Molecular Biology - Enzymes

Enzymes are globular proteins that act as biological catalysts by lowering the activation energy ($\Delta G^{\ddagger}$ or $E_a$) of a chemical reaction without being consumed.

The active site is a specific region of the enzyme where the substrate binds. The 'Induced Fit Model' describes how the enzyme undergoes a conformational change to improve the fit with the substrate $S$ to form the enzyme-substrate complex ($ES$).

Collision theory states that for a reaction to occur, the substrate must collide with the active site with sufficient energy and at the correct orientation. Factors like temperature ($T$) and concentration affect these collision rates.

Factors affecting rate: (1) Temperature increases kinetic energy until the optimum; beyond this, thermal agitation breaks hydrogen bonds leading to denaturation. (2) $pH$ affects the ionization of $R$-groups, altering the protein's tertiary structure. (3) Substrate concentration $[S]$ increases rate until all active sites are saturated ($V_{max}$).

Inhibition: Competitive inhibitors bind to the active site and can be overcome by increasing $[S]$. Non-competitive (allosteric) inhibitors bind to a site other than the active site, changing the enzyme's shape and reducing $V_{max}$ regardless of $[S]$.

Immobilized enzymes are often used in industry (e.g., using lactase to produce lactose-free milk). Advantages include easier separation of the product and increased stability against changes in $T$ and $pH$.

End-product inhibition is a form of negative feedback where the final product of a metabolic pathway acts as a non-competitive inhibitor for the first enzyme in the chain (e.g., Threonine to Isoleucine pathway).