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Hunter and Markert (1957) defined isozymes as distinct variations of the same enzyme that have identical functions and are present in the same person. (1) enzyme variations that are the product of separate genes and hence represent various loci (described as isozymes) and (2) enzymes that are the result of different alleles of the same gene are included in this definition (described as allozymes).
What are Isozymes?
Isozymes (also known as isoenzymes) are enzymes with varied amino acid sequences that catalyse the same chemical process. Various kinetic parameters (i.e. different KM values) or regulatory characteristics are frequently displayed by these enzymes.
Isozymes allow metabolism to be fine-tuned to fit the specific demands of a certain tissue or developmental stage (for example, lactate dehydrogenase (LDH)). Isozymes (or isoenzymes) are enzyme isoforms (closely related variations) in biochemistry. They are coded for by homologous genes that have diverged through time in many situations. Although the terms allozymes and isozymes technically refer to distinct alleles of the same gene and separate genes whose products catalyse the same process, the two terms are frequently interchanged.
Isozymes are most commonly caused by gene duplication, although they can also be caused by polyploidization or hybridization. If the function of the new variation remains the same as the original over time, one or the other will most likely be lost as mutations accumulate, resulting in a pseudogene. If, on the other hand, the mutations do not instantly stop the enzyme from working, but instead change its function or gene expression pattern, the two versions may be favoured by natural selection and specialise in distinct roles. They may, for example, be expressed at various phases of development or in various tissues.
Point mutations or insertion-deletion (indel) events that alter the gene’s DNA coding sequence can result in allozymes. A new allozyme might do one of three things, just like any other new mutation:
a. The new allele is most likely to be non-functional, in which case it will cause poor fitness and be eliminated from the population by natural selection.
b. If the altered amino acid residue is in a relatively insignificant region of the enzyme, such as a long distance from the active site, the mutation may be selectively neutral and vulnerable to genetic drift.
c. In rare circumstances, the mutation may produce a more efficient enzyme or one that can catalyse a slightly different chemical reaction, in which case the mutation may enhance fitness and be favoured by natural selection.
Glucokinase, a form of hexokinase that is not inhibited by glucose 6-phosphate, is an example of an isozyme. Its unique regulatory characteristics and reduced affinity for glucose (as compared to other hexokinases) allow it to perform a variety of activities in cells of certain organs, such as regulating insulin release in pancreatic beta cells or initiating glycogen synthesis in liver cells. Both of these procedures must take place only when glucose is plentiful or when difficulties arise.