Isozyme
In
In many cases, isozymes are encoded by
; the two terms are often used interchangeably.Introduction
Isozymes were first described by
Isozymes are usually the result of gene duplication, but can also arise from polyploidisation or nucleic acid hybridization. Over evolutionary time, if the function of the new variant remains identical to the original, then it is likely that one or the other will be lost as mutations accumulate, resulting in a pseudogene. However, if the mutations do not immediately prevent the enzyme from functioning, but instead modify either its function, or its pattern of expression, then the two variants may both be favoured by natural selection and become specialised to different functions.[3] For example, they may be expressed at different stages of development or in different tissues.[4]
Allozymes may result from point mutations or from insertion-deletion (indel) events that affect the coding sequence of the gene. As with any other new mutations, there are three things that may happen to a new allozyme:
- It is most likely that the new allele will be non-functional—in which case it will probably result in low fitness and be removed from the population by natural selection.[5]
- Alternatively, if the amino acid residue that is changed is in a relatively unimportant part of the enzyme (e.g., a long way from the active site), then the mutation may be selectively neutral and subject to genetic drift.[6]
- In rare cases, the mutation may result in an enzyme that is more efficient, or one that can catalyse a slightly different chemical reaction, in which case the mutation may cause an increase in fitness, and be favoured by natural selection.[6]
Examples
An example of an isozyme is glucokinase, a variant of hexokinase which is not inhibited by glucose 6-phosphate. Its different regulatory features and lower affinity for glucose (compared to other hexokinases), allow it to serve different functions in cells of specific organs, such as control of insulin release by the beta cells of the pancreas, or initiation of glycogen synthesis by liver cells. Both these processes must only occur when glucose is abundant.
1.) The enzyme lactate dehydrogenase is a tetramer made of two different sub-units, the H-form and the M-form. These combine in different combinations depending on the tissue:[7]
Type | Composition | Location | Electrophoretic Mobility | Whether destroyed by
Heat (at 60 °C) |
Percentage of normal
serum in humans |
---|---|---|---|---|---|
LDH1 | HHHH | Heart and Erythrocyte | Fastest | No | 25% |
LDH2 | HHHM | Heart and Erythrocyte | Faster | No | 35% |
LDH3 | HHMM | Brain and Kidney | Fast | Partially | 27% |
LDH4 | HMMM | Skeletal Muscle and Liver | Slow | Yes | 8% |
LDH5 | MMMM | Skeletal Muscle and Liver | Slowest | Yes | 5% |
2.) Isoenzymes of creatine phosphokinase:[7] Creatine kinase (CK) or creatine phosphokinase (CPK) catalyses the interconversion of phospho creatine to creatine .
CPK exists in 3 isoenzymes. Each isoenzymes is a dimer of 2 subunits M (muscle), B (brain) or both[7]
Isoenzyme | Subunit | Tissue of Origin |
---|---|---|
CPK1 | BB | Brain |
CPK2 | MB | Heart |
CPK3 | MM | Skeletal muscle |
3.) Isoenzymes of alkaline phosphatase:[7] Six isoenzymes have been identified. The enzyme is a monomer, the isoenzymes are due to the differences in the carbohydrate content (sialic acid residues). The most important ALP isoenzymes are α1-ALP, α2-heat labile ALP, α2-heat stable ALP, pre-β ALP and γ-ALP. Increase in α2-heat labile ALP suggests hepatitis whereas pre-β ALP indicates bone diseases.
Distinguishing isozymes
Isozymes (and allozymes) are variants of the same enzyme. Unless they are identical in their biochemical properties, for example their
While isozymes may be almost identical in function, they may differ in other ways. In particular, amino acid substitutions that change the electric charge of the enzyme are simple to identify by gel electrophoresis, and this forms the basis for the use of isozymes as molecular markers. To identify isozymes, a crude protein extract is made by grinding animal or plant tissue with an extraction buffer, and the components of extract are separated according to their charge by gel electrophoresis. Historically, this has usually been done using gels made from potato starch, but acrylamide gels provide better resolution.
All the proteins from the tissue are present in the gel, so that individual enzymes must be identified using an assay that links their function to a staining reaction. For example, detection can be based on the localised
Isoenzymes differ in kinetics (they have different
Isozymes and allozymes as molecular markers
Other major examples
- The steroidogenesis.
- The multiple forms of phosphodiesterase also play major roles in various biological processes. Although more than one form of these enzymes have been found in individual cells, these isoforms of the enzyme are unequally distributed in the various cells of an organism. From the clinical standpoint they have been found to be selectively activated and inhibited, an observation which has led to their use in therapy.
References
- Hunter, R. L.; Merkert, C.L. (1957). "Histochemical demonstration of enzymes separated by zone electrophoresis in starch gels". Science. 125 (3261): 1294–1295. PMID 13432800.
- Weiss, B.; Hait, W.N. (1977). "Selective cyclic nucleotide phosphodiesterase inhibitors as potential therapeutic agents". Annu. Rev. Pharmacol. Toxicol. 17: 441–477. PMID 17360.
- Wendel, JF, and NF Weeden. 1990. "Visualisation and interpretation of plant isozymes." pp. 5–45 in P. S. Soltis, eds. Isozymes in plant biology. Chapman and Hall, London.
- Weeden, NF, and JF Wendel. 1990. "Genetics of plant isozymes". pp. 46–72 in P. S. Soltis, eds. Isozymes in plant biology. Chapman and Hall, London
- Crawford, DJ. 1989. "Enzyme electrophoresis and plant systematics". pp. 146–164 in P. S. Soltis, eds. Isozymes in plant biology. Dioscorides, Portland, Oregon.
- Hamrick, JL, and MJW Godt. 1990. "Allozyme diversity in plant species". pp. 43–63 in A. H. D. Brown, M. T. Clegg, A. L. Kahler and B. S. Weir, eds. Plant Population Genetics, Breeding, and Genetic Resources. Sinauer, Sunderland
- Biochemistry by jeremy M. Berg, John L. Tymoczko, Lubert Stryer (Intro taken from this textbook)
- Specific
- PMID 16590440.
- ^ Kearney (2014). Fundamental Genetics (3rd ed.). McNaughton Publishing. pp. 413–414.
- ^ Gerald, Gerald (2015). The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology. Sterling. p. 79.
- ^ Huang, Le (2009). Genome. Grady-McPherson. p. 299.
- ^ Alberts (2017). Molecular Biology of the Cell (6th ed.). Garland Science. p. 649.
- ^ a b Walstrom, Ford; et al. (2014). "Models of genetics and natural selection: a current biomolecular understanding". Biomolecular Ecology. 70 (2): 1021–1034.
- ^ OCLC 71209231.
External links
- Allozyme Electrophoresis Techniques – a complete guide to starch gel electrophoresis
- Development of new isozyme specific therapeutics – Fatty Acid Dioxygenases and Eicosanoid Hormones (Estonia)