Different forms of the same enzyme that catalyzing the same biochemical reactions are called isoenzymes or isozymes. All the isozymes of a given enzyme are coded by different alleles of the same gene. Isozymes differ in their subunit composition, isoelectric point (PI), electrophoretic mobility, amino acid sequence and kinetic parameters (Km and Vmax). Due to the difference in subunit composition and isoelectric point, isoenzymes can be separated by native gel electrophoresis and ion exchange chromatography.
The isozymes are often evolved to suit for different tissues of the body, therefore they exhibit variations in kinetics properties. the most well-studied isozymes are those of lactate dehydrogenase the enzyme that is responsible for the reduction of pyruvate to lactate.
LDH is a tetrameric enzyme that can be homo tetramer or hetero tetramer
There are five possible isozymes of LDH has been studied
- a) LDH- I (H4) is a homo tetramer of ‘H’ subunits; present in Heart cells
- b) LDH-II (H3M), LDH-III (H2M2) and LDH-IV (HM3) are tetramers distributed in different tissues
- c) LDH- V (M4) is a homo tetramer of ‘M’ subunits present in skeletal muscles.
The heart form of LDH has very high Km for substrate and very low affinity and Vmax also low. These properties are very suitable for heart form since pyruvate accumulation rarely occurs due to the aerobic nature of heart muscle.
The skeletal muscle form has very low Km, very high affinity for the substrate and high Vmax. Skeletal muscle form is well studied for rapid conversion of pyruvate to lactate, which is needed due to the anaerobic nature of skeletal muscles, the rapid activity of LDH in skeletal muscle leads to the accumulation of lactate resulting muscle fatigue.
Isozymes of LDH are also used in clinical biochemistry as diagnostic markers. LDH-I is a marker for myocardial infection, as well as LDH-V, is a marker for muscular dystrophy.
Questions for evaluation
Q Which is the following about isoenzymes is correct
1) Isozymes same in their subunit composition and catalyzes the same biochemical reaction
2) Isoenzymes have same isoelectric point and catalyze the different biochemical reaction
3) isoenzymes have different amino acid sequence and catalyze the same biochemical reaction
4) Isoenzymes differ in kinetic parameters and show different catalytic activity
Q Isoenzymes be separated by
1) Column chromatography and Ion exchange chromatography
2) Gel electrophoresis and Ion exchange chromatography
3) Gel electrophoresis and Column chromatography
4) None of the above