Phosphorylation of 3-phosphoglycerate

The rest of the Calvin cycle is involved in interconversion of carbohydrates to make glucose (or starch) and the regeneration of the ribulose-bisphosphate acceptor. The reactions are also found in the pathways for gluconeogenesis and the pentose phosphate shunt (see Volume 1, Chapters 10 and 12). The first step is the phosphorylation of 3-phosphoglycerate by the same reactions involved in gluconeogenesis. 

Direct Phosphorylation of the Substrate, There are several examples of this type of reaction. In the reversal of glycolysis pyruvate is phos-phorylated by ATP to give phosphoenol pyruvate, and the phosphorylation of 3-phosphoglyceric acid occurs prior to its reduction to the aldehyde. 

The remaining steps in the glycolytic pathway prepare for synthesis of the second ATP equivalent. This begins with the phosphoglycerate mutase reaction (Eigure 19.23), in which the phosphoryl group of 3-phosphoglycerate is moved... 

In subsequent steps, ATP is utilized to phosphorylate the carboxylate group of 3-phosphoglycerate to create 1,3-diphosphoglycerate (a mixed anhydride of glyceric and phosphoric acids). This substance then is reduced by NADPH to glyceraldehyde 3-phosphate ... 

Glycine is then transported to the mitochondrial matrix where the conversion of two glycines to one serine occurs with the loss of CO2 and NH3 from the pool of fixed molecules. The serine is transported into the peroxisome, where it is deaminated to glycerate. The glycer-ate is transported back to the chloroplast, where it is phosphorylated to 3-phosphoglycerate for the Calvin-Benson cycle. 

What is the energy expenditure for synthesizing a hexose Six rounds of the Calvin cycle are required, because one carbon atom is reduced in each round. Twelve molecules of ATP are expended in phosphorylating 12 molecules of 3-phosphoglycerate to 1,3-bisphosphoglycerate, and 12 molecules... 

Phosphoglycerate is isomerized to produce 2-phosphoglycerate in a reaction catalyzed by the enzyme phosphoglycerate mutase. The phosphoryl group attached to the third carbon of 3-phosphoglycerate is transferred to the second carbon. 

Scheme 11.2. The phosphorylation of the carboxylate group of 3-phosphoglycerate to produce a mixed carboxylate-phosphate anhydride, 1,3-bisphosphoglycerate. The phosphoglycerate kinase-catalyzed (EC 2.7.2.3) formation of the anhydride involves the removal of a phosphate group from adenosine triphosphate (ATP), yielding adenosine diphosphate (ADP) and the bisphosphoglycerate. The stereochemistry of the attack at phosphorus is backside to the leaving group.

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