Enzymes are catalysts that accelerate the rates of biochemical reactions but at the end of the reaction remain unchanged themselves.

Like all catalysts, an enzyme does not affect the equilibrium point of a reaction and cannot bring about a reaction that is energetically unfavourable. Rather, an enzyme decreases the time it takes for the reaction to reach equilibrium by lowering its activation energy.

Within the folds of an enzyme’s protein chain is the active site—the region where the reaction takes place.

• The active site has the specific shape and chemical reactivity needed to catalyze the reaction. One or more substrates are held in place by intermolecular forces to groups that line the active site.

The extent to which an enzyme’s activity is limited to a certain substrate and a certain type of reaction is referred to as the specificity of the enzyme. Enzymes differ greatly in their specificity.

The catalytic activity of an enzyme is measured by its turnover number, the maximum number of substrate molecules acted upon by one molecule of enzyme per unit time.

Cofactor is a nonprotein part of an enzyme that is essential to the enzyme’s catalytic activity; a metal ion or a coenzyme.

Coenzyme is an organic molecule that acts as an enzyme cofactor.

Lock-and-key model is a model of enzyme action in which the enzyme is a rigid lock that exactly fits the substrate, the key for the reaction.

Induced-fit model is a model of enzyme action in which the enzyme has a flexible active site that changes shape to best fit the substrate and catalyze the reaction.

Enzyme-catalyzed reactions begin with migration of the substrate (S) or substrates into the active site of the enzyme (E) to form an enzyme–substrate complex (ES).

• The substrate is first drawn into position by the same kinds of noncovalent forces that govern the shapes of protein molecules. Before forming an enzyme–substrate complex, the substrate molecule is in its most stable, lowest-energy shape.

• Within the enzyme–substrate complex, the substrate is forced into a less stable shape, and bonding electrons may be drawn away from some bonds in preparation for breaking them and forming new bonds.

• The result is that the activation energy barrier between substrate and product is lowered without the need for a large energy input.

Enzymes act as catalysts because of their following abilities to:

• Bring substrates and catalytic sites together (proximity effect)

• Hold substrates at the exact distance and in the exact orientation necessary for reaction (orientation effect)

• Provide acidic, basic, or other types of groups required for catalysis (catalytic effect)

• Lower the energy barrier by inducing strain in bonds in the substrate molecule (energy effect)

With fixed enzyme concentration, reaction rate first increases with increasing substrate concentration and then approaches a fixed maximum at which all active sites are occupied. In the presence of excess substrate, reaction rate is directly proportional to enzyme concentration.

With increasing temperature, reaction rate increases to a maximum and then decreases as the enzyme protein denatures. Reaction rate is maximal at a pH that reflects the pH of the enzyme’s site of action in the body.

Activation (of an enzyme) is any process that initiates or increases the action of an enzyme.

Inhibition (of an enzyme) is any process that slows or stops the action of an enzyme.

The inhibition of an enzyme can be reversible or irreversible. In reversible inhibition, the inhibitor can leave, restoring the enzyme to its uninhibited level of activity. In irreversible inhibition, the inhibitor remains permanently bound and the enzyme is permanently inhibited.

• The inhibition can also be competitive, uncompetitive, or mixed, depending on whether the inhibitor binds to the active site, the substrate, or some combination of enzymes.

In uncompetitive inhibition, the inhibitor does not compete with the substrate for the active site and cannot bind to enzyme alone. An uncompetitive inhibitor exerts control by binding to the enzyme–substrate complex so that the reaction occurs less efficiently or not at all. This type of inhibition is reversible and often occurs in reactions where two substrates are involved.

Competitive (enzyme) inhibition is the enzyme regulation in which an inhibitor competes with a substrate for binding to the enzyme active site.

Irreversible (enzyme) inhibition is the enzyme deactivation in which an inhibitor forms covalent bonds to the active site, permanently blocking it.

Allosteric control is an interaction in which the binding of a regulator at one site on a protein affects the protein’s ability to bind another molecule at a different site.

Allosteric enzyme is an enzyme whose activity is controlled by the binding of an activator or inhibitor at a location other than the active site.

Feedback control is the regulation of an enzyme’s activity by the product of a reaction later in a pathway.

There are two modes of enzyme regulation by covalent modification—removal of a covalently bonded portion of an enzyme or addition of a group. Some enzymes are synthesized in inactive forms that differ from the active forms in composition.

• Activation of such enzymes, known as zymogens or proenzymes, requires a chemical reaction that splits off part of the molecule.

Genetic control is exercised by regulation of the synthesis of enzymes specific to the stage of life and need of the organism.

Vitamins are organic molecules required in small amounts in the body that must be obtained from the diet. The water-soluble vitamins are coenzymes or parts of coenzymes. The fat-soluble vitamins have diverse and less well-understood functions.

In general, excesses of water-soluble vitamins are excreted and excesses of fat-soluble vitamins are stored in body fat, making excesses of the fat-soluble vitamins potentially more harmful.

Vitamin C, bacarotene (a precursor of vitamin A), vitamin E, and selenium work together as antioxidants to protect biomolecules from damage by free radicals.

Minerals are chemical elements needed in small amounts in the diet. Minerals function as macronutrients (calcium and phosphorus for bone), electrolytes, and micronutrients used primarily as enzyme cofactors.