BL/CH 401 Lecture #15A
Introduction to Enzyme Mechanisms -- Part I
Part I. Features of Enzymatic Catalysis.
Enzymes cause rate increases in reactions that are 10 times to 10,000 times greater than the best chemical catalysts. Enzymes, since they are proteins and must maintain their folded shape or conformation, operate under mild conditions (370C/pH 7) and have high specificity. Enzymes achieve specificity via geometric and physical complementarity between the active site and the substrate.
Figure 1. 3-D Geometric Fit of Substrate to Enzyme's Active Site.
Of course, it is somewhat obvious that both the substrate and the enzyme are 3-D and that the shape of the substrate must fit the shape of the active site for the substrate to bind to the enzyme and the E-S complex to form. But an important aspect of this reality is that the enzyme and its active site are both assymetric in the chemical sense that they are chiral. Amino acids are chiral and so the enzyme and its active site are formed by chiral structures and are themselves chiral. So if the substrate has an degree of chirality (ie either it has a chiral center or even one partially chiral - usually called pro-chiral, then the enzyme will bind the one form or stereo-isomer of the substrate. Let's take the example of an amino acid which is present in both the D- and L-forms, the enzyme will bind only one of these forms, which in most cases will be the L-form of the amino acid.
But how do enzymes achieve enormous rate accelerations? Why are enzymes such good catalysts?
The answer seems to be that enzymes are better designed for catalyzing a reaction than chemical catalysts. On the other hand, a review of chemical catalysis and the concept of a mechanism for reaction is useful for laying the foundation for understanding an enzyme's mechanism of catalysis.
©Wilbur H. Campbell, 1995; wcampbel@mtu.edu