Harper's Illustrated Biochemistry, Volume 1 - Murray R. 1993

Structure and Function of Proteins and Enzymes
Enzymes: General Properties
Enzyme Specificity

The ability of an enzyme to catalyze one and only one specific reaction is arguably its most important property. This allows the rates of specific metabolic processes to be regulated by altering the catalytic activity of specific Enzymes. However, Many enzymes catalyze reactions of a single type (such as phosphate transfer or oxidation-reduction), with substrates comprising a small number of structurally similar compounds. Reactions with alternative substrates occur when these substrates are present in high concentrations. Whether all reactions possible with a given enzyme actually take place in living organisms depends on the relative concentration of alternative substrates within The Cell and the relative affinity of the enzyme for those substrates. Below, we examine some general aspects of Enzyme Specificity.

Optical Specificity of Enzymes

With the exception of epimerases (racemases), which catalyze the interconversion of optical isomers, enzymes generally exhibit absolute optical specificity, at least with respect to one portion of the substrate molecule. For instance, Enzymes of the glycolytic and direct oxidative pathways catalyze the transformation of D-phosphosugars exclusively, ignoring their L-counterparts. With rare exceptions (such as renal D-Amino Acid Oxidase), the majority of mammalian enzymes catalyze the transformation solely of L-amino acid isomers.

Optical specificity may apply either to a specific fragment of a molecule or to the molecule as a whole. A classic illustration of this is the specificity of glycosidases. These enzymes catalyze the Hydrolysis of glycosidic bonds between a sugar and an alcohol group: they are highly specific for both the sugar moiety and the configuration of the glycosidic bond (a or ß), yet relatively nonspecific regarding the alcohol portion of the molecule.

Group Specificity of Enzymes

Lytic enzymes act on specific chemical groupings: glycosidases target glycosidic bonds, Pepsin and Trypsin act on peptide bonds, and esterases hydrolyze ester bonds. The action of these enzymes extends across a wide array of substrates, enabling the Organism to get by with a relatively small Complement of digestive enzymes—otherwise, an immense number would be required. Many proteases are also capable of catalyzing the hydrolysis of esters. Although the ability of proteases to hydrolyze ester linkages lacks physiological significance, The Use of synthetic ester substrates has proven exceptionally valuable for studying their MECHANISM OF ACTION.

Certain lytic enzymes display a much higher degree of group specificity. For example, Chymotrypsin predominantly hydrolyzes peptide bonds in which the carboxyl group belongs to aromatic Amino Acids—phenylalanine, Tyrosine, or Tryptophan. Carboxypeptidases and aminopeptidases cleave off amino acids one by one from the carboxyl or amino terminus, respectively.

Some oxidoreductases can utilize both NAD⁺ and NADP⁺ as electron acceptors, but the majority employ only one of them. In general, mammalian oxidoreductases involved in biosynthetic processes (such as fatty acid or steroid synthesis) typically use NADPH as a reductant, whereas enzymes involved in degradative pathways (Glycolysis, Fatty acid oxidation) predominantly utilize NAD⁺ as an oxidant.