Enzyme Classification

We will use the comprehensive classification from the Commission on Enzymes of the International Union of Biochemistry, which classifies enzymes into six main groups. All reactions involving degradation or synthesis are catalyzed by one of these six types. Theoretically, all enzyme reactions are reversible, but conditions do not always exist to drive the reaction in reverse.

1. Oxidoreductases

These enzymes carry out the specific energy-releasing reactions for the cell. This is most often accomplished through enzymes called dehydrogenases, which, by removing hydrogen, also remove electrons. As these electrons are passed to an electron (or hydrogen) acceptor, energy is released that the cell is able to trap and store as chemical energy. Oxidation is the only major chemical source of energy for a cell, and biological oxidation is most frequently accomplished by the removal of hydrogen. It involves a loss of electrons from highly electronegative atoms and a gain of electrons by atoms of lesser electronegativity. As electrons move from one molecule to another on this trip, one reactant is oxidized while the other is reduced. Oxidation and reduction are described, therefore, as coupled reactions. Over 200 different oxidoreductases have been described.

2. Transferases

During degradation and synthesis of compounds in cells, functional chemical groups are frequently transferred from one substrate to another. This does not cause the liberation of energy from the substrate but instead converts a substrate to a compound that may then be oxidized or used for the synthesis of cellular materials. A number of special names are used to indicate reaction types (e.g. kinase) to indicate a phosphate transfer from ATP or other phosphate donor, to the named substrate.

3. Hydrolases

These enzymes break large molecules into smaller ones. In bacteria and some fungi these enzymes are excreted by the cell into its external environment (and are called exoenzymes). In this way large insoluble compounds can be broken down in the presence of water into soluble molecules that can enter the bacterial or fungal cell and serve as nutrients. General examples of hydrolytic enzymes include:

< Cellulases - hydrolyze cellulose to glucose

< Amylases - hydrolyze starch to maltose

< Proteases - hydrolyze proteins to amino acids

< Lipases - hydrolyze fats to glycerol and fatty acids

< Nucleases - hydrolyze ribonucleic acid (RNA) and deoxyribonucleic

acid (DNA) into smaller soluble molecules

Many of the exoenzymes or hydrolases of bacteria are really digestive juices. Similar exoenzymes in human are the digestive juices that break food down into substances that are usable by the body. Some exoenzymes of bacteria are potent toxins and contribute to the disease-producing potential of bacteria by catalyzing reactions that are harmful to the host.

4. Lyases

These enzymes either (1) remove groups from substrates nonhydrolytically, usually leaving double bonds, or, (2) add groups to both atoms involved in a double bond thus converting the double bond to a single bond.

5. Isomerases

The isomerases catalyze reactions where reactants and products do not differ in their chemical composition, but in the way chemical groups are arranged on the molecule. This is easiest to see in the case of epimerases or racemases which simply catalyze a rearrangement of the groups attached to an asymmetric carbon atom. There are epimerases, for example, that interconvert sugars by changing the positions of specific hydroxyl groups, and there are racemases the interconvert the L and D isomers of amino acids such as glutamic acid, alanine, and lysine. Mutases transfer a group such as a phosphate from one place in a molecule to another.

6. Ligases

Ligases catalyze reactions in which two molecules are linked together with the consumption of energy from ATP. These are synthesis reactions and the enzymes have been known for years as synthetases. Ligases take part in many of the steps involved in the synthesis of macromolecules such as proteins and many other compounds used as intermediates in nucleic acid biosynthesis.

It should be stressed that the action of cellular enzymes is neither sporadic or disorganized. All cells, including bacteria, have enzyme systems in which the enzymes work in an orderly sequence until a particular series of reactions has been completed. Many enzyme systems act in a kind of chain reaction; the product of one reaction become the substrate for the next reaction in the series and so on.