Classification by reaction type

Enzymes are typically categorized by the type of reaction they catalyze.

Examples:

• Oxidoreductases (catalyze redox reactions)
• Transferases (transfer functional groups)
• Hydrolases (cleave bonds using water)
• Lyases (break bonds without hydrolysis or oxidation)
• Isomerases (rearrange atoms within a molecule)
• Ligases (form new bonds, often using ATP)

Mechanism

Enzymes lower the activation energy by stabilizing the transition state, positioning substrates in the right orientation, and/or using catalytic residues that promote electron flow. Through this general catalysis, enzymes increase the reaction rate constant without changing the equilibrium constant or the overall free energy change.

• Substrates and enzyme specificity Enzymes show high specificity for their substrates because the enzyme and substrate fit together in three dimensions. Binding pockets in the enzyme recognize particular shapes, charges, and functional groups. This specificity helps ensure that only the intended substrates react, which reduces unwanted side reactions.

• Active site model The active site is the region of the enzyme where the substrate binds and the chemical reaction occurs. In the lock-and-key model, the active site is treated as a rigid cavity that already matches the substrate’s structure.

• Induced-fit model The induced-fit model extends the lock-and-key idea by allowing flexibility: when the substrate binds, the enzyme changes shape. This conformational change can position catalytic residues and the substrate more precisely, improving transition state stabilization and promoting the chemical transformation.

• Cofactors, coenzymes, and vitamins Some enzymes require additional non-protein components called cofactors. Cofactors can be metal ions (e.g., Fe²⁺, Zn²⁺) or coenzymes (organic cofactors), which are often derived from vitamins (e.g., B-complex vitamins). These components enable reaction chemistry that amino acid side chains alone can’t carry out, such as redox transfers (e.g., NAD⁺ ⇌ NADH).

Kinetics

Enzyme kinetics describes how variables such as substrate concentration and enzyme concentration affect reaction rate. The Michaelis-Menten equation is a common model for this behavior, showing how velocity increases with substrate concentration until it reaches a maximum rate (Vmax).

General (catalysis)

Enzymes speed up reactions by lowering the activation energy. They do not change the reaction’s equilibrium position or overall thermodynamics. Both the forward and reverse reactions become faster, while the net free energy change (ΔG) stays the same.

Michaelis-Menten

In the Michaelis-Menten framework, reaction velocity (v) increases as substrate concentration [S] increases, then approaches Vmax when the enzyme becomes saturated with substrate. Two key parameters describe this relationship:

• Km: the substrate concentration at which v is half of Vmax
• Vmax: the maximum reaction rate at saturating [S]

Cooperativity

Some multimeric enzymes show cooperativity, meaning that substrate binding to one subunit changes the binding affinity of other subunits. With positive cooperativity, binding at one site increases binding at additional sites, producing a sigmoidal relationship between velocity and [S].

Effects of local conditions on enzyme activity

Local conditions such as pH, temperature, and ionic strength strongly affect enzyme structure and activity. Each enzyme has an optimal pH and temperature range; moving away from these conditions can reduce activity or cause denaturation. Cells often maintain specific microenvironments or use localization strategies to keep enzymes near their preferred conditions.

Inhibition

Enzymes can be inhibited in several ways:

• Competitive inhibitors compete for the active site, increasing the substrate concentration needed to reach a given velocity.
• Non-competitive inhibitors bind allosterically, reducing maximal activity regardless of substrate concentration.
• Mixed or uncompetitive inhibitors have more specific effects by binding either the free enzyme or the enzyme-substrate complex, changing kinetic parameters in distinct ways.

Regulatory enzymes

Many metabolic pathways depend on regulatory enzymes to control pathway flux. Regulation can occur through:

• Allosteric control: effectors bind at sites other than the active site, causing conformational changes that increase or decrease activity.
• Covalent modification: enzymes are switched on or off through phosphorylation, methylation, or acetylation.
• Zymogens: inactive precursors that require a chemical change (e.g., peptide cleavage) to become catalytically active.