Depending on the types of reactions the enzymes catalyse, enzymes are classified into six classes, namely, oxidoreductases or dehydrogenases, transferases, hydrolases, lyases, isomerases and ligases.
Oxidoreductases or dehydrogenases catalyse the transfer of electrons and hydrogen ions from one molecule, the reductant to another molecule, the oxidant. An example is lactic dehydrogenase, which converts pyruvic acid into lactic acid and vice versa.
Transferases catalyse the transfer of a functional group G other than hydrogen, between the two substrates S and S prime. An example is transminase, which transfers the amino group from alanine to oxaloacetic acid, forming pyruvic acid and aspartic acid.
Hydrolases catalyse the hydrolysis of ester, ether, peptide, glycoside, carbon-carbon bonds, halide bonds or phosphorous and nitrogen bonds. An example is sucrase, which hydrolyse the glycosidic bonds in sucrose and forms glucose and fructose.
Lyases catalyse by forming double bonds or a new ring structure for catalysis. An example is aldolases, which cleave the aldol groups in fructose one six biphosphate to form dihydroxyacetone phosphate and glyceraldehyde three phosphate.
Isomerases catalyse the inter-conversion of optical, positional and geometric isomers. An example is phosphohexose isomerase that catalyses the conversion of glucose six phosphate to fructose six phosphate during glycolysis.
Ligases catalyse the process of linking together of two compounds. An example is ligase, which joins carbon with oxygen, carbon with sulphur, carbon with nitrogen and phosphate with oxygen.
Sometimes, non-protein chemical compounds called co-factors bind with the enzymes and make them catalytically active. The protein part of such enzymes is known as apoenzyme. There are three types of cofactors – prosthetic groups, co-enzymes and metal ions.
Prosthetic groups are organic compounds, which bound to the apoenzyme tightly. For example, in the enzyme complex succinate dehydrogenase, FAD is a prosthetic group that oxidises succinate to fumarate in the eighth step of the citric acid cycle.
Co-enzymes are also organic compounds, which bound to apoenzyme for a brief period during catalysis. Examples of coenzymes are NAD and coenzyme A.
Co-factors are metal ions, required by enzymes to form coordination bonds with side chains at the active site and one or more coordination bond with the substrate. For example, magnesium is a cofactor for glucose-6 phosphatase and manganese is a cofactor for arginase.