The following has been excerpted from a very popular Worthington publication which was originally published in 1972 as the Manual of Clinical Enzyme Measurements. While some of the presentation may seem somewhat dated, the basic concepts are still helpful for researchers who must use enzymes but who have little background in enzymology.
Enzymes are catalysts and increase the speed of a chemical reaction without themselves undergoing any permanent chemical change. They are neither used up in the reaction nor do they appear as reaction products.
The basic enzymatic reaction can be represented as follows
where E represents the enzyme catalyzing the reaction, S the substrate, the substance being changed, and P the product of the reaction.
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PDF version of Introduction to Enzymes