Enzymes

Enzyme
I OVERVIEW
Virtually all reactions in the body are mediated by enzymes, which are protein catalysts that increase the rate of reactions without being changed in the overall process. Among the many biological reactions that are energetically possible, enzymes selectively channel reactants (called substrates) into useful pathways. Enzymes thus direct all metabolic events.
II. NOMENCLATURE
Each enzyme is assigned two names. The first is its short, recommended name, convenient for everyday use. The second is the more complete systematic name, which is used when an enzyme must be identified without ambiguity.
A. Recommended name
Most commonly used enzyme names have the suffix “-ase” attached to the substrate of the reaction (for example, glucosidase, urease, sucrase), or to a description of the action performed (for example, lactate dehydrogenase and adenylyl cyclase). [Note: Some enzymes retain their original trivial names, which give no hint of the associated enzymic reaction, for example, trypsin and pepsin.]
B. Systematic name
The International Union of Biochemistry and Molecular Biology (IUBMB) developed a system of nomenclature in which enzymes are divided into six major classes (Figure 5.1), each with numerous subgroups. The suffix -ase is attached to a fairly complete description of the chemical reaction catalyzed, including the names of all the substrates; for example D-glyceraldehyde 3-phosphate:NAD oxidoreductase. The IUBMB names are unambiguous and informative, but are frequently too cumbersome to be of general use.



Enzyme Commission Number (EC number)
The Enzyme Commission number (EC number) is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze. As a system of enzyme nomenclature, every EC number is associated with a recommended name of enzyme.
Every enzyme code consists of the letters "EC" followed by four numbers separated by periods. Those numbers represent a progressively finer classification of the enzyme.
For example, the tripeptide aminopeptidases have the code "EC 3.4.11.4", whose components indicate the following groups of enzymes:
• EC 3 enzymes are hydrolases (enzymes that use water to break up some other molecule)
• EC 3.4 are hydrolases that act on peptide bonds
• EC 3.4.11 are those hydrolases that cleave off the amino-terminal amino acid from a polypeptide
• EC 3.4.11.4 are those that cleave off the amino-terminal end from a tripeptide