3- Phosphoglyceric acid, PGA

Recently, we have developed a method for preparing PEP in situ from commercially available 3-phosphoglyceric acid (PGA), using phosphoglyceromutase and enolase (Scheme 9) (28). 

Fig. 34. Experiment to determine which reaction is indirectly light-dependent in the secondary processes of photosynthesis. When it becomes dark the amount of 3-phosphoglyceric acid (PGA) increases, in contrast to that of the other components (modified from Baron 1967).

Abbreviations RuBP, Ribulose 1,5-bephosphate 3-PGA, 3-phosphoglyceric acid PEP, phosphoenolpyruvate OAA, oxaloacetic acid AcCoA, acetyl coenzyme A Glu, glutamic acid Gin, glutamine ATP, adenosine triphosphate, Cys, cysteine. 

For the moment, we will focus on photoautotrophs that utilize rubisco. Rubisco is the official name of an enzyme for which the systematic name—ribulose-l,5-bisphosphate carboxylase oxygenase—is inconveniently long. As indicated by the first activity specified in the systematic name, this enzyme catalyzes the carboxylation of ribulose-1,5-bisphosphate, RuBP, a five-carbon molecule. A six-carbon product is formed as a transient intermediate, but the first stable products are two molecules of PGA, 3-phosphoglyceric acid, C3H7O7P. The carbon number of this compound gives the process its shorthand name, C3 photosynthesis. At physiological pH, the acidic functional groups on the reactants and products are ionized as shown below. 

PGA represents 3-phosphoglyceric acid. Six moles of NADPH + are required to provide the 12 electrons required to reduce three moles of carbon initially in the form of CO2 to the level of carbohydrate. Energy provided by the hydrolysis of ATP (not shown) is required to drive the overall process. 

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... 

Fig. 4. Radioautogram of products of 60 seconds photosynthesis with C 0-. Radioautograph of two-dimensional paper chromatogram of products formed by Chlorella pyrenoidosa during 60 seconds of photosynthesis with C 0 . Abbreviations P, POaH" UDPG, uridine diphosphoglucose PGA, 3-phosphoglyceric acid PEPA, phosphoenolpyruvic acid. Sugar diphosphate includes ribulose-l,5-diphosphate, sedoheptulose-1,7-diphosphate, and fructose-1,6-diphosphate.

The 6-C intermediate in Eq. 39.11 is the highly unstable 3-keto-