Isozymes usually catalyze the exact same reaction. That is, they use the same substrates and produce the same products. They often differ in the rate of reaction and their response to allosteric activators and inhibitors. This difference in reactivity and response is usually do to a difference in the amino acid sequence in one or more of the polypeptide chains making up the molecule. The isoenzymes are usually separated upon electrophoresis.

Isozymes often result when the cell makes two similar but different polypeptide chains. For example, Creatine phosphokinase (CPK) is a dimer (two peptide chains) made from two genes. One polypeptide chain is called B because it is heavily expressed in Brain. The Other polypeptide is called M because it is heavily expressed in Muscle. When a cell expresses both genes both the B and the M chains are produced. They combine to make the following isozymes:

MM, MB, BB

The amount of each isozyme depends upon the relative synthesis of the M and B chains.

Another example is the Lactate dehydrogenase enzyme (LDH). The LDH molecule always contains 4 polypeptide subunits and each subunit can be either a H (heart) or a M (muscle) subunit. When a cell expresses both genes both the H and the M chains are produced. They combine to make the following 5 isozymes:

HHHH, HHHM, HHMM, HMMM, MMMM