Useful Notes on Isoenzymes and Zymogens (With Diagram)

Occasionally, several different enzyme molecules, all of which catalyze the same chemical reaction, have been isolated from a single tissue.

Such families of en­zymes are called isoenzymes or isozymes. Among the various isoenzymes the lactic dehydrogenases have been most extensively studied, and five different forms have been identified.

All are composed of four polypeptide chains of two types called M and H subunits. Thus the lactic dehydro­genase isoenzymes may take any one of the following forms: M₄, M₃H, M₂H₃, or H₄. Similar arrange­ments are believed to exist for other groups of isoen­zymes.

Isozymes should be distinguished from allelozymes, which are multiple forms of single polypeptide enzymes resulting from variations in a single allelic or structural gene pair.

Although allelozymes act on the same substrate, and in this regard are similar to iso­zymes, isozymes result from combinations of polypep­tide chain products of two or more separate pairs of structural genes.

Zymogens:

A number of enzymes arise from an inactive precur­sor form called a zymogen or proenzyme. A number of the alimentary digestive enzymes belong to this group, including pepsin, trypsin, and chymotrypsin. The conversion of the zymogen to the active enzyme involves the preliminary cleavage of one or more of the zymogen’s peptide bonds, followed occasionally by re­moval of a portion of the original protein molecule. This phenomenon may best be understood by consid­ering a few examples.

Pepsin, the major protein-digesting enzyme of the stomach, is synthesized in the form of the precursor polypeptide pepsinogen (molecular weight 42,000). On entering the acidic gastric juice, the pepsinogen mole­cule is hydrolyzed at several positions to yield a num­ber of small peptides and the active enzyme pepsin (molecular weight 35,000). The activation of pepsino­gen can also be carried out by pepsin itself.

Trypsin, another proteolytic digestive enzyme, is produced in the pancreas as the zymogen trypsinogen and secreted into the duodenum (the anterior portion of the small intestine). In the duodenum, another en­zyme, enterokinase, catalyzes the removal of six amino acids from the N-terminus of trypsinogen, thereby yielding trypsin. Trypsin itself can convert trypsinogen molecules to more trypsin.

The enzyme chymotrypsin also participates in the alimentary digestion of protein and is produced in the pancreas as the inactive proenzyme chymotrypsino­gen. The activation of this zymogen involves a series of peptide bond cleavages catalyzed by trypsin already present in the duodenum and also by chymotrypsin it­self. These cleavages split the chymotrypsinogen mole­cule into three polypeptides that remain intercon­nected in the activated enzyme through disulfide bridges that were part of the molecule’s primary structure.

The activation of the proenzyme trypsinogen is shown in Figure 8-21 and illustrates some of the gen­eral features of zymogen activation. The active site of the enzyme is devoid of binding and/or catalytic activ­ity until peptide bonds of the zymogen are broken. Following this peptide cleavage, the remaining por­tion of the molecule is reorganized with a consequent unmasking of the active site, which can now bind and act on the substrate.

Not only proenzymes but also other proteins may be activated by a preliminary proteolysis. For example, during the coagulation of blood, the formation of the matrix of the clot is brought about through a cascade of proteolytic activations that finally convert inactive, soluble protein monomers (fibrinogen) in blood plasma to the active, polymerizable form, which then pro­duces the insoluble protein threads (fibrin). Some pro­tein hormones are also synthesized as inactive precur­sors that are activated only on peptide cleavage (e.g., the conversion of proinsulin to insulin).