Triose phosphate formation from pentose

Several investigators had observed the formation of hexose phosphates during nucleoside and pentose metabolism by cell extracts. It was considered until recently that these hexose phosphates arose from a reversal of the Embden-Meyerhof scheme operative on triose phosphate derived from pentose. It has now been found by Dische that more hexose may be generated from adenosine in hemolysates than would be expected from the proportion of 3-carbon fragments in the pentose molecule, i.e., 0.75 mole hexose per mole pentose instead of a maximal 0.6. Furthermore, much of the carbon of the pentose was in hexose monophosphate produced under conditions in which hexose-6-phosphate and hexose-1,6-diphosphate were not interconvertible. 

Photosynthesis. The formation of carbohydrates in green plants by the process of photosynthesis is described in ihc entry on Photosynthesis. The synthetic mechanism involves the addition of carbon dioxide to ribulose-1,5-diphosphate and the subsequent formation of two molecules of 3-phosphoglyccric acid which are reduced to glyceraldehyde-3-phosphate. The triose phosphates are utilized to again from ribulose-5-phosphates by enzymes of the pentose phosphate cycle Phosphorylation or ribulose-5-phosphate with ATP regenerates ribulose-1.5-diphosphate to accept another molecule of carbon dioxide. See also Phosphorylation (Photosynthetlc). 

The action of an enzyme, transketolase, from rat liver or from spinach, on pentose 5-phosphate results in the formation of a triose phosphate and an ester of sedoheptulose, which is presumably sedoheptulose 7-phosphate. The heptulose has been identified by the preparation of sedoheptulosan tetrabenzoate (227). 

Aldolase catalyses the formation of heptulose diphosphate from a molecule of tetrose phosphate and a molecule of triose phosphate (C + C3 = C7). Phosphatase converts the heptulose-PP to heptulose-P which, with a further molecule of triose-P, in the presence of transketolase, forms a molecule of ribose-P and a molecule of ribulose-P (Q + Cj = 2Cj). The isomerase for pentose phosphates converts this mixture to ribulose-P which, in the presence of ATP and phosphopentokinase, gives ribulose-PP. 

From my laboratory and the laboratories of Ef Racker, Frank Dickens and Melvin Calvin, the years that followed witnessed a series of parallel and often highly synergistic discoveries on the nature of the pentose phosphate pathway and the path of carbon in photosynthesis. Andrew Benson and others in Calvin s laboratory, had shown that phosphate esters of ribulose and sedoheptulose were early products of CO2 fixation in photosynthesis,and the immediate precursor of phosphoglyceric acid, and therefore the primary CO2 acceptor, appeared to be ribulose diphosphate. The major problems became (1) to find the enzyme or enzymes that catalyzed the formation of phosphoglyceric acid from ribulose diphosphate and (2) to define the reactions leading to the synthesis of ribulose diphosphate from triose and hexose phosphates. 

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