Enzymes and Bioenergetics

An enzyme catalyses a reaction by lowering the activation energy needed to start the reaction. It does not change the equilibrium constant.

Pepsin has an acidic optimum pH, about pH 1–2. When it enters the duodenum, the medium becomes alkaline, so pepsin loses its activity and becomes inactive.

Most coenzymes are derived from vitamins. Vitamins are organic nutrients required in small amounts in the diet, and they act as precursors of many enzyme cofactors. So, vitamins provide the materials from which coenzymes are formed.

The catalytic activity of an enzyme is affected by:

- Temperature: reaction rate increases with temperature up to an optimum temperature, then falls. Most enzymes work best around 40–45°C, and human enzymes around 37°C.
- pH: each enzyme has an optimum pH. Activity is maximum at this pH, and it decreases on either side. At extreme pH, the enzyme becomes inactive.
- Substrate concentration: as substrate concentration increases, the rate increases at first. After saturation of enzyme molecules, further increase in substrate does not increase the rate, and the reaction reaches maximum velocity.

In Michaelis-Menten kinetics, the rate-determining step is the dissociation of the ES complex into product and enzyme, i.e., the slow step after complex formation.

**$K_m$ is the Michaelis constant. It is the substrate concentration at which the reaction velocity is half of the maximum velocity** $(v_0 = V_{max}/2)$.

Significance:
- It indicates the affinity of an enzyme for its substrate.
- A **lower $K_m$ means higher affinity, because half-maximal velocity is reached at a lower substrate concentration.
- A higher $K_m$** means lower affinity.