Enzymes epimerization

Transition from the D-gluco to the D-galacto configuration occurs through enzymic epimerization at C-4 of the hexosyl group in glycosyl nucleotides. Such reactions were observed for UDP-Glc,14,102-104 UDP-GlcNAc,44,105-107... 

The conversion includes at least three enzymic reactions.169-171 In the first stage, which requires pyridoxamine 5 -phosphate as a cofactor,171,172 dehydration of 7b occurs through intermediate formation of the Schiff base.173 Reduction of the resulting, unsaturated derivative with NADPH, the mechanism of which is not completely clear,174 leads to CDP-3,6-dideoxy-D-eryf/iro-hexos-4-ulose,169 and, in the third stage, further reduction of the latter at C-4 of the hexosyl group produces the derivatives of paratose or abequose the stereochemistry of the reaction is determined by the source of the enzyme.168 The tyvelose derivative is formed as a result of enzymic epimerization at C-2 of the hexosyl group in CDP-paratose.175... 

Biosynthesis of the most common constituent of the bacterial-lipopolysac-charide core, L-glycero-D-manno-heptose, was shown239,240 to occur through enzymic epimerization at C-6 of the heptosyl group in 9. Genetic evidence241,242 supports this pathway of biosynthesis. 

Examples of enzymic epimerization are given in Refs. 22 and 23 above. 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

It is likely that the madurastatins are biosynthesized on a nonribosomal peptide synthetase, from salicylic acid as the starter acid. L-Serine is probably the precursor to the aziridine moiety, with epimerization occurring on the enzyme-bound amino acid as found for other nonribosomal peptides, with aziridine formation occurring at a late stage. Compounds 120 and 123 could therefore be biosynthetic precursors to 119 and 122, respectively. 

Extensive studies have indicated that only pyruvate is acceptable as the NeuA donor substrate, with the exception of fluoropyruvate [49], but that the enzyme displays a fairly broad tolerance for stereochemically related aldehyde substrates as acceptor alternatives, such as a number of sugars and their derivatives larger or equal to pentoses [36,48,50,51]. Permissible variations include replacement of the natural D-manno configured substrate (4) with derivatives containing modifications such as epimerization, substitution, or deletion at positions C-2, -4, or -6 [16,27]. Epimeriza-tion at C-2, however, is restricted to small polar substituents owing to strongly... 

It is important to note that the foregoing, biosynthetic-polymer modification is usually incomplete. In fact, only a fraction of the heparin precursor undergoes all of the transformations shown in Scheme 1. However, as the product of each enzymic reaction constitutes the specific substrate for the succeeding enzyme, the biosynthesis of heparin is not a random process. Thus, sulfation occurs preferentially in those regions of the chain where the amino sugar residues have been N-deacetylated and N-sulfated, and where D-glucuronic has been epimerized to L-iduronic acid.20... 

Even when hydrolysis and epimerization can be avoided during sample preparation and handling, it is not possible to conclude definitively whether the compounds found in plasma and urine are true metabolites or simply degradation products. Indeed, chemical degradation can also occur within the body since urine and plasma contain a wide variety of potential catalysts, including metal ions, phosphate ions, proteins, and sugars (see Sect. 5.2.6). Whereas the existence of mammalian enzymes that act on penicillins and cephalosporins is considered possible [155], no such mammalian enzyme appears to have been identified to date. 

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. 

As well as activating the amino acids and catalysing formation of the peptide linkages, the enzyme may possess other domains that are responsible for epimerizing L-amino acids to o-amino acids,... 

Thus, racemic 62 can be resolved enzymatically to give either C15 diaste-reomer of 61, simply by using the appropriate enzyme. Because it was observed that the C15 stereocenter of 62 epimerizes faster in the presence of base [87, 88] (e.g., EtsN) than that of 61, even an enantioconvergent process is possible, where 62a and 62b are in constant equilibrium, but only 62a is converted to the corresponding acetate. Enantiomerically pure 61a is then readily converted to 64. 

---