state that enzymes are globular proteins that catalyse reactions inside cells (intracellular enzymes) or are secreted to catalyse reactions outside cells (extracellular enzymes)

Mode of Action of Enzymes

Learning Objective

State that enzymes are globular proteins that catalyse reactions inside cells (intracellular enzymes) or are secreted to catalyse reactions outside cells (extracellular enzymes).

Key Points

• Enzymes are biological catalysts composed of globular proteins.
• They lower the activation energy (\$E_a\$) of biochemical reactions.
• Enzyme activity is highly specific to substrate(s) due to the shape of the active site.
• Enzymes can function either within the cell (intracellular) or after being secreted (extracellular).

Intracellular vs Extracellular Enzymes

Mechanism of Catalysis

Enzymes bind substrates at the active site, forming an enzyme–substrate complex. This stabilises the transition state and reduces the activation energy required for the reaction:

\$\$

\text{E} + \text{S} \rightleftharpoons \text{ES} \rightarrow \text{E} + \text{P}

\$\$

Where E = enzyme, S = substrate, ES = enzyme–substrate complex, and P = product.

Factors Influencing Enzyme Activity

1. Temperature – increases rate up to an optimum, then denatures the protein.
2. pH – each enzyme has an optimal pH; deviation can affect ionisation of active‑site residues.
3. Substrate concentration – follows Michaelis–Menten kinetics.
4. Inhibitors – competitive, non‑competitive, or uncompetitive.
5. Presence of cofactors or coenzymes – required for activity of many enzymes.

Suggested Diagram