Helper molecules referred to as coenzymes play an essential part in enzyme-catalyzed reactions. Enzymes with covalently or noncovalently bound coenzymes are referred to as holoenzymes. A holoenzyme without a coenzyme is termed an apoenzyme. Coenzymes are divided into two categories. Soluble coenzymes bind to the protein moiety of the enzyme, undergo a chemical change, and are ultimately released. Prosthetic groups are tightly bound to and remain associated with the enzyme during the entire catalytic cycle. Most coenzymes are vitamin derivatives. Derivatives of the B vitamins, niacin and riboflavin, act as coenzymes and are involved in oxidoreductase reactions. The structure and function of coenzymes will be described in later chapters. Some enzymes require inorganic (metal) ions, frequently termed cofactors, for their activity, e.g. blood-clotting enzymes that require Ca2+ and oxidoreductases, which use iron, copper, and manganese.
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Table 5-2.
Enzyme classification. |
| Body_ID: None |
| Enzyme classification |
| Body_ID: T005002.50 |
| Class | Reaction | Enzymes |
| Body_ID: T005002.100 |
| 1. Oxidoreductases | Ared + Box → Aox + Bred | dehydrogenase, peroxidase |
| Body_ID: T005002.150 |
| 2. Transferases | A-B + C → A + B-C | hexokinase, transaminase |
| Body_ID: T005002.200 |
| 3. Hydrolases | A-B + H2O → A-H + B-OH | alkaline phosphatase, trypsin |
| Body_ID: T005002.250 |
| 4. Lyases (synthases) | A(XH)-B → A-X + B-H | carbonic anhydrase, dehydratases |
| Body_ID: T005002.300 |
| 5. Isomerases | A Iso-A | triose phosphate isomerase, phosphoglucomutase |
| Body_ID: T005002.350 |
| 6. Ligases (synthetases) | A + B + ATP → A-B + ADP + Pi | pyruvate carboxylase, DNA ligase |
| Body_ID: T005002.400 |
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| Body_ID: T005002.450 |
Major classes of enzymes.
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| PROPORTION OF ENZYME GENES IN WHOLE HUMAN GENOME |
| Original data (Venter et al., Science 291:1335, 2001) are quoted here, and so classification does not exactly match nomenclature in Table 5.2. About a quarter of genes encodes enzymes. Names of enzyme groups with number and proportion (percentage in parenthesis) in a total of 26 383 human genes were as follows: transferase, 610 (2.0); synthase and synthetase, 313 (1.0); oxidoreductase, 656 (2.1); lyase, 117 (0.4); ligase, 56 (0.2); isomerase, 163 (0.5); hydrolase, 1227 (4.0); kinase, 868 (2.8); nucleic acid enzyme, 2308 (7.5). |
| Isozyme profiles are often performed in the clinical laboratory for diagnostic purposes (see Fig. 5.3). The definition of isozymes is often operational, i.e. based on simple and reproducible assay methods that sometimes do not require precise analysis of enzyme structure. The term isozyme is commonly used to refer to: (1) genetic variants of an enzyme; (2) genetically independent proteins with little homology; (3) heteropolymers of two or more noncovalently bound polypeptide chains; (4) unrelated enzymes that catalyze similar reactions, e.g. enzymes conjugated with different prosthetic groups or requiring different coenzymes or cofactors; (5) different forms of a single polypeptide chain, e.g. varying in carbohydrate composition, deamination of amino acids, or proteolytic modification. |
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