Sialic acid biosynthesis in bacteria is less complex than in mammals. There are only three enzymes involved in Neu5Ac biosynthesis; they are the Neu5Ac-synthase, CMP-Neu5Ac-synthetase, and sialyltransferase. Although these enzymes were cloned only within the past 10 years, their straightforward overexpression in bacteria has allowed extensive studies to be performed.

It was not until 4 years ago that the first bacterial Neu5Ac-synthase was cloned [136]. This enzyme, derived from E. coli K1, is unlike the mammalian enzyme in that it does not accept the ManNAc-6-PO;T as a substrate. Rather, it condenses ManNAc with phosphoenolpyruvate (PEP) to produce Neu5Ac. Furthermore, 6-N3 ManNAc can be converted to 9-N3 Neu5Ac, although only with 59% efficiency compared to the native substrate [137]. The E. coli synthase has also been shown to accept ManN as a substrate, albeit with only a minimal production of the sialic acid

Table 10 A Limited Number of CMP-Sialic Acid Derivatives Are Accepted as Substrates by «2,3- and «2,6-Sialyltransferases from Bovine and Human Liver

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