Most biotransforming enzymes are high molecular weight proteins, composed of chains of amino acids linked together by peptide bonds. A wide variety of biotransforming enzymes exist. Most enzymes will catalyze the reaction of only a few substrates, meaning that they have high "specificity". Specificity is a function of the enzyme's structure and its catalytic sites. While an enzyme may encounter many different chemicals, only those chemicals (substrates) that fit within the enzymes convoluted structure and spatial arrangement will be locked on and affected. This is sometimes referred to as the "lock and key" relationship. As shown in Figure 1, when a substrate fits into the enzyme's structure, an enzyme-substrate complex can be formed. This allows the enzyme to react with the substrate with the result that two different products are formed. If the substrate does not fit into the enzyme, no complex will be formed and thus no reaction can occur.
The array of enzymes range from those that have absolute specificity to those that have broad and overlapping specificity. In general, there are three main types of specificity: