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2 with many enzymes that catalyze different reactions. It is thus primarily the nature of the apoenzyme rather than that of the coenzyme that determines the specificity of the reaction. Only a small portion of the enzyme protein, the active site (Fig. 2), comes into contact with the substrate and is directly involved in catalyzing the reaction. The active site consists of a few amino acid residues, which are not necessarily adjacent to each other on the polypeptide chain, but are brought into proximity by the folding of the chain or chains. The active site also includes any coenzyme that may be required for activ- ity. Part of the active site is involved in binding the substrate, whereas another part is responsible for the making or breaking of chemical bonds. There is much evidence to indicate that, for some enzymes, the binding of the substrates produces a conformational change that brings some reactive group of the enzyme into the proper position for the reaction to take place. Classification and nomenclature Enzymes are usually classified and named according to the reaction that they catalyze. There are six principal classes: oxi- doreductases, transferases, hydrolases, lyases, isomerases, and ligases (also called synthetases). Some representative enzymes, their sources, and their reaction specificities are shown in the table. Enzyme (continued) Enzyme Sources Reaction catalyzed Pepsin Gastric juice Hydrolysis of proteins to peptides and amino acids Urease Jack bean, bacteria Hydrolysis of urea to ammonia and carbon dioxide Amylase Saliva, pancreatic juice Hydrolysis of starch to maltose Phosphorylase Muscle, liver, plants Reversible phosphorolysis of starch or glycogen to glucose-1-phosphate Transaminases Many animal and plant tissues Transfer of an amino group from an amino acid to a keto acid Phosphohexose isomerase Muscle, yeast Interconversion of glucose-6-phosphate and fructose-6-phosphate Pyruvic carboxylase Yeast, bacteria, plants Decarboxylation of pyruvate to acetaldehyde and carbon dioxide Catalase Erythrocytes, liver Decomposition of hydrogen peroxide to oxygen and water Alcohol dehydrogenase Liver Oxidation of ethanol to acetaldehyde Xanthine oxidase Milk, liver Oxidation of xanthine and hypoxanthine to uric acid Table — Some representative enzymes, their sources, and their reaction specificities Fig. 2: Illustration of the enzyme catalytic cycle. A substrate binds to the active site of a enzyme, forming an enzyme-substrate complex via temporary bonds. The binding of the substrate and enzyme weakens the original bonds of the substrate, allowing them to break. Products are released, whereas the enzyme is free to again bind other substrates. (Copyright © McGraw Hill) + ward ' s science