Enzymes

What enzymes are and what they are not

Enzymes are biological catalysts, almost all of which are globular proteins (the exception being catalytic RNA, ribozymes, discovered by Cech and Altman, Nobel 1989). Punjab Textbook Chapter 5 and Campbell Chapter 8 stress three things every MDCAT paper hammers: enzymes lower the activation energy E_a without changing ΔG; they are not consumed in the reaction; and they are highly specific for substrates by virtue of the active site. The lock-and-key model (Fischer, 1894) is a useful first picture, but the induced-fit model (Koshland, 1958) is the answer the MDCAT actually wants because the active site reshapes around the substrate.

Cofactors, coenzymes, and prosthetic groups

Many enzymes are inactive on their own (apoenzyme) and require a non-protein helper to become a holoenzyme. Inorganic helpers (Mg²⁺ for kinases, Zn²⁺ for carbonic anhydrase, Fe²⁺/³⁺ for cytochromes) are cofactors; organic helpers derived mostly from B-vitamins (NAD⁺ from niacin, FAD from riboflavin, coenzyme A from pantothenic acid, TPP from thiamine) are coenzymes. A prosthetic group is a coenzyme covalently bound to the enzyme — haem in catalase is the canonical example.

Factors affecting enzyme rate

Rate rises with substrate concentration up to V_max, when all active sites are saturated; the Michaelis constant K_m is the substrate concentration at ½V_max and is a measure of enzyme-substrate affinity (lower K_m = higher affinity). Temperature optimum for human enzymes is 37°C, with denaturation accelerating above 40°C; bacterial enzymes from Thermus aquaticus (the source of Taq polymerase used in PCR) work happily at 72°C. pH optima are tightly tuned: pepsin in the stomach pH 2, salivary amylase pH 6.8, trypsin in the small intestine pH 8. Enzyme concentration scales rate linearly when substrate is in excess.

Inhibition: competitive, non-competitive, and feedback

Competitive inhibitors resemble the substrate and bind the active site reversibly; they raise the apparent K_m without changing V_max, and adding more substrate overcomes them. Classic example: malonate inhibiting succinate dehydrogenase. Non-competitive inhibitors bind an allosteric site, distort the active site, lower V_max without changing K_m, and cannot be relieved by extra substrate. Heavy metals like Pb²⁺ and Hg²⁺ act this way. Feedback inhibition is the cellular control where the end product of a pathway switches off the first committed enzyme — isoleucine on threonine deaminase in E. coli is the textbook case.

Worked numerical patterns and traps

A reaction with K_m = 2 mM proceeds at half its maximum velocity when [S] = 2 mM, regardless of enzyme amount. Doubling enzyme doubles V_max but does not change K_m. A 10°C rise typically doubles rate up to the optimum (Q₁₀ ≈ 2). Common MDCAT traps: enzymes do not change ΔG or the equilibrium position, only the rate; activation energy is lowered by stabilising the transition state, not the substrate; competitive inhibitors do not bind the substrate, they bind the active site of the enzyme. Match the Punjab Textbook terminology to Campbell's Michaelis-Menten graph and you will recognise every variant.

Allosteric regulation and industrial enzymes

Allosteric enzymes are typically multi-subunit proteins that bind regulators at sites distinct from the active site, switching between low-affinity (T) and high-affinity (R) states. ATCase (aspartate transcarbamoylase) and phosphofructokinase are textbook examples — both inhibited allosterically by end products (CTP and ATP, respectively) and activated by precursors. Industrially, enzymes appear everywhere on MDCAT: amylase in detergents, rennet (chymosin) in cheese-making, pectinase to clarify fruit juice, lactase to make lactose-free milk, Taq polymerase from Thermus aquaticus running PCR at 72°C, and restriction endonucleases (EcoRI, HindIII) cutting DNA at palindromic sequences for genetic engineering — work for which Werner Arber, Hamilton Smith, and Daniel Nathans shared the 1978 Nobel Prize.