Glucose-6-phosphate synthesis

Figure 5.5 The coupled reaction in which ATP supplies the phosphoryl group for glucose-6-phosphate synthesis in contrast, phosphoenolpyruvate has a phosphoryl-transfer potential sufficiently elevated to enable it to donate its phosphoryl group to ADP, generating ATP.

PHOSPHOGLUCOSE ISOMERASE Glucose-6-phosphate isomerase, PHOSPHOGLUCOISOMERASE D-Glucose 6-phosphate synthesis, 6-PHOSPHO-/3-GLUCOSIDASE GLUCOSE-1-PHOSPHATE URIDYLYLTRANS-FERASE... 

FIGURE 20-27 Regulation of sucrose phosphate synthase by phosphorylation. A protein kinase (SPS kinase) specific for sucrose phosphate synthase (SPS) phosphorylates a Ser residue in SPS, inactivating it a specific phosphatase (SPS phosphatase) reverses this inhibition. The kinase is inhibited allosterically by glucose 6-phosphate, which also activates SPS allosterically. The phosphatase is inhibited by Pi, which also inhibits SPS directly. Thus when the concentration of glucose 6-phosphate is high as a result of active photosynthesis, SPS is activated and produces sucrose phosphate. A high P, concentration, which occurs when photosynthetic conversion of ADP to ATP is slow, inhibits sucrose phosphate synthesis. 

The energy stored in the a(l 4) glycosidic bond during the condensation reaction in glycogen synthesis is sufficient to permit the formation of a glucose-phosphate bond without using ATP. 

Occurrence and preparation. The tetrose (4) participates as D-erythrose 4-phosphate in the pentose phosphate pathway of carbohydrate metabolism.11 D-Erythrose can be prepared using a method of degradation from the pentose, D-arabinose,19 or from D-glucose.20 Synthesis of the 4-phosphate has been reported.21... 

UDP-glucose transfers glucose to dolichol phosphate to form dolichol phospho-glucose in synthesis of N-linked glycoproteins. 

Blood this step because, under conditions that favor gluconeogenesis, the concentrations of the compounds that activate the glycolytic enzyme PFK-1 are low. These same compounds, fructose 2,6-bisphosphate (whose levels are regulated by insulin and glucagon) and AMP, are allosteric inhibitors of fructose 1,6-bisphosphatase. When the concentrations of these allosteric effectors are low, PFK-1 is less active, fructose 1,6-bisphosphatase is more active, and the net flow of carbon is toward fructose 6-phosphate and, thus, toward glucose. The synthesis of fructose 1,6-bisphosphatase is also induced during fasting. 

T3 has an amplifying or facilitatory effect on the lipolytic action of epinephrine on the fat cell. Yet thyroid hormone has a bipolar effect on lipid storage, because it increases the availability of glucose to the fat cell, which serves as a precursor for fatty acid and glycerol 3-phosphate synthesis. The major determinant of the rate of lipogenesis, however, is not T3, but rather the amount of glucose and insulin available to the adipocyte for triacylglycerol synthesis. 

FIGURE 19.2 2-Deoxyribose 5-phosphate synthesis from glucose and acetaldehyde hy baker s yeast and a deoxyriboaldolase-expressing E. coli. ADP, adenosine diphosphate. 

Horinouchi, N., 1. Ogawa, T. Sakai, T. Kawano, S. Matsumoto, M. Sasaki, Y. Mikami, and S. Shimizu. 2003. Construction of deoxyriboaldolase-overexpressing Escherichia coli and its application to 2-deoxyribose 5-phosphate synthesis from glucose and acetaldehyde for 2 -deoxyribonucleoside production. Appl. Environ. Microbiol. 69 3791-3797. 

The glycerol from fat breakdown feeds into the glucose breakdown/synthesis pathways at the level of glyceraldehyde 3-phosphate. It is important, however, to note that the fatty acids themselves cannot yield glucose. They form acetyl CoA, but in humans there is no route back from acetyl CoA to pyruvate. The conversion of pyruvate into acetyl CoA is effectively a one-way valve. Thus, it is easy to convert excess carbohydrate into fat but not the other way round. 

For the synthesis of vanillin itself, there follows, in a separate step, a further enzymatic reduction ofthe carboxylic function. To recover the NADP+, the reaction product is stirred for 7 hours at 30 °C together with glucose in the presence of the arylaldehyde-dehydrogenase from Neurospora crassa and glucose phosphate dehydrogenase. 

This is an enzyme of glycogen synthesis which can be estimated by u.v. spectroscopy using phosphoglucomutase in an auxiliary reaction and glucose -phosphate dehydrogenase in an indicator reaction [644]. 

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