Mechanisms involved in the biosynthesis of polysaccharides

Abstract

Abstract:The mechanisms for the biosynthesis of three polysaccharides are presented: (i) starch synthesized by starch synthase and adenosine diphospho glucose; (ii) dextran synthesized by Leuconostoc mesenteroides B-512FMC dextransucrase and sucrose; and (iii) Acetobacter xylinum cellulose synthesized by cellulose synthase, uridine diphospho glucose, and bactoprenol phosphate. All three enzymes were pulsed with substrates, containing 14 C-glucose and chased with the same nonlabeled substrates. When the polysaccharides were isolated, reduced, and hydrolyzed, the pulsed reactions gave 14 C-glucitol, which was significantly decreased in the chase reaction. These experiments definitively show that all three polysaccharides are biosynthesized by the addition of glucose to the reducing-ends of the growing polysaccharides and not by the addition to the nonreducing-ends of primers. Additional evidence indicates that glucose and the polysaccharides are covalently attached to the active-sites of the enzymes. A two catalytic-site insertion mechanism at one active-site is proposed for the biosyntheses. Two of the polysaccharides are α-linked glucans, starch and dextran, and cellulose is a β-linked glucan, known for several years to require a bactoprenol lipid phosphate intermediate. It is shown how this intermediate is involved in determining that β-linkages are synthesized. Other β-linked polysaccharides: bacterial cell wall peptidomurein, Salmonella O-antigen polysaccharide, and Xanthanomonas camprestris xanthan, are heteropolysaccharides, with the later two also being hetero-linked polysaccharides, with the β-linkage at the reducing-end of the repeating unit. All three require bactoprenol lipid phosphate intermediates and are biosynthesized by the addition of the repeating units to the reducing-end of a growing polysaccharide chain, with the formation of a β-linkage.Key words: starch; dextran; cellulose; starch synthase; dextransucrase; cellulose synthase; reducing-end synthesis.Abbreviations: ADPGlc, adenosine diphospho glucose; CP, conversion period, which is the theoretical time necessary to convert the substrate into product for the amount of enzyme present; d.p., degree of polymerization; α-Glc-1-P, α-Dglucose-1-phosphate; Pi, inorganic phosphate; UDPGlc, uridine diphospho glucose.

Two-Dimensional Size/Branch Length Distributions of a Branched Polymer

“…At present, there seem to be no data in the literature for size distributions of waxy (100% amylopectin) native starch obtained under conditions such there was no degradation of the starch during the extraction and dissolution process (although there are data for commercially extracted starch which may have suffered degradation). The implications of the observation that there are probably very small amylopectin molecules are not yet apparent; if this observation is indeed found to be artifact-free, then it needs to be taken into account in descriptions of starch biosynthesis, for which there are significant unresolved questions. ,,, …”

“…The implications of the observation that there are probably very small amylopectin molecules are not yet apparent; if this observation is indeed found to be artifact-free, then it needs to be taken into account in descriptions of starch biosynthesis, for which there are significant unresolved questions. 22,37,44,45 Interpretation of the 2D number distributions is difficult at this point. It is important to realize that while size and number distributions are trivially related for linear polymer, eq 4, there is no such relation for branched polymers: the number and weight distributions include complementary information.…”