Saccharide biosynthesis

Biosynthesis of the polypeptide chain is realised by a complicated process called translation. The basic polypeptide chain is subsequently chemically modified by the so-called posttranslational modifications. During this sequence of events the peptide chain can be cleaved by directed proteolysis, some of the amino acids can be covalently modified (hydroxylated, dehydrogenated, amidated, etc.) or different so-called prosthetic groups such as haem (haemoproteins), phosphate residues (phosphoproteins), metal ions (metal-loproteins) or (oligo)saccharide chains (glycoproteins) can be attached to the molecule by covalent bonds. Naturally, one protein molecule can be modified by more means. 

This great structural variety, however, complicates the specific biosynthesis of complex oligosaccharides. In general, the formation of each saccharide linkage requires specific enzymes ( one linkage—more than one enzyme ) and thus, in comparison with the enzymic synthesis of proteins and nucleic acids, much more effort is needed. 

As a complement to the comprehensive Chapter on the biosynthesis of complex saccharides by Nikaido and Hassid in Vol. 26, where the products of biosynthesis were stressed, Kochetkov and Shibaev (Moscow) here emphasize the nucleotide sugar biosynthetic intermediates, themselves, in a detailed article on their chemistry and enzymology. 

Scheme 1.—Pathway of the Biosynthesis oi Dolichol Phosphate and ot Doliehol-1 inked Saccharides. (Reactions requiring more than one enzyme are indicated by dashed arrows. The Scheme is based on work cited in Refs. 1, 3, 8—25, 35, and 50.)...

Nbiikeld, Elizabeth I1 ., uiul Hahhiu, W. Z., Biosynthesis of Saccharides from Glycopyranosyl Esters of Nucleotides ( Sugar Nucleotides ), 18, 300-356... 

Three monosaccharides of the group were identified which contain an amino group at both C-2 and C-3. These include acylated 2,3-diamino-2,3-dideoxy-D-glucose, which is a component of the lipid A region of lipopoly-saccharides from Rhodopseudomonas,144 and 2,3-di(acetamido)-2,3-dideoxy-D-glucuronic145 and -D-mannuronic146 acids, identified as constituents of P. aeruginosa O-specific polysaccharides. The pathway of their biosynthesis, and their activated forms, remain unknown. 

The material presented in previous sub-sections clearly shows that both of the possible mechanisms of polysaccharide chain-assembly may operate in the biosynthesis of bacterial polysaccharides. There is no clearcut, mechanistic difference in the biosynthesis of O-specific chains of lipopoly-saccharides, exocellular polysaccharides, and carbohydrate chains of Grampositive, cell-wall polymers for every class of polymer, the existence of both mechanisms of chain assembly was demonstrated. 

Glycoproteins Fonnd in Plants, Type of Linkage between the Peptide and Saccharide Moieties, and Involvement of Lipid Intermediates in Their Biosynthesis... 

Chapters 17 through 21 deal with carbohydrate-enzyme systems. Hehre presents some new ideas on the action of amylases. Kabat presents some new immunochemical studies on the carbohydrate moiety of certain water-soluble blood-group substances and their precursor antigens. Hassid reviews the role of sugar phosphates in the biosynthesis of complex saccharides. Pazur and co-workers present information obtained by isotopic techniques on the nature of enzyme-substrate complexes in the hydrolysis of polysaccharides. Gabriel presents a common mechanism for the production of 6-deoxyhexoses. An intermediate nucleoside-5 -(6-deoxyhexose-4-ulose pyrophosphate) is formed in each of the syntheses. 

The Role of Sugar Phosphate in the Biosynthesis of Complex Saccharides... 

Glycosyl—Enzyme Complex Intermediates in Biosynthesis of Complex Saccharides. The synthesis of nucleoside diphosphate sugars involves the transfer of a nucleotidyl group from a nucleoside triphosphate to a sugar 1-phosphate with the simultaneous release of pyrophosphate according to the following general reaction (11) ... 

Since the discovery of UDP-D-glucose by Leloir,17,18 in 1951, this compound has been implicated in the biosynthesis of a number of D-glucose-containing saccharides.4 It is now well accepted that such gly-cosyl esters are the most important intermediates in the biosynthesis of carbohydrates.1 3,4... 

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