How enzymes catalyse reactions

Transition-state stabilisation

Rate depends on the free-energy gap between the bound substrate and the transition state. Pauling's insight was that an enzyme binds the transition state more tightly than the substrate, so the barrier falls. This is why transition-state analogues, stable molecules that mimic the geometry and charge of the transition state, are often extremely potent inhibitors. Contributions include precisely placed hydrogen-bond donors (such as an oxyanion hole), electrostatic preorganisation of the active site and the loss of translational entropy when reacting groups are held together (proximity and orientation).

Catalytic strategies

• General acid–base catalysis. Side chains such as His (pK_a near 6–7), Asp, Glu, Lys, Tyr and Cys donate or accept protons in the transition state, avoiding high-energy charged intermediates. Histidine is ideal because it can act as acid or base near neutral pH. Bell-shaped pH–rate profiles often reveal the pK_a values of the catalytic groups.
• Covalent catalysis. A nucleophilic residue (Ser, Cys, Lys, His) forms a transient covalent intermediate, such as an acyl-enzyme or an imine (Schiff base), splitting one difficult step into two easier ones.
• Metal-ion catalysis. Metal ions act as Lewis acids that polarise carbonyl groups, lower the pK_a of bound water to generate a nucleophilic hydroxide at neutral pH (as in carbonic anhydrase with Zn^{2+}), stabilise negative charge, or shuttle electrons.

Worked example: serine proteases

Chymotrypsin, trypsin and elastase hydrolyse peptide bonds using a catalytic triad of Ser195, His57 and Asp102.

1. Substrate binds with the side chain of the residue before the scissile bond in the S1 pocket, which sets specificity: hydrophobic and deep for chymotrypsin (aromatic residues), with an Asp at the bottom for trypsin (Lys/Arg), and small for elastase. 2. His57, its imidazolium oriented and charge-stabilised by Asp102, acts as a general base, deprotonating Ser195 as its oxygen attacks the amide carbonyl. 3. A tetrahedral intermediate forms; its oxyanion is stabilised by backbone N–H groups of Gly193 and Ser195 in the oxyanion hole. 4. The intermediate collapses, with His57-H^+ acting as general acid to protonate the departing amine. The C-terminal fragment leaves and an acyl-enzyme ester remains. 5. Water, activated by His57, attacks the ester; a second tetrahedral intermediate collapses to release the carboxylic acid and regenerate the free enzyme.

Evidence includes burst kinetics with ester substrates (fast acylation, slow deacylation), irreversible inhibition by diisopropyl fluorophosphate, which phosphorylates only Ser195, and large rate losses when any triad residue is mutated. The same triad has evolved independently in subtilisin, a striking case of convergent evolution.

How enzymes achieve their rate enhancements remains actively debated. Warshel's electrostatic preorganisation view, that the active site's dipoles are already arranged to stabilise the transition state so less reorganisation energy is needed than in water, is supported by quantum mechanics/molecular mechanics (QM/MM) free-energy calculations and is now widely accepted as the dominant factor. Proposals that protein dynamics or conformational sampling are directly coupled to the chemical step, and that hydrogen tunnelling is promoted by specific vibrations, remain contested; heavy-enzyme experiments, in which the protein is isotopically labelled, have been used to probe such coupling.

Mechanisms are established by combining approaches: steady-state and pre-steady-state kinetics (stopped-flow, rapid quench), kinetic isotope effects, including heavy-atom KIEs that reveal transition-state structure, linear free-energy relationships, site-directed mutagenesis, and time-resolved or neutron crystallography, which locates protons directly. Vibrational Stark spectroscopy measures active-site electric fields and correlates them with rate.

These insights feed enzyme design and engineering. Computational design creates catalysts for non-natural reactions, though initial designs are usually weak and are improved by directed evolution, which repeatedly diversifies and screens variants. Engineered enzymes now perform carbene and nitrene transfers, asymmetric reductive aminations and industrial-scale routes to pharmaceuticals. Transition-state analogue design has produced picomolar inhibitors, and covalent inhibitors targeting catalytic serine or cysteine residues are an established class of drugs, including protease inhibitors used against viral infections.