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BASIC DEFINITIONS

A catalyst is a substance that increases the rate of a reaction without modifying the overall standard Gibbs-energy change in the reaction. This definition is equivalent to the statement that the catalyst does not appear in the stoichiometric expression of the complete reaction. Catalysts are said to exert a catalytic action, and a reaction in which a catalyst is involved is called a catalysed reaction.

Kinetic equations are commonly expressed in terms of the amount-of-substance concentrations of the chemical species involved The amount-of-substance concentration is the amount of substance (for which the SI unit is the mole, symbol mol) divided by the volume. As it is the only kind of concentration commonly used in biochemistry it is usually abbreviated to concentration and this shorter form will be used in the remainder of this document without further discussion. The unit almost invariably used for concentration is mol dm^-3, which is alternatively written as to mol L^-1, mol l^-1, or simplyКMК(molar).

An enzyme is a protein that acts as a catalyst.

A substrate is a reactant (other than a catalyst) in a catalysed reaction.

An inhibitor is a substance that diminishes the rate of a chemical reaction and the process is called inhibition. In enzyme-catalysed reactions an inhibitor frequently acts by binding to the enzyme, in which case it may be called an enzyme inhibitor. An activator is a substance, other than the catalyst or one of the substrates, that increases the rate of a catalysed reaction. An activator of an enzyme-catalysed reaction may he called an enzyme activator if it acts by binding to the enzyme.

The terms effector and modifier are general terms that apply to substances that interact with enzymes and either increase or decrease their catalytic action. Enzyme inhibitors and enzyme activators are therefore special cases of effectors and modifiers. The term effector is more commonly used when the substance produces effects of physiological significance, whereas the term modifier is more appropriate for a substance that is artificially added to an enzyme system being studied in vitro.

Sometimes added substances increase or decrease the rate of an enzyme-catalysed reaction without interacting with the enzyme itself; they may interact with substrates or with modifiers or effectors that are already present in the system. Such substances may be referred to as activators or inhibitors, but should not be referred to as enzyme activators, enzyme inhibitors, modifiers or effectors.

A typical overall enzyme-catalysed reaction involving a single substrate and.a single product may be written as

E + A E + Z

where E is the enzyme, A the substrate and Z the product. The double arrows indicate that the reaction occurs in both directions. When two or more substrates and two or more products are involved, the overall reaction may be written as

E + A + B +... E + Z + Y +...

It is convenient to use early letters of the alphabet for substrates and late letters for products. However, the letter S is frequently used for the substrate of a one-substrate reaction, and P, Q, R are often used for products. In general, any consistent system may be used and the symbols should always be defined. Although the enzyme E is included for completeness in these two examples, it is usually omitted because it is not necessary for specifying the overall stoichiometry of a reaction.

Many of the terms defined in this document, especially in this section, such as catalyst, substrate, order of reaction, steady state, etc., are not unique to enzyme kinetics, but have a wide currency in chemistry. The definitions given here are not intended to conflict with those recommended for use by chemists [5], but because of the special needs of enzyme kinetics they are not necessarily identical.

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