Glucose 6-phosphate accumulation

Substrate level control - In this control mechanism, high levels of the product of a reaction inhibit the ability of the small amounts of substrate present to react. An example is the first step in glycolysis, catalyzed by hexokinase. It is inhibited by the product of its action, glucose-6-phosphate. If glycolysis is blocked for any reason, glucose-6-phosphate accumulates. 

The toxic effect of fluoroacetate was studied in an experiment using intact isolated rat heart. After the heart was perfused with 0.22 mM fluoroacetate, the measured rate of glucose uptake and glycolysis decreased, and glucose 6-phosphate and fructose 6-phosphate accumulated. Examination of the citric acid cycle intermediates revealed that their concentrations were below normal, except for citrate, with a concentration 10 times higher than normal. 

Answer In galactokinase deficiency, galactose accumulates in UDP-glucose galactose 1-phosphate uridylyltransferase deficiency, galactose 1-phosphate accumulates (see Fig. 14-12). The latter metabolite is clearly more toxic. 

The aldolase that cleaves fructose phosphates is deficient. Fructose 1-phosphate accumulates and inhibits glucose production, causing severe hypoglycemia if fructose is ingested. 

Polyphosphate can replace ATP in the phosphorylation of glucose in numerous microorganisms [23,24]. Since the equilibrium of Reaction (1) is on the side of polyphosphate synthesis, this enzyme is not employed in polyphosphate degradation. Instead, a poly-P AMP phosphotransferase activity was found in cell-free extracts of the phosphate-accumulating bacterium Acinetobacter johnsonii which catalyzes Reaction (2)... 

In plant plastids, GGPP is formed from products of glycolysis and is eight enzymatic steps away from central glucose metabolism. The MEP pathway (reviewed in recent literature - ) operates in plastids in plants and is a preferred source (non-mevalonate) of phosphate-activated prenyl units (IPPs) for plastid iso-prenoid accumulation, such as the phytol tail of chlorophyll, the backbones of carotenoids, and the cores of monoterpenes such as menthol, hnalool, and iridoids, diterpenes such as taxadiene, and the side chains of bioactive prenylated terpenophe-nolics such as humulone, lupulone, and xanthohumol. The mevalonic pathway to IPP that operates in the cytoplasm is the source of the carbon chains in isoprenes such as the polyisoprene, rubber, and the sesquiterpenes such as caryophyllene. 

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