Isozyme 1079368 212965746 2008-05-17T02:27:23Z SamatBot 7027852 robot Adding: [[hu:Izoenzim]] Modifying: [[cs:Izoenzym]] '''Isozymes''' (also known as '''isoenzymes''') are [[enzyme]]s that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters (i.e. different [[Michaelis-Menten kinetics|''K''<sub>M</sub>]] values), or different regulatory properties. The existence of isozymes permits the fine-tuning of metabolism to meet the particular needs of a given tissue or developmental stage (for example [[lactate dehydrogenase]] (LDH)). In [[biochemistry]], isozymes (or isoenzymes) are [[isoform]]s (closely related variants) of enzymes. In many cases, they are coded for by [[homology (biology)|homologous]] genes that have diverged over time. Although, strictly speaking, '''allozymes''' represent enzymes from different alleles of the same [[gene]], and isozymes represent enzymes from different genes that process or [[catalyse]] the same reaction, the two words are usually used interchangeably. ==Introduction== Isozymes were first described by [[R. L. Hunter]] and [[Clement Markert]] (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 [[locus (genetics)|loci]] (described as ''isozymes'') and (2) enzymes that are the product of different alleles of the same gene (described as ''allozymes''). Isozymes are usually the result of [[gene duplication]], but can also arise from [[polyploidy|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 [[gene expression]], then the two variants may both be favoured by [[natural selection]] 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 [[point mutation]]s or from insertion-deletion ([[mutation|''indel'']]) events that affect the [[DNA]] coding sequence of the [[gene]]. As with any other new [[mutation]], there are three things that may happen to a new allozyme: # It is most likely that the new allele will be non-functional &mdash; in which case it will probably result in low [[fitness (biology)|fitness]] and be removed from the population by [[natural selection]]. # Alternatively, if the [[amino acid]] residue that is changed is in a relatively unimportant part of the enzyme, for example a long way from the [[active site]] then the mutation may be [[Neutral theory of molecular evolution|selectively neutral]] and subject to [[genetic drift]]. # 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]]. ==An example of an isozyme== 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), allows it to serve different functions in cells of specific organs, such as control of [[insulin]] release by the [[beta cell]]s of the [[pancreas]], or initiation of [[glycogen]] synthesis by [[liver]] cells. Both of these processes must only occur when glucose is abundant, or problems occur. ==Distinguishing isozymes == Isozymes (and allozymes) are variants of the same enzyme. Unless they are identical in terms of their biochemical properties, for example their [[Substrate (biochemistry)|substrates]] and [[enzyme kinetics]], they may be distinguished by a [[biochemical assay]]. However, such differences are usually subtle (particularly between ''allozymes'' which are often [[Neutral theory of molecular evolution| neutral variants]]). This subtlety is to be expected, because two enzymes that differ significantly in their function are unlikely to have been identified as ''isozymes''. Whilst 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 (such as replacing [[aspartic acid]] with [[glutamic acid]]) are simple to identify by [[gel electrophoresis]], and this forms the basis for the use of isozymes as [[molecular marker]]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 [[potato]] starch, however, [[acrylamide]] gels provide better resolution, and [[cellulose acetate]] gels are now ([[as of 2005]]) the norm. 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 [[precipitation (chemistry)|precipitation]] of soluble indicator [[dye]]s such as [[tetrazolium salt]]s which become insoluble when they are [[Redox|reduced]] by [[Cofactor (biochemistry)|cofactors]] such as [[Nicotinamide adenine dinucleotide|NAD]] or [[NADP]], which generated in zones of enzyme activity. This assay method requires that the enzymes are still functional after separation ([[native gel electrophoresis]]), and provides the greatest challenge to using isozymes as a laboratory technique. ==Isozymes and allozymes as molecular markers == [[Population genetics]] 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 [[Molecular marker]]s for this purpose. Although they have now been largely superseded by more informative [[DNA]]-based approaches (such as direct [[DNA sequencing]], [[single nucleotide polymorphism]]s and [[microsatellites]]), they are still amongst the quickest and cheapest marker systems to develop, and remain ([[as of 2005]]) an excellent choice for projects that only need to identify low levels of genetic variation, e.g. quantifying [[mating system]]s. ==Other major examples== *The [[cytochrome P450]] isozymes play important roles in [[metabolism]] and [[steroidogenesis]]. == References == * Hunter, R. L. and C.L. Merkert. (1957) Histochemical demonstration of enzymes separated by zone electrophoresis in starch gels. ''Science'' 125: 1294-1295 * Wendel, JF, and NF Weeden. 1990. "Visualisation and interpretation of plant isozymes." Pp. 5-45 in D. E. Soltis and 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 D. E. Soltis and P. S. Soltis, eds. ''Isozymes in plant biology.'' Chapman and Hall, London * Crawford, DJ. 1989. "Enzyme electrophoresis and plant systematics". Pp. 146-164 in D. E. Soltis and 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 ==External links== * [http://www.genetics.cf.adfg.state.ak.us/techfac/electro2.php Allozyme Electrophoresis Techniques] - a complete guide to starch gel eletrophoresis * [http://www.chem.ttu.ee/teadus/teema_en?t=T499 Development of new isozyme specific therapeutics] - Fatty Acid Dioxygenases and Eicosanoid Hormones (Estonia) [[Category:Enzymes]] [[cs:Izoenzym]] [[de:Isoenzym]] [[es:Isoenzima]] [[eo:Izoenzimo]] [[fr:Isoenzyme]] [[id:Isoenzim]] [[it:Isoenzima]] [[he:איזואנזים]] [[hu:Izoenzim]] [[ja:アイソザイム]] [[pl:Izoenzymy]] [[ru:Изоферменты]] [[fi:Isoentsyymi]]