'''Epimerases''' and '''racemases''' are isomerase enzymes that catalyze the inversion of stereochemistry in biological molecules.<ref name=":0">{{cite journal | author = Tanner, ME. | title = Understanding nature's strategies for enzyme-catalyzed racemization and epimerization | journal = Acc. Chem. Res. | year=2002 | volume=35 | issue=4 | pages=237–246 | pmid = 11955052 | doi = 10.1021/ar000056y }}</ref> Racemases catalyze the stereochemical inversion around the asymmetric carbon atom in a substrate having only one center of asymmetry, i.e. they catalyze interconversion between two enantiomers. On the other hand, epimerases catalyze the stereochemical inversion of the configuration about an asymmetric carbon atom in a substrate having more than one center of asymmetry, thus interconverting epimers.<ref name=":1">{{Citation |title=Design and evaluation of substrate–product analog inhibitors for racemases and epimerases utilizing a 1,1-proton transfer mechanism |date=2023-01-01 |work=Methods in Enzymology |volume=690 |pages=397–444 |url=https://www.sciencedirect.com/science/chapter/bookseries/abs/pii/S0076687923002227 |access-date=2026-04-08 |publisher=Academic Press |language=en-US}}</ref> Due to this, both epimerases and racemases lack the usual stereospecificity of enzymes, because they are able to catalyze the inversion in both directions.<ref name=":0" /><ref name=":1" /> Unlike most other enzymes, which return to their resting state after they catalyze a reaction and can catalyze the same reaction again, epimerases and racemases instead finish in a state which is available for the catalysis of the reverse reaction.<ref>{{Cite journal |last=Holliday |first=Gemma L. |last2=Mitchell |first2=John B. O. |last3=Thornton |first3=Janet M. |date=2009-07-17 |title=Understanding the Functional Roles of Amino Acid Residues in Enzyme Catalysis |url=https://www.sciencedirect.com/science/article/pii/S0022283609005798 |journal=Journal of Molecular Biology |volume=390 |issue=3 |pages=560–577 |doi=10.1016/j.jmb.2009.05.015 |issn=0022-2836|url-access=subscription }}</ref>
== Structure == Epimerases and racemases can either be confactor-independent, i.e. only rely on the protein to catalyze their reaction, or can be cofactor-dependent, using a non-proteinaceous structure for catalysis, e.g. metal ions or PLP.<ref name=":1" />
== Importance == Human epimerases include methylmalonyl-CoA epimerase, involved in the metabolic breakdown of the amino acids alanine, isoleucine, methionine and valine,<ref>{{Cite web|url=http://www.britannica.com/science/isomerase|title = Isomerase | enzyme | Britannica}}</ref> and UDP-glucose 4-epimerase, which is used in the final step of galactose metabolism – catalyzing the reversible conversion of UDP-galactose to UDP-glucose.
Racemases are important to obtain <small>D</small>-amino acids which have various uses,<ref name=":0" /> highlighting the importance of enzymes such as alanine racemase.<ref name=":0" /> Phosphopentose epimerase participates in the pentose phosphate pathway used to generate energy in the form of NADH.<ref name="Alfarouk 2020">{{cite journal |last1=Alfarouk |first1=Khalid O. |last2=Ahmed |first2=Samrein B. M. |last3=Elliott |first3=Robert L. |display-authors=etal |date=2020 |title=The Pentose Phosphate Pathway Dynamics in Cancer and Its Dependency on Intracellular pH |journal=Metabolites |volume=10 |page=285 |doi=10.3390/metabo10070285 |pmc=7407102 |pmid=32664469 |doi-access=free}}</ref>
==See also== * Galactose epimerase deficiency
==References== {{reflist}}
==External links== * http://medical-dictionary.thefreedictionary.com/racemase * http://medical-dictionary.thefreedictionary.com/epimerase * {{MeshName|Entry+Term+Epimerases}}
{{Racemases and epimerases}}
Category:Isomerases
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