Sucrose phosphorylase reaction with

When a mixture of UDPG and D-fructose is subjected to the reaction of enz3nne preparations from these plant sources in the presence of inorganic phosphate buffer at pH 7.0, a nonreducing disaccharide, identified as sucrose, is formed. The equilibrium constant, Ky for this reaction at 37°C was found to be between 2 and 8, and the AF was estimated to be approximately —1000 cal. as compared to +1770 cal. for the sucrose phosphorylase reaction. In contrast to the sucrose phosphorylase reaction, the equilibrium of the reaction starting with UDPG and D-fructose is in favor of sucrose synthesis. 

Bacillus subtilis levansucrase (sucrose 2,6-/8-D-fructan 6-)8-D-fructosyltrans-ferase, EC 2.4.1.10) catalyzes fructosyl transfer from sucrose to levan (154). In the absence of a fructosyl acceptor, the primary reaction is sucrose hydrolysis, although a limited amount of self-initiated levan synthesis occurs as well (155). As with sucrose phosphorylase, acceptor specificity is broad a number of saccharides and other nucleophiles are suitable fructosyl acceptors (154-158). The complete amino acid sequence of the approximately SO-kDa enzyme has been determined by both protein (159) and gene (160) sequence analyses. The three-dimensional structure at 3.8 A reveals a rod- or ellipsoid-shaped protein with a length some four times the diameter (161). 

This enzyme is very specific it does not react with sorbose, fructose phosphates, glucose, or any other compound tested in place of fructose, and glucose-l-phosphate cannot replace UDPG. Unlike the case of sucrose phosphorylase, the equilibrium of this reversible reaction favors sucrose synthesis, and the presence of this enzyme in the cells of higher plants implicates this reaction in sucrose synthesis. 

The fact that sucrose can be synthesized from -D-glucose 1-phosphate and D-fructose is consistent with the evidence presented by Isbell and Pigman 172) and Gottschalk 173) that D-fructose exists as an equilibrium mixture of the pyranose and furanose forms. The total system of sucrose synthesis can be represented by the equation (Fig. 4) which includes the equilibrium reaction of the two ring forms of D-fructose. The occurrence of D-fructose in the sucrose molecule as D-fructofuranose is an indication that sucrose phosphorylase is specific for the furanose configuration of that ketose 17A, 175),... 

It was at this state of knowledge that an incidental observation with extracts of C. kluy-veri led to a development which played a significant role in elucidating the mechanism of CoA action. It was observed that arsenate inhibited the ability of extracts to catalyze the oxidation of butyrate, as well as the reduction of acetyl-P and acetate to butyrate. The inhibition was ultimately explained by the fact that arsenate stimulated the enzymic hydrolysis of acetyl-P [reaction (18)]. By analogy to the postulated role of arsenate in the hydrolysis of glucose-l-P by sucrose phosphorylase,< > it was proposed that the arsenolysis of acetyl-P could be the result of a reversible transfer of the acetyl moiety to an acetyl acceptor, x , catalyzed by a phosphotransacetylase [reaction (21)]. 

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