Of hexose phosphate

Fig. 3.—The Interconversion of Hexose Phosphates by Yeast and Liver Enzymes.

The above transketolase and transaldolase reactions were found inadequate to explain the metabolism of D-ribose 5-phosphate, because of the non-accumulation of tetrose phosphate, the 75 % yield of hexose phosphate, and the results of experiments with C14 (the distribution of which differed markedly from the values predicted for such a sequence). 24(b) Thus, with D-ribose-l-C14, using rat-liver enzymes, any hexose formed should have equal radioactivity at Cl and C3, whereas, actually, 74% appeared at Cl. Furthermore, D-ribose-2,3-Cl42 should have given material having equal labels at C2 and C4 in the resultant hexose, whereas, in fact, it had 50% of the activity at C4, C3 was nearly as active as C2, and Cl had little activity. Similar results were obtained with pea-leaf and -root preparations.24 The following reactions, for which there is enzymic evidence,170(b) were proposed, in addition to those involving D-aftro-heptulose, to account for these results.24(b) (o) 200... 

The reader can easily show that the same enzymes will catalyze the net conversion of hexose phosphate to erythrose 4-phosphate or to sedoheptulose 7-phos-phate (Eq. 17-17) ... 

The final energy payoff in the glycolytic pathway occurs in the hydrolysis of phosphoenolpyruvate to pyruvate and the concomitant phosphorylation of ADP to ATP. Two molecules of ATP are produced for each molecule of hexose phosphate consumed, bringing the net yield of ATP to two molecules for each molecule of glucose (two molecules of ATP are regenerated in the phosphoglycerate kinase step and two in this step, and two are consumed in the hexoki-nase and phosphofructokinase steps). 

Because of its roles in the synthesis of glycogen, in isomerization of hexose phosphates, and as a precursor for numerous biosynthetic intermediates, UDP-glucose is regarded as a central hexose derivative in mammalian metabolism. In bacteria and plants, both ADP-glucose (production of storage polysaccharide) and UDP-glucose (sugar interconversions and biosynthesis) play important roles as precursors. 

Because of the possible alternative pathways, which probably occur simultaneously, no single set of reactions can uniquely describe the pentose phosphate pathway. One possible set of pathways is shown in figure 12.34. If the triose phosphate formed is converted to hexose phosphate, the overall pathway can be seen as regenerating five molecules of hexose phosphate for each six used initially. 

Bublitz C, Lawler CA. The levels of nicotinamide nucleotides in liver microsomes and ttieir possible significance to the function of hexose phosphate dehydrogenase. Biochem. J. 1987 245 263-267... 

Williams and Loewus (7) prepared l-[4- C]ascorbic acid by the method of Bakke and Theander (8) and showed that this form of specifically labeled ascorbic acid, like L-[l- C]ascorbic acid, was an effective precursor of tartaric acid in grape berries and grape leaves (Table I) (9). Over 98% of the was located in the carboxyl groups of labeled tartaric acid from l-[1- C]- or L-[4- C]ascorbic acid labeled leaves or berries. Only L-(-f)-tartaric acid was formed (10). The C2 fragment of this cleavage, as judged by studies with l-[6- C] ascorbic acid, was recycled into products of hexose phosphate metabolism (5,6,11,12). 

There is a cascade in which the cAMP-dependent protein kinase catalyzes a phosphorylation ofphosphorylase kinase. This activates phosphorylase kinase, which catalyzes the phosphorylation of phosphorylase from the inactive to the active form, which can then rapidly catalyze the mobilization of glycogen. The advantage of the cascade system is a multiplicative effect. That is, each cAMP-dependent protein kinase can catalyze the phosphorylation of a large number of phosphorylase kinases, which in turn can catalyze the phosphorylation of a large number of inactive phosphorylase molecules (i.e., phosphorylase b) to the active phosphorylated form (i.e., phosphorylase a), and each of these active phosphorylases can accelerate production of hexose phosphates from glycogen. This amplifies the stimulatory effect of cAMP and causes a more rapid mobilization of glycogen than would result from catalysis of a direct phosphorylation of phosphorylase by the cAMP-dependent protein kinase. 

