Phosphoryl-3-phosphoglyceric acid

Another enzyme of the carbon reduction cycle activates PGA, converting it to phosphoryl-3-phosphoglyceric acid (IV). This acid anhydride can then be reduced in a subsequent enzymatic step mediated by triose phosphate dehydrogenase. For its reducing agent, this enzyme uses nicotinamide adenine dinucleotide phosphate (NADPH) and thereby converts the carboxylic acid to... 

Phosphoenolpyruvate/pyruvate kinase. Phosphoenolpyruvate (PEP 2)/pyruvate kinase (PK E.C. 2.7.1.40) is the most efficient system for the regeneration of ATP from ADP. The phosphorylating agent PEP can be prepared in a mole scale [47L Starting from crude pyruvic acid, the crystalline monopotassium salt PEP-K is synthesized in a three-step procedure. For transformations on a scale <1 mol, PEP can be prepared from commercially available 3-phosphoglyceric acid in an enzyme-catalyzed reaction1641. This method is more expensive than the chemical preparation, but is more convenient because it requires less time and produces less organic waste (see Section 13.2.1.2 Fig. 13-5). 

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. 

COz-acceptor. The inability to isolate glycolaldehyde or indeed any 2-carbon fragments as such in the experiments on cleavage of ribulose-5-phosphate, described in the previous section, suggests the formation of a 2-carbon enzyme complex. Indeed one may conceive that this complex at the oxidation-reduction level of glycolaldehyde is condensed with CO2 to form 3-phosphoglyceric acid after steps involving phosphorylation and reduction. 

A. Glycolysis 3-phosphoglyceraldehyde 1.3- diphosphoglyceric acid 1.3- diphosphoglyceric acid — 3-phosphoglyceric acid 2NADH+H+— 6 ATP 2 ATP (substrate chain phosphorylation)... 

At first two molecules of triose phosphate combine to form hexose diphosphate fructose diphosphate), which is then hydrolyzed by a specific phosphatase to fructose 6-phosphate and phosphate. Hexose phosphate and triose phosphate interact in a transketolase reaction to produce erythrose 4-phosphate and xylulose 5-phosphate, which then rearranges to ribulose 5-phosphate (the first pentose molecule). In a type of aldol condensation, erythrose phosphate and triose phosphate combine to form sedoheptulose diphosphate (C4 - - C3 = C7), which is subsequently dephos-phorylated. The enzyme transketolase then transfers a C2 fragment from heptulose to triose phosphate yielding 2 moles of pentose xylulose 5-phosphate and ribose 5-phosphate) both must be rearranged to ribulose 5-phosphate. Having ribulose 5-phosphate available, the cycle can commence again first, phosphorylation with ATP to the diphosphate, then acceptance of CO2 by the diphosphate, and production of 2 moles of phosphoglyceric acid. 

Like all anhydrides (Section 21.5), the mixed carboxylic-phosphoric anhydride is a reactive substrate in nucleophilic acyl (or phosphoryl) substitution reactions. Reaction of 1,3-bisphosphoglycerate with ADR occurs in step 7 by substitution on phosphorus, resulting in transfer of a phosphate group to ADP and giving ATP plus 3-phosphoglycerate. The process is catalyzed by phospho-gjvcerate kinase and requires Mg2+ as cofactor. Together, steps 6 and 7 accomplish the oxidation of an aldehyde to a carboxylic acid. 

Figure 15-6 Generation of ATP coupled to oxidation of an aldehyde to a carboxylic acid. The most important known example of this sequence is the oxidation of glyceraldehyde 3-phosphate to 3-phosphoglycerate (Fig. 10-3, steps a and b). Other important sequences for "substrate-level" phosphorylation are shown in Eq. 14-23 and in Fig. 15-16.

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 ... 

The mixed anhydride of phosphoric acid and glyceric acid then is used to convert ADP to ATP and form 3-phosphoglycerate. Thereafter the sequence differs from that in photosynthesis. The next few steps accomplish the formation of pyruvate by transfer of the phosphoryi group from C3 to C2 followed by dehydration to phosphoenolpyruvate. Phosphoenolpyruvate is an effective phosphorylating agent that converts ADP to ATP and forms pyruvate ... 

The Conversion of Triose Phosphates to Phosphoglycerates Occurs in Two Steps The Three-Carbon Phosphorylated Acids Constitute a Third Metabolic Pool... 

The same active site of ribulose bisphosphate carboxylase also catalyzes a competing reaction in which 02 replaces C02 as a substrate. The products of this oxygenase reaction are 3-phosphoglycerate and a two-carbon acid, 2-phosphoglycolate (fig. 15.27). Phosphoglycolate is oxidized to C02 by 02 in additional reactions involving enzymes in the cytosol, mitochondria, and another organelle, the peroxisome. This photorespiration is not coupled to oxidative phosphorylation, and it appears to constitute a severe... 

---