Enzyme activity, classes and co-factors

Each enzyme works best at a particular temperature and pH: its optimum. On either side, activity falls. Low temperature only makes an enzyme inactive for a while; it works again when warmed. High temperature destroys it (denaturation), because the protein's shape is lost.

Adding more substrate speeds the reaction at first. But once every enzyme molecule is busy (saturated), adding more substrate cannot help: the rate reaches its maximum, Vmax.

Some chemicals slow enzymes down (inhibitors). A competitive inhibitor looks so much like the substrate that it competes for the active site and blocks it. Malonate looks like succinate, so it blocks the enzyme succinic dehydrogenase. Such inhibitors are used to fight disease-causing bacteria.

Enzymes fall into six classes, each with 4 to 13 subclasses and named by a four-digit number:

• Oxidoreductases/dehydrogenases: oxidation and reduction.
• Transferases: move a group from one molecule to another.
• Hydrolases: break bonds using water.
• Lyases: remove groups to form double bonds.
• Isomerases: rearrange a molecule into an isomer.
• Ligases: join two compounds together.

Many enzymes need a non-protein helper, a co-factor; the protein part alone (the apoenzyme) is inactive. Three kinds: prosthetic groups, bound tightly (haem in peroxidase and catalase); co-enzymes, bound only for a moment, often made from vitamins (NAD and NADP contain the vitamin niacin); and metal ions (zinc for carboxypeptidase).