The increase in cAMP is a hormonal message which has an effect in two major target organs, liver and muscle, but different hormones affect increased cAMP in each. In the liver, the hormone is glucagon, whereas in the muscle, the hormone is epinephrine. In the liver, glycogen breakdown (i.e., glycogenolysis) can serve as a source of hexose phosphate, a precursor of blood glucose in the... 

The pentose-phosphate pathway. The early steps of the pentose-phosphate pathway are shown in detail as for a pentose-phosphate formation. This pathway produces two NADPHs and ribose-5-phosphate. The reformation of hexose phosphate involves C3,... 

When a hexose phosphate is hydrolyzed to free hexose and inorganic phosphate, the ratio of the concentration of hexose to the concentration of hexose phosphate at equilibrium is 99 to 1. What is the free-energy change for the reaction under standard conditions ... 

Like all juggling acts, this leaves one a bit dizzy, but if we look back carefully at what has happened it is really very crafty Overall we have taken three molecules of hexose phosphate. Initially, we have converted these into three molecules of pentose phosphate plus three molecules of CO2. In the process, we have achieved our initial objective by reducing six molecules of NADP+ to NADPH. Then we have used two enzymes to reshuffle the atoms of the three pentose molecules to give us back, in effect, 2j hexose phosphate molecules, back on the mainline glycolytic pathway (as we have seen in Topic 26, triose phosphates are readily reconvertible to hexose phosphates). 

Integration of Pathways of Synthesis and Degradation of Hexose Phosphates r. ap Bees... 

If the formation of hexose phosphates occurred via a reversal of the glycolytic pathway from PGA, both carbon atoms 3 and 4 of the hexose should be formed from the carboxyl of PGA. Carbon atoms 2 and 5 would come from the a carbons of PGA, while carbon atoms 1 and 6 would derive from the carbons of PGA. When Calvin et al. (1951) degraded hexose molecules in such a way as to obtain these pairs of carbon atoms, they found the distribution... 

Clearly it is incorrect to consider the nonoxidative phase of the P.p.c. as a fixed, albeit fairly complex mechanism for the conversion of 3 molecules of pentose phosphate into a triose phosphate and 2 molecules of hexose phosphate. There exists a network of possible reactions, which can change in emphasis, depending on the tissue, and possibly on the physiological state of the tissue. Thus, in rat epididymal fat pad, isotopic labeling suggests that the older scheme of Horecker operates for the metabolism of pentose phosphates. Operation of the new scheme in plants would provide an explanation of the Gibbs effect (see). Other workers [T.Wood A. Gascon Archives of Biochemistry and Biophysics 2Q3 (1980) 727-733] have reported their failure to demonstrate the interconversion of D-arabinose S-phosphate and D-ribose S-phosphate, or the role of D-arabinose S-phosphate as an acceptor for transketolase in baker s yeast, Candida ulilis, or rat liver. 

Bacterial cell wall lipopolysaccharides of Salmonella are some of the most interesting examples of the coordinated use of sugar nucleotides involving both different sugars and different nucleotide bases. To a backbone of hexose, phosphate, and ketodeoxyoctanoate, are added in a sequential manner the sugars galactose, abequose, rhamnose, mannose, and A-acetyl-... 

Constit. of resting muscle and of the crude mixt. of hexose phosphates obt. by yeast fermentation. 

A number of hexose phosphates have been isolated from natural sources. Of these oTructose 1,6-diphosphate (Harden-Young ester) (I), D-fructose 6-phosphate (Neuberg ester) (II), D-glucose 6-phosphate (Robison ester) (III), and a-D-glucose 1-phosphate (IV) are of particular biochemical importance in the processes of fermentation and glycolysis. These hexose phosphates are interconvertible in the presence of the proper enzymes and cofactors. 

H-N.m.r. spectroscopic studies on the phosphoglucoisomerase-catalyzed interconversion of hexose phosphates are covered in Chapter 21. 

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