Hexulose 6-phosphate

Similar to DHAP aldolases, the 3-hexulose 6-phosphate aldolase found in Methylomonas Ml 5 is highly specific for the aldol donor component D-ribulose 5-phosphate, but accepts a wide variety of aldehydes as replacement for formaldehyde as the acceptor. With propanal,... 

Like the DHAP aldolases, the class II 3-hexulose 6-phosphate aldolase from a unique formaldehyde-fixing system of the methylotrophic bacterium Methyl-omonas Ml5 utilizes a ketose phosphate, i.e. D-ribulose 5-phosphate (128), as the aldol donor component for which it has a stringent requirement [374], On the... 

For preparative applications, the expensive and configurationally unstable donor 128 can be simply prepared in situ by the action of ribose 5-phosphate isomerase (EC 5.3.1.6) on D-ribose 5-phosphate (39). This technique was applied to the stereoselective synthesis of d-[1-13C] fructose 6-phosphate 38 from [13C] formaldehyde [376,377] which also included a second enzymatic isomerization of the D-arafrino-3-hexulose 6-phosphate intermediate 129 into the more stable 2-hexulose derivative 38. Notable are the conflicting demands for high substrate levels (necessary to shift the fully reversible multi-component equilibrium) versus the notorious enzyme inactivation that occurs at higher formaldehyde concentrations. 

Rabbit muscle aldolase (RAMA) furnished ketose l ho hates with 3(S)/4(i ) stereochemistry from dihydioxyacetone phosphate (DHAP) and simple aldehydes. When a-hydroxyaldehydes were used kinetic resolution occurred, provided there was a negative charge (e.g. CCX) ) 4 or 5 atoms away from the aldehyde group. Fructose 1,6-diphosphate aldolase from spinach leaves catalysed similar condensations between DHAP and aldehydes with a substrate specifity somewhat different from that of RAMA. A reinvestigation of the stmc course of the hexulose phosphate synthetase-catalysed condensation between ribulose S-phosphate and simple aldehydes is covered in Chapter 2. 

Meyerhof and Schulz86 studied this reaction in trisodium phosphate solution, and regarded it as coming to a triose-hexose equilibrium containing 92 % of hexose. Berl and Feazel67 examined the kinetics of hexose formation from trioses in alkaline solution, and noted that 75-90% of hexulose is formed from DL-glycerose alone, but that the yield is lower (about 60%) when dihydroxyacetone is added in equivalent quantity. Paper chromatog-... 

These conclusions are borne out by the data on some phosphorylated ketoses83 that cannot form pyranoses or furanoses. 5,6-Dideoxy-D-fhreo-hexulose 1-phosphate (1) contains 96% (by i.r.) or 91% (by n.m.r.) of the keto form, and D-en/fhro-pentulose 1,5-bisphosphate (2) contains 84% (by i.r.) or 88% (by n.m.r. spectroscopy80 ) in aqueous solution in the hydrates, there would be 1,3-parallel interactions. There are no such interactions in the hydrate of l,5-dihydroxy-2-pentanone 1,5-bisphosphate (3), but here one of the carbon atoms vicinal to the carbonyl group carries no hydroxyl group, and there is 84% of the keto form in equilibrium. l,3-Dihydroxy-2-propanone phosphate has two neighboring hydroxyl groups and no 1,3-parallel interactions only 55% is in the keto form and 45% is present as the hydrate.83,84... 

Reactions with D-fructose 1,6-bisphosphate Propanal 5,6-dideoxy-D-fAreo-hexulose 1-phosphate 1100 73 Ac 1.4 TW1 1.8 32... 

Another feature common to all hexoses is their fragmentation to trioses and related products (such as lactic acid, l-hydroxy-2-propanone, and pyruvaldehyde). l-Hydroxy-2-propanone is formed by heating a solution of D-fructose in potassium acid phosphate buffer solution of pH 6.7 it was proposed that an intermediary 3,4-enediol (s) rearranges to a /3-diketone, which yields (t) by fragmentation of the molecule. Studies by Wolfrom and Schumacher and by Blair and Sowden showed that this mechanism, followed by recombination and aldolization, accounts for some of the observed products. The identification of L-a 2/io-hexulose and DL-a yZo-hexulose was conclusive in this respect, but deuterium studies by Sowden and Thompson proved the minor role of this recombination mechanism in the transformations. 

Occurrence. D-Tagatose (D-Zyxo-hexulose, 32) has been found only as a hydrolytic product from a gum exudate of the tropical tree Sterculia setigera.154 A biological role of its 1-phosphate and 1,6-bisphosphate has been described in biotransformations of D-galactose in several microorganisms.155... 

Deoxy-lyxo -hexose D-form, D-204 4-Deoxy-xy/o-hexose D-form, D-210 4-Deoxy-/ reo-2-hexulose D-form 6-Phosphate, D-223 4-Deoxy-// rco-2-hexulose D-form, D-223 4-Deoxy-// reo-pentose L-Pyranose-ybmi, D-350 4-Deoxy-c/7i/iro-pentose DL-form, D-347 4-Deoxy-c/7i/iro-pentose D-form, D-347 4-Deoxy-crji/iro-pentose L-form, D-347 4-Deoxysucrose, D-365... 

Deoxy-D-ara6Bio-hexose-6-phosphate, D-199 6-Deoxy-aro6Bio-2-hexulose D-form 1-Phosphate, D-218... 

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