The below mentioned article highlights the major functions of isoenzymes in plants.
Some enzymes occur in multiple or more than one form in the same species, tissue or even in the same cell which catalyse the same biochemical reaction but have different molecular structure and kinetic properties. They are generally made up of similar or dissimilar sub-units (polypeptide chains) in varying combinations which can be separated and distinguished by suitable methods like electrophoresis.
Such multiple forms of enzymes with the same catalytic activity but different structure are known as isoenzymes or isozymes. When isoenzymes consist of many sub-units or polypeptide chains, they are called as polymers. Isoenzymes having similar subunits are said to be homopolymers while those with dis-similar units are called as heteropolymers.
It is now generally held that there are different genes that code for each different isoenzyme. In case, the isoenzyme is a heteropolymer then there are separate genes for each of its different subunits or polypeptide chain. If the isoenzyme is a monomer even then it may exists into two different isozymes forms due to genetic coding by each of the two allelic genes each derived from a different parent.
Other genetic possibilities may also be there. According to Trewavas (1976) sometimes there might be minor differences in structure of isoenzymes which occur only after they have been synthesized. Such differences are independent of gene coding and may be due to glycosylation, methylation, acetylation or phosphorylation etc., of some of the amino acid residues of the polypeptide chain of the isoenzymes.
Major function of isoenzymes is in the control of metabolic activities of the cell under different metabolic or environmental conditions which exist at different sites within the cell, in the same cell at different stages of its development or in different tissues and organs of the organisms. More than a hundred different enzymes are now known to exist in isozyme forms among living organisms. They have been investigated in detail especially in animals although a number of them occur in plants too.
One of the first enzymes discovered in isozyme forms is lactate dehydrogenase (abbreviated as LDH) which catalyses the following reversible oxidation reduction reaction:
Lactate + NAD+ <==> Pyruvate + NADH + H+
This isoenzyme lactate dehydrogenase has been thoroughly studied in animal tissues such as skeletal muscles and heart tissues of vertebrates. This enzyme has been separated electrophoretically into five distinctly different forms each with the same mol. wt. (Ca. 1, 35,000 Dalton) and with essentially the same catalytic activity. These different forms of LDH are designated as LDH-1, LDH-2, LDH-3, LDH-4 and LDH-5. Each of these isozyme forms of LDH in turn is a tetramer consisting of four polypeptide chains or sub-units, each sub-unit having a mol. wt. of Ca. 33500 Dalton.
Different forms of LDH contain varying ratios of two kinds of polypeptides chains which differ in composition and sequence of amino acids and are designated as A or M (M for muscle) and B or H (H for heart). In skeletal tissue the LDH isozyme that predominates contains four A type i.e., M type chains while LDH predominating in heart contains four B-type i.e., H-type chains and they are designated as LDH-A4 and LDH-B4 respectively. Other tissues contain a mixture of five possible forms which are designated as follows:-
The different LDH isozymes differ significantly in the maximum activities (Vmax), and in the Michaelis constant (Km) for their substrates especially for pyruvate. Presence of isoenzymes is not uncommon in plants too. For instance, 18 different peroxidase isoenzymes have been obtained from maize.
In plants, aspartate kinase which is involved in the biosynthesis of amino acids such as lysine and alanine from aspartate is known to exist in two isozyme forms. Ting et al, 1975 in their experiments showed the separation of three isozyme forms of the enzyme malate dehydrogenase by starch-gel electrophoresis from spinach leaf cells. Two of these isozymes corresponded one each to isozymes extracted from isolated peroxisomes and mitochondria while the third existed in cytosol.