Enzymes epimerases

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

The turnover number of methylmalonyl-CoA epimerase is 100 sec and thus the enzyme enhances the reaction rate by a factor of 10. ... 

Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-glucose by galactose 4-epimerase, where UDP = uridylyl diphosphate (a ribonucleotide diphosphate Section 28.1). The enzyme requires NAD+ for activity (Section 17.7), but it is not a stoichiometric reactant. and NADH is not a final reaction product. Propose a mechanism. 

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

The GAGs are synthesized by the sequential actions of a battery of specific enzymes (glycosyltransferases, epimerases, suhotransferases, etc) and are degraded by the sequential action of lysosomal hydrolases. Genetic deficiencies of the latter result in mucopolysaccharidoses (eg, Hurler syndrome). 

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below ... 

Subsequent studies196 on crystalline transketolase have revealed the presence of a contaminating enzyme termed pentulose 5-phosphate waldenase (or epimerase) the presence of which had led to the erroneous conclusion that d-erythro-pentulose 5-phosphate was the substrate for transketolase. d-erythro-Pentulose 5-phosphate is virtually unattacked by transketolase prepared from spinach or liver. In subsequent discussions of experiments involving the use of transketolase, in this article, the enzymic reactions must be viewed as the result of action of transketolase and pentulose 5-phosphate waldenase (epimerase). 

As an alternative to chemical epimerization, NAG epimerase may be used to maintain a constant NAM NAG ratio in a one-pot reaction with pyruvate and NANA aldolase. The epimerase is itself inhibited by pymvate, which must, therefore, be added continuously or via aliquots to the reaction. In a refined version of this reaction at laboratory scale, Kragl et al produced NANA by a continuous process, using a membrane reactor to contain both enzymes in solution. 

Tabata, K., Koizumi, S., Endo, T. and Ozaki, A., Production of A-acetyl-D-neuraminic acid by coupling bacteria expressing N-acetyl-o-glucosamine 2-epimerase and N-acetyl-o-neuraminic acid synthetase. Enzyme Microb. Technol. 2002, 30, 327-333. 

Once transported into tissues, galactose is phosphorylated (galactokinase), trapping it in the cell. Galactose 1-phosphate is converted to glucose 1-phosphate by galactose 1-P uridyltrans-ferase and an epimerase. The pathway is shown in Figure 1-12-5 important enzymes to remember are ... 

Eor shaping the 1,3-diaminocyclitol into its final configuration and into the actinamine structure, first an epimerization step has to invert the stereochemical placement of the 2-hydroxyl group, which is proposed to be catalyzed by the SpcD enzyme, being related to sugar epimerases second, the actinamine branch... 

This enzyme catalyzes the epimerization at the 2-position of A-acetylglucosamine 6-phosphate. See also N-Acyl-glucosamine-6-phosphate 2-Epimerase... 

This enzyme [EC 5.1.3.8], also known as A-acetylglucos-amine 2-epimerase, catalyzes the epimerization of N-acylglucosamine to A-acylmannosamine. ATP is reported to be required in this reaction. 

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