Enzymes and Metabolism
How enzyme structure controls catalysis, how conditions change reaction rate, and how inhibitors regulate metabolic pathways.
Metabolism is the network of chemical reactions in cells. Catabolic pathways break larger molecules into smaller ones and can release usable energy; anabolic pathways build larger molecules and require energy or reducing power.
Enzyme
A biological catalyst that increases reaction rate by lowering the activation energy required to reach the transition state. The enzyme is not consumed by the reaction.
Active sites and induced fit
Substrates bind to an enzyme at its active site through interactions determined by shape, charge, polarity, and other chemical properties. Binding can slightly change enzyme shape, producing an induced fit that positions reacting groups effectively.
Enzymes do not supply energy to a reaction or change its overall free-energy difference. They provide a lower-activation-energy pathway.
Temperature
Increasing temperature initially raises collision frequency and can increase enzyme activity. Above an optimum, heat disrupts interactions that maintain protein structure, changing the active site and reducing activity.
pH
pH changes the protonation and charge of amino-acid side chains. If these changes disrupt substrate binding, catalytic residues, or protein structure, enzyme activity falls. Different enzymes therefore have different pH optima.
Substrate and enzyme concentration
At low substrate concentration, adding substrate often increases rate because more active sites are occupied. At high substrate concentration, active sites become saturated and the rate approaches a maximum. Increasing enzyme concentration can increase the maximum rate when enough substrate is available.
Competitive inhibition
A competitive inhibitor binds at the active site and competes with substrate. Raising substrate concentration can reduce the inhibitor’s relative effect because substrate has more opportunities to occupy the active site.
Non-competitive and allosteric inhibition
An inhibitor that binds away from the active site can change enzyme activity without directly competing for the same binding site. In the simple non-competitive model, increasing substrate concentration does not restore the original maximum rate.
| Feature | Competitive | Non-competitive/allosteric |
|---|---|---|
| Binding site | Active site | Separate regulatory/allosteric site |
| Direct competition with substrate | Yes | No |
| Effect of much higher substrate concentration | Can reduce inhibition | Does not fully overcome inhibition in the simple model |
Feedback inhibition
In many pathways, a final product binds allosterically to an enzyme acting earlier in the pathway. When product accumulates, pathway activity falls; when product concentration decreases, inhibition is relieved. This prevents unnecessary use of substrates and energy.
Some inhibitors are converted by the enzyme into a reactive form that binds very tightly or covalently, producing long-lasting inhibition. They should not be treated as ordinary reversible competitive inhibitors.