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Isoenzymes, also referred to as isozymes, are variant forms of the same enzyme.

Enzymes which catalyse the same reaction, but have structural differences as have a different amino acid sequence, they are coded for by different genes and are expressed in different parts of the body.
Isoenzymes may have different physical and/or kinetic properties. The optimal pH for one isoenzyme may differ to another.

Lactate dehydrogenase (LDH) is an enzyme which has isoenzyme forms. LDH is composed of two different types of sub-unit which have slight alterations in their amino acid sequence. These sub-units, H and M, form random combinations with each other to form the final 4 unit polymer (tetramer).

There are five possible isoenzymes: LDH-1 (4H) - found in heart and RBCs; LDH-2 (3H1M) - found in the reticuloendothelial system, LDH-3 (2H2M) - found in the lungs; LDH-4 (1H3M) - found in the kidneys, placenta, and pancreas; and LDH-5 (4M) - found in the liver and skeletal muscles. From this, it is clear that specific isoenzymes of LDH are found in different tissues. 

During medical diagnoses, cell death in the affected tissue will cause contents of the cell to be present in the blood. For instance, if there is a myocardial infarction, the affected tissues will be in the heart, therefore a higher concentration of LDH-1 will be present in the blood if tested. Testing of the isozymes present in the blood can lead doctors to diagnose which tissues are being affected and also keep an eye on how effective a proposed treatment is.

Another instance of isoenzymes is PKC, protein kinase C. This family of kinases has 11 known isozymes. After being activated, each isozyme ‘translocates and binds to a specific receptor for activated C kinase (RACK)’.

Following stimulation via glucose, two isozymes (alpha PKC, epsilon PKC) will go to the periphery of the cell, whereas two others (delta PKC and zeta PKC) will move to perinuclear sites. Thus, showing how different isozymes take effect in different areas and can be influenced by other molecules such as glucose [1][2].


  1. Page 75. Hames, B.D., Hooper, N.M, 2000, Instant Notes Biochemistry. Second Edition. Ofxord: BIOS Publishers Limited.
  2. Internet source: Translocation inhibitors define specificity of protein kinase C isoenzymes in pancreatic beta-cells. [available at] accessed 30/11/11

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