Glucose-6-phosphatase substrate concentration

Lakshmi and Balasubramanian (1980) showed the presence of a new multiple form of arylsulfohydrolase B in human and monkey brain. Arylsulfohydrolase B, can be separated by DEAE-cellulose chromatography (Mathew and Balasubramanian, 1984). The B, form totally binds to Sephadex G-200 and was not eluted with 1.0 M NaCl, 0.5 M glucose, 0.5 M glucose plus 0.5 M NaCl, 0.5 M KSCN, 1 M urea, or 1% Triton X-100. The treatment of arylsulfohydrolase B with Escherichia coli alkaline phosphatase results in the formation of a less acidic form, presumably due to dephosphorylation. The dephosphorylated form does not bind to DEAE-cellulose. Inorganic phosphate and serine phosphate but not mannose 6-phosphate can inhibit this dephosphorylation. The kinetic properties of the phosphorylated and dephosphorylated arylsulfohydrolase are quite similar. The possibility that arylsulfohydrolase B is a dephosphorylated form of B, has been ruled out by the significant differences between substrate concentration and activity curves of these enzymes. 

The hexokinase step is bypassed in the same maimer as is the phosphofmctokinase reaction, by a phosphatase that is activated by high concentrations of glucose-6-phosphate (an example of substrate-level control). Note that this is the opposite of the effect of glucose-6-phosphate on the rate of the hexokinase step. 

Substrate-Limited Reactions. As individual steps in a sequence approach their equilibria, the rate of reaction decreases. The fermentation of glucose catalyzed by yeast extracts illustrates the effect of product accumulation. Normally ATP is considered a desirable product, and, as it plays a catalytic role in glycolysis, is not considered in estimating the progress of the reaction. In most crude extracts active phosphatases hydrolyze ATP and prevent its accumulation. Glycolyzing yeast extracts, however, were found to be stimulated by the addition of apyrase, which formed ADP from ATP. Thus glycolysis was shown to be limited at the phosphokinase steps by limiting concentrations of ADP, the phosphate acceptor, or by excessive concentrations of ATP that favored the back reactions. 

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