Key Points: Enzyme Kinetics, Inhibition, and Cofactors

Factors Affecting Enzyme Activity:

• Temperature: activity rises to optimum (~37°C for human enzymes), then falls sharply due to denaturation. Bell-shaped curve
• pH: each enzyme has an optimal pH. Pepsin = pH 2 (stomach); Trypsin = pH 8 (small intestine). Deviation causes denaturation
• Substrate concentration: activity rises with [S] to reach maximum (Vmax) at enzyme saturation

Michaelis-Menten Kinetics:

• Km = substrate concentration at which v = ½ Vmax. Low Km = HIGH affinity for substrate
• Vmax = maximum rate when all enzyme active sites are saturated
• At [S] = Km → v = ½ Vmax (definition)

Competitive Inhibition:

• Inhibitor binds ACTIVE SITE (structurally similar to substrate)
• Km INCREASES (apparent — enzyme needs more substrate to displace inhibitor)
• Vmax UNCHANGED (overcome by very high substrate concentration)
• OVERCOME by excess substrate. REVERSIBLE
• Classic example: Malonate inhibits succinate dehydrogenase

Non-Competitive Inhibition:

• Inhibitor binds ALLOSTERIC SITE (different from active site)
• Km UNCHANGED (substrate binding unaffected)
• Vmax DECREASES (catalytic efficiency reduced by conformational change)
• CANNOT be overcome by excess substrate
• Classic example: Heavy metals ($Hg^{2+}$, $Pb^{2+}$)

Cofactors:

• Coenzymes: organic, loosely bound. N$AD^{+}$ (from niacin/B3), FAD (from riboflavin/B2). Electron carriers
• Prosthetic groups: tightly/covalently bound. Haem (in haemoglobin, cytochromes)
• Metal ions: $Zn^{2+}$ (carbonic anhydrase), $Mn^{2+}$, $Cu^{2+}$
• Apoenzyme (protein, inactive) + Cofactor = Holoenzyme (complete, active)
• Ribozyme: RNA molecule with catalytic activity. Discovered by Cech and Altman (Nobel 1989). NOT a protein