Enzymes with slightly different molecular structure but performing identical activity are called isoenzymes.

Explanation:
Enzymes are biological catalysts that facilitate and speed up biochemical reactions in living organisms. They are usually proteins with a specific three-dimensional structure that allows them to bind to specific substrates and convert them into products. However, enzymes can also have slightly different molecular structures while performing the same catalytic activity. These enzymes are called isoenzymes.

Definition of Isoenzymes:
Isoenzymes, also known as isozymes, are enzymes that catalyze the same chemical reaction but have slightly different amino acid sequences or molecular structures. They are encoded by different genes but perform identical functions in the same organism or different tissues. Isoenzymes are often found in multiple forms and can be distinguished from each other through various methods such as electrophoresis or immunoassay.

Reasons for Different Molecular Structures:
1. Genetic Differences: Isoenzymes are encoded by different genes, which can result in variations in the amino acid sequence and overall structure of the enzyme.
2. Post-translational Modifications: Isoenzymes may undergo different post-translational modifications, such as phosphorylation or glycosylation, which can alter their structure and function.
3. Tissue-Specific Expression: Different tissues within an organism may require slightly different versions of an enzyme to perform the same activity optimally. These tissue-specific isoenzymes may have distinct molecular structures.

Importance of Isoenzymes:
1. Diagnostic Tool: Isoenzymes can be used as diagnostic markers for certain diseases or conditions. For example, the measurement of different forms of creatine kinase isoenzymes can help diagnose heart attacks.
2. Tissue-Specific Function: Isoenzymes may have different kinetic properties or substrate specificities, allowing them to perform specialized functions in specific tissues.
3. Evolutionary Adaptation: Isoenzymes can evolve to perform similar functions under different conditions or in different organisms, allowing for adaptation and survival in diverse environments.

In conclusion, isoenzymes refer to enzymes with slightly different molecular structures but identical catalytic activities. They play important roles in various biological processes and have diagnostic and evolutionary significance.

These enzyme catalyze the rearrangement of molecular structure to form isomers. isomers are molecules or molecular compounds in that they have same molecular formula but different arrangement of atoms or groups of atom .they catalyse interconversion of optical , gymetrical ,positional isomers.. example _isomerase