Roles of Enzymes in Chemical Reactions : Cofactors and Coenzymes

Enzymes are biomolecules (usually proteins) that act as catalysts. Mechanism of Enzyme Action      Enzymes act by binding with reactant molecules (referred to as substrates) and converting them into product. The reaction is represented in this way: E+S ® ¬ ES ® P+E ¬      The first step in which the substrate (S) binds with the enzyme (E) is the binding step and is reversible. The second step is the catalytic step and results in the substrate being converted into product (P) and the enzyme being released. The enzyme can then be reused again and again.      This is an oversimplification of how enzymes act since enzymes usually act on more than one substrate and produce more than one product. Substrate Specificity      Enzymes act upon specific substrate molecules or specific types of substrate molecules. The basis of substrate specificity is that the substrate molecule has a shape that is complementary to a site on the enzyme molecule called the active site.      The preciseness of the fit between substrate molecules and the enzyme is described by a lock and key model because of the way a key fits into a lock by its complementary shape.      A more realistic model is called the induced-fit model. According to this model the binding of the substrate molecule to the enzyme induces in the enzyme a conformational change (a change in the molecule's shape). A good analogy for this model is the way a foot induces a change of shape of a sock which is already of a size and shape to accommodate the foot. Cofactors and Coenzymes      Some enzymes can function properly only when non-protein components called cofactors are also present. Cofactors may help the enzyme to hold its normal conformation. Cofactors may be metal ions such as iron, copper and zinc, organic compounds or organometallic compounds (e.g. heme of hemoglobin). Sometimes the cofactor is a vitamin, or is derived from a vitamin (an organic molecule obtained in the diet and necessary in only trace amounts).      Some cofactors function as coenzymes which temporarily bind to the enzyme. Coenzymes are organic molecules that generally serve as carriers of electrons or chemical groups. A coenzyme can function with different enzymes and hence be involved in different reactions.      Some coenzymes include: 1. Flavin Adenine Dinucleotide (FAD)    Derived from vitamin B2 (riboflavin). FAD is a hydrogen (electron) carrier in oxidation-reduction reactions FAD + 2H ® FADH2 2. Nicotinamide Adenine Dinucleotide (NAD+)    Derived from the vitamin B3 (niacin). NAD+ also carries electrons in oxidation-reduction reactions both directly by carrying an electron which neutralizes the positive charge and indirectly by carrying a H atom NAD+ + 2H ® NADH  +  H+ 3. Coenzyme A (CoA)    CoA is derived from vitamin B5 (pantothenic acid). The coenzyme is involved in glucose oxidation and carries acetyl groups (-CH2COOH) in certain metabolic reactions. CoA becomes covalently bonded to acetyl groups to form acetyl coenzyme A.