Mechanism of Enzyme Catalysed Reaction

The generally accepted mechanism for enzyme catalysis proceeds through a short-lived enzyme–substrate complex:

where E is the enzyme, S is the substrate (the reactant being acted on), ES represents the activated enzyme–substrate complex, and P is the product. The enzyme's active site has a shape that fits the substrate closely (often pictured as a lock-and-key fit); binding the substrate into this active site forms the ES complex, within which the substrate is held in a strained, energetically activated orientation that greatly lowers the barrier to reaction. The complex then breaks down to release the product P, and — exactly as any catalyst must — the enzyme E itself is regenerated completely unchanged, free to bind a fresh substrate molecule and repeat the cycle.

Enzyme-catalysed reactions show several special characteristics that set them apart from ordinary catalysed reactions.

1. Extraordinary efficiency. A single enzyme molecule can transform on the order of a million substrate molecules into product within a single minute. The decomposition of hydrogen peroxide, $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$, illustrates this dramatically by its activation energy: without any catalyst, the activation energy is about 18 kcal/mole; with colloidal platinum as catalyst, it drops to about 11.7 kcal/mole; but with the appropriate enzyme catalyst, the activation energy falls to LESS THAN 2 kcal/mole — a far bigger reduction than an ordinary inorganic catalyst achieves.
2. High specificity. Urease, which efficiently catalyses the hydrolysis of urea ($\text{H}_2\text{N-CO-NH}_2 + \text{H}_2\text{O} \rightarrow 2\text{NH}_3 + \text{CO}_2$), does NOT catalyse the closely related hydrolysis of methylurea ($\text{H}_2\text{N-CO-NH-CH}_3 + \text{H}_2\text{O} \rightarrow$ no reaction) — even a small structural change to the substrate is enough to prevent the enzyme's active site from binding it, illustrating just how narrowly targeted enzyme specificity really is.
3. Dependence on optimum temperature. The rate of an enzyme-catalysed reaction rises with temperature at first, exactly like an ordinary reaction, but reaches a maximum at the enzyme's optimum temperature (typically around 40°C) and then falls away sharply — potentially all the way to zero — because heating beyond this point denatures the enzyme's delicate protein structure and destroys its catalytic activity. Enzymes in the human body have an optimum temperature of about 37°C (98°F); this is exactly why a high fever, by pushing body temperature well above this optimum, can cause enzymatic activity to collapse and become dangerous.
4. Dependence on optimum pH. The rate of an enzyme-catalysed reaction similarly varies with the pH of the medium, reaching a maximum at a characteristic optimum pH ($\text{pH}_{opt}$) and falling away on either side of it, because the active site's shape and charge distribution — set by the ionisation states of its constituent amino-acid side chains — is only correctly configured for efficient substrate binding within a narrow pH window. …