The problem of enzyme catalysis is considered in terms of the behavior of a single hydrolytic enzyme.
Carl NiemannEnzymes are molecularly homogeneous proteins (1) recognized by their ability to accelerate (catalyze) certain chemical reactions. They share this property with other molecular species but differ from them in possessing a greater degree of selectivity (specificity) in both a structural and a stereochemical sense. Enzymes are further characterized by their effectiveness under essentially ambient conditions and by their ability to function either singly or as components of a multi-enzyme system. These properties permit enzymes to perform roles of singular importance in the development, maintenance, and reproduction of the living organism.There is a large, and ever-increasing, number of known enzymes (2). However, in basic studies experimental considerations dictate the choice of enzyme to be studied. It is this feature of experimental accessibility, coupled with other desirable characteristics, that leads me, in this article, to center attention on a single hydrolytic enzyme, the bovine pancreatic proteinase a-chymotrypsin. In doing so I do not imply that all features of enzyme catalysis, even among the hydrolases, are to be found in a-chymotrypsin, although its properties include many that are common to all enzymes.The Chymotrypsinogens a-Chymotrypsin is one of a family of proteinases that arise from precursors (zymogens) known as chymotrypsinogens, which are enzymatically inactive (3). The existence of such zymogens provides the biologist with an answer to the question, How can proteinases be synthesized and stored in a protein environment? The zymogens are in an inactive form and are activated, when needed, by structural alteration of the inactive molecule at sites removed from those involved in their synthesis and storage. The availability of zymogens of several proteinases permits the chemist to isolate, purify, and otherwise manipulate these precursors, which, in contrast to the proteinases, are incapable of self-destruction.There are two known bovine pancreatic chymotrypsinogens, A (3) and B (4). The two are present in bovine pancreas in relatively large and nearly equal amounts (5, 6). However, chymotrypsinogen A is more readily crystallized and purified than chymotrypsinogen B. This property has facilitated isolation of the former protein and has led to its availability in relatively large quantities and at modest cost. No crystalline zymogen is more accessible, and there are few crystalline proteins that can be obtained in a higher state of purity. The accessibility of bovine chymotrypsinogen A has fostered many studies on the structure of this zymogen. Many of the results have been summarized by Desnuelle and Rovery (6). It appears (6-8) that bovine chymotrypsinogen A is a linear polypeptide of 240 to 250 a-amino acid residues cross-linked through five disulfide bridges. Its molecular weight is approximately 25,000; its nitrogen content is 16.5 percent; and its empirical formula, in terms of c...