An isoenzyme (or isozyme) is one of a group of enzymes that are very similar in catalytic properties, but may be differentiated by variations in physical properties, such as isoelectric point or electrophoretic mobility.
An enzyme performing the same function as another enzyme but having a different amino acid composition. Isoenzymes can differ in their kinetic properties. [Source: NHBLI/NCBI Glossary
One of many forms of a protein catalyst differing in characteristics (chemical, physical, immunological) but catalyzing the same reaction. For example, lactate dehydrogenase may exist in five different forms.