isoenzyme
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In , isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are s that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. different values), or are regulated differently. They permit the fine-tuning of metabolism to meet the particular needs of a given tissue or developmental stage. In many cases, isozymes are encoded by genes that have diverged over time. Strictly speaking, enzymes with different amino acid sequences that catalyse the same reaction are isozymes if encoded by different genes, or s if encoded by different s of the same ; the two terms are often used interchangeably.


Introduction

Isozymes were first described by and (1957) who defined them as ''different variants of the same enzyme having identical functions and present in the same individual''. This definition encompasses (1) enzyme variants that are the product of different genes and thus represent different (described as ''isozymes'') and (2) enzymes that are the product of different of the same gene (described as ''allozymes''). Isozymes are usually the result of , but can also arise from or . 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 s accumulate, resulting in a . However, if the mutations do not immediately prevent the enzyme from functioning, but instead modify either its function, or its pattern of , then the two variants may both be favoured by and become specialised to different functions. For example, they may be expressed at different stages of development or in different tissues. Allozymes may result from s or from insertion-deletion () 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 and be removed from the population by . * Alternatively, if the residue that is changed is in a relatively unimportant part of the enzyme (e.g., a long way from the ), then the mutation may be and subject to . * In rare cases, the mutation may result in an enzyme that is more efficient, or one that can catalyse a slightly different , in which case the mutation may cause an increase in fitness, and be favoured by natural selection.


Examples

An example of an isozyme is , a variant of which is not inhibited by . 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 release by the s of the , or initiation of synthesis by cells. Both these processes must only occur when glucose is abundant. 1.) The enzyme is a tetramer made of two different sub-units, the H-form and the M-form. These combine in different depending on the tissue: 2.) Isoenzymes of creatine phosphokinase: 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 3.) Isoenzymes of alkaline phosphatase: 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 and , they may be distinguished by a . However, such differences are usually subtle, particularly between ''allozymes'' which are often . This subtlety is to be expected, because two enzymes that differ significantly in their function are unlikely to have been identified as ''isozymes''. While isozymes may be almost identical in function, they may differ in other ways. In particular, substitutions that change the of the enzyme are simple to identify by , and this forms the basis for the use of isozymes as s. 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 , but 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 of soluble indicator s such as which become insoluble when they are by such as or , which generated in zones of enzyme activity. This assay method requires that the enzymes are still functional after separation (), and provides the greatest challenge to using isozymes as a laboratory technique. Isoenzymes differ in kinetics (they have different values).


Isozymes and allozymes as molecular markers

is essentially a study of the causes and effects of genetic variation within and between populations, and in the past, isozymes have been amongst the most widely used s for this purpose. Although they have now been largely superseded by more informative -based approaches (such as direct , s and ), they are still among the quickest and cheapest marker systems to develop, and remain () an excellent choice for projects that only need to identify low levels of genetic variation, e.g. quantifying s.


Other major examples

*The isozymes play important roles in and . *The multiple forms of 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

* * {{cite journal , last1 = Weiss , first1 = B. , last2 = Hait , first2 = W.N. , year = 1977 , title = Selective cyclic nucleotide phosphodiesterase inhibitors as potential therapeutic agents , journal = Annu. Rev. Pharmacol. Toxicol. , volume = 17 , pages = 441–477 , doi=10.1146/annurev.pa.17.040177.002301 , pmid = 17360 * Wendel, JF, and NF Weeden. 1990. "Visualisation and interpretation of plant isozymes." pp. 5–45 in and , eds. ''Isozymes in plant biology.'' Chapman and Hall, London. * Weeden, NF, and JF Wendel. 1990. "Genetics of plant isozymes". pp. 46–72 in and , eds. ''Isozymes in plant biology.'' Chapman and Hall, London * Crawford, DJ. 1989. "Enzyme electrophoresis and plant systematics". pp. 146–164 in and , 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


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) Biochemistry