Co-factors: prosthetic group, co-enzyme, metal ion.

An apoenzyme is inactive until its co-factor docks: a haem prosthetic group stays bound, NAD comes and goes between enzymes, zinc bridges side chains and substrate.

Make each of the three holoenzymes turn over 20 substrates, then remove the co-factor and show the rate falls to zero.

NAD called a prosthetic group; haem called a co-enzyme.

• Enzymes are grouped by the type of reaction they catalyse into 6 classes, each with 4–13 subclasses, and are named with a four-digit number.
• Oxidoreductases (dehydrogenases) catalyse oxidoreduction between two substrates. Transferases move a group G, other than hydrogen, from one substrate to another.
• Hydrolases break bonds by adding water: peptide, ester, ether, glycosidic, P–N, C–halide or C–C bonds. Lyases take groups off substrates without hydrolysis, which leaves double bonds behind.
• Isomerases interconvert optical, geometric or positional isomers. Ligases join two compounds, forming bonds such as C–O, C–S, C–N and P–O.
• Many enzymes need a non-protein co-factor bound to them to be catalytically active; the protein part is then called the apoenzyme. The three kinds of co-factor are prosthetic groups, co-enzymes and metal ions.
• Prosthetic groups are organic and tightly bound to the apoenzyme. In peroxidase and catalase, which break hydrogen peroxide into water and oxygen, haem is the prosthetic group and forms part of the active site.
• Co-enzymes are organic too, but they attach only briefly, during catalysis, and serve many different enzymes. Many contain vitamins: NAD and NADP contain the vitamin niacin.
• Metal ions make coordination bonds with the side chains in the active site and with the substrate at the same time; zinc is the co-factor of the protein-digesting enzyme carboxypeptidase.
• Removing the co-factor stops catalysis, which shows how crucial it is.

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