Hexose diphosphates

Hexose diphosphate was found by Harden and Young69 in cell-free alcoholic-fermentation liquors. In 1930, it was observed that addition of fluoride to fermenting-yeast extracts leads to an accumulation of 0-phospho-D-glyceronic acid,60 which is also a metabolite of muscle extracts.61 Attention was turned, therefore, to the pathway from hexose diphosphate to 0-phos-pho-D-glyceronic acid. In 1932, Fischer and Baer62 described the synthesis of D-glycerose 3-phosphate, and, in 1933, Smythe and Gerischer63 noted... 

D-afe-o-Heptulose (sedoheptulose) (XXXVII) has been synthesized from D-erythrose (XXXVIII) plus triose phosphate, using an aldolase preparation from peas.169 Aldolases from yeast and from rat liver also form heptu-lose phosphate from these substrates.7S(o) 170(a) Crystalline muscle aldolase causes the formation of L-jrZwco-heptulose (XXXVIIa) from a mixture of L-erythrose (XXXVTIIa) and hexose diphosphate.170(b)... 

The natural substrate for the dehydrogenase, glyceraldehyde-3-phosphate (G-3-P), had been synthesized earlier by Hermann Fischer, Emil Fischer s son, and Baer in 1932. In 1934 Meyerhof and Lohmann synthesized hexose diphosphate, establishing it to be fructose 1,6 bisphosphate (F-l, 6 bis P). With F-1,6 bisP as substrate and hydrazine to trap the aldehydic and ketonic products of the reaction, G-3-P was identified in the mixture of G-3-P and dihydroxyacetone phosphate which resulted. Triose phosphate isomerase was then isolated and the importance of phosphorylated 3C derivatives established. 

Glycogen — hexose — hexose monophosphate —> hexose diphosphate —> glycerose phosphate + dihyroxyacetone phosphate —> glyceric acid phosphate + glycerol phosphate. Glyceric acid... 

The long-known stimulating effect of mono- and polynitro com-pounds on the onset of fermentation in yeast maceration juice has been reinvestigated by Vandendriessche. The induction time is shortened significantly by 2,4- or 2,5-dinitrophenol, while 2,6-dinitro-phenol did not show such an effect. The influence is evident when using as substrates the fermentable hexoses and D-fructose-6-phosphate, but not hexose diphosphate. According to MarkoviCev a stimulation of the oxidation processes can be proved thereby. It is probable that these effects are related to the known phytochemical reduction of nitro compounds (see pp. 98 and 99). 

The arsenate considerably accelerates the fermentation of hexose diphosphate,10 and inhibits the combination of sucrose with phosphoric acid (phosphorylation).11... 

In further experiments (Jam et ci., 1944) it was shown that, with barley saps to which hexose diphoqihate and ascorbic acid were added, an increased oxygen uptake occurred in excess of that caused by the addition of ascorbic acid alone. This oxygen uptake could be still further increased by the addition of coenzyme I. The breakdown of hexose diphosphate to phosphoglyceric acid which was observed was stimulated by the addition of ascorbic acid. The course of hydrogen transport was therefore believed to be triose phosphate —> coenzyme I ascorbic acid — Oj. 

Whether coenzyme I was concerned in the earlier experiments was not determined and has not mnce been determined. The evidence from these studies was suggestive of the participation of ascorbic acid as a respiratory carrier. It was, however, only suggestive and not conclusive, for the authors did not demonstrate either with their lactate dehydrogenase or hexose diphosphate systems the direct reduction of dehydroascorbic acid to ascorbic acid. 

This reaction was recognized as a potential intracellular source of deoxyribose 1-phosphate because the mutase-catalyzed interconversion of 1-and 5-phosphate esters of deoxyribose was known. Boxer and Shonk (25) showed that extracts of rat liver formed deoxyribose phosphate when incubated with hexose diphosphate and threonine it was evident that the threonine aldolase activity of the extracts provided acetaldehyde for the in vitro synthesis of deoxyribose phosphate and it was postulated that this might also occur in vivo ... 

Fructose-6-phosphate is further phosphorylated to yield fructose-1,6-diphosphate, and then hexose diphosphate is split to yield two triose phosphate sugars—dihydroxyacetone phosphate and D-phos-phoglyceraldehyde. The phosphofructokinase and aldolase reactions are discussed when fructose metabolism is reviewed. We are concerned here only with the fate of the triose phosphates (see Fig. 1-6). 

Aldolase (see Fig. 1-7) is the enzyme that catalyzes the splitting of hexose diphosphate to yield triose phosphates [49-51]. This reaction is reversible, and, in fact, the equilibrium favors hexose diphosphate formation. Thus, aldolase condenses dihydroxyacetone phosphate and D-glyceraldehyde phosphate in a typical aldol condensation this type of reaction explains the origin of... 

Phosphofructokinase. Fructose-6-phosphate is phosphorylated on position 1 by the action of phosphofructokinase (X), which can use ATP, UTP, or ITP. As with other kinases, Mg++ is an essential cofactor. The enzyme contains essential SH groups. This is the second essentially irreversible reaction of glycolysis, since the equilibrium lies far to the side of hexose diphosphate formation. 

The further metabolism of hexose diphosphate (HDP) was elucidated by Meyerhof and his collaborators. Initially they found that crude enzyme preparations converted HDP to dihydroxyacetone phosphate, and named the enzyme system involved zymohexase. This term is no... 

Sedoheptulose Diphosphate. The transaldolase reaction requires phos-phoglyceraldehyde, which has become available as a sjmthetic compound only recently. A convenient method for supplying those phosphate is to add fructose diphosphate and aldolase. When this device was used in a study of the transaldolase reaction, the reaction products of the system containing both aldolase and transaldolase, and hexose diphosphate and sedoheptulose phosphate were expected to include tetrose phosphate, as shown in equation (VI) above. Tetrose phosphate failed to accumulate, however. Instead, sedoheptulose-1,7-diphosphate was found to accumulate as a result of condensation of the tetrose ester with dihydroxy-acetone phosphate in the presence of aldolase. This diphosphate reacts rapidly with aldolase, and it is not known whether it can react in any other systems, or only shuttles back and forth in response to changes in tetrose phosphate level. 

Efficient utilization of the fructose requires phosphorylation of the glyceraldehyde. Tracer experiments show that the carbon 1 of fructose appears as both carbons 1 and 6 of glucose. This is the result of triose phosphate isomerization followed by (conventional) aldolase condensation to hexose diphosphate. The conversion of fructose diphosphate to glucose-6-phosphate requires a phosphatase and an isomerase, as discussed in the pentose phosphate pathway. 

Evidence from a number of sources indicated that pentose phosphates were metabolized in a series of reactions that resulted in the formation of hexose monophosphates and hexose diphosphates. Several enzyme steps are involved in these transformations. The reaction between D-ribulose 5-phosphate and D-ribose 5-phosphate to form D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate is catalyzed by an enzyme known as transketolase (91). This enzyme is found in plant, animal, and bacterial cells. Thiamine pyrophosphate (TPP) and Mg ions are required as cofactors. The mechanism of the reaction was suggested (92) as shown in reaction (28). 

Enzymatic analysis of the E. coli methionine-synthesizing system has progressed. E. coli 121-176, which requires methionine or Bi2, yielded an extract whose synthesis of methionine from homocysteine plus serine was increased six- to eightfold by cyanocobalamin (Helleiner et al., 1958). The compound formed by mixing equimolar HCHO and FH4 acted directly as donor of the Ci unit to homocysteine. The system requires (Kisliuk and Woods, 1958a) DPN, ATP, Mg++, hexose diphosphate, and inorganic P. 

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