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Isoenzymes or Isozymes- Definition and Examples

Isoenzymes Definition

Different forms of the same enzyme that catalyzes 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). As they differ in their subunit composition and isoelectric point, the isoenzymes can easily be separated by native gel electrophoresis and ion-exchange chromatography.

The isozymes are often evolved as per the requirements of different tissues, therefore they exhibit variations in kinetic properties.

Isoenzyme Definition and Examples
Isoenzyme Definition and Examples

What are Isoenzymes or Isozymes and Examples?

The well-studied isozymes are those of lactate dehydrogenase (LDH), the enzymes that are responsible for the reduction of pyruvate to lactate.

LDH is a tetrameric enzyme that can be found in homotetrameric or heterotetrameric forms.

There are five possible isozymes of LDH that have been studied.

a) LDH- I (H4) is a homotetramer 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 homotetramer of ‘M’ subunits present in skeletal muscle cells.

Figure: What are Isoenzymes or Isozymes and Examples

The heart form of LDH has very high Km for substrate and very low affinity and Vmax. These properties are very common for heart form since pyruvate accumulation rarely occurs due to the aerobic nature of heart muscle cells.

The skeletal muscle form has very low Km, high Vmax, and a very high affinity for the substrate. Skeletal muscle is well studied for rapid conversion of pyruvate to lactate. This conversion is very crucial as the skeletal muscle cells are bound to anaerobic respiration. More activity of LDH in skeletal muscle cells leads to an accumulation of lactate, resulting in muscle fatigue.

Isozymes of LDH are also used in clinical biochemistry as diagnostic markers. LDH-I is a marker for myocardial infection, while LDH-V is a marker enzyme for muscular dystrophy.

Frequently asked questions

Q Which is the following about isoenzymes is correct

1) Isozymes are the same in their subunit composition and catalyze the same biochemical reaction.

2) Isoenzymes have the same isoelectric point and catalyze different biochemical reactions.

3) Isoenzymes have different amino acid sequences and catalyze different biochemical reactions.

4) Isoenzymes differ in kinetic parameters and show different catalytic activity.

Answer: 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.

Answer: 2 (Gel electrophoresis and Ion exchange chromatography).