Add more enzyme and the plateau moves up.

Binding is not a rigid lock and key: the active site changes shape slightly as the substrate enters. This induced fit is what brings the catalytic groups into position.

Two kinds of inhibitor

• Competitive — the inhibitor resembles the substrate and binds the active site. Substrate and inhibitor compete, so flooding the system with substrate wins the competition back. The maximum rate is unchanged; it just takes more substrate to reach it.
• Non-competitive (allosteric) — the inhibitor binds a different site and changes the enzyme's shape, so the active site no longer fits. Extra substrate cannot help, and the maximum rate falls.

Feedback inhibition

The end product of a pathway binds allosterically to an early enzyme in the same pathway and switches it off. The cell stops making what it already has, and stops it at the first committed step, so no intermediate is wasted. This is negative feedback at the molecular level, and it is the standard example on the exam.

Allosteric sites also work the other way: an activator can lock the enzyme in its active shape. Cooperative binding in haemoglobin follows the same logic — the first oxygen bound makes the next one easier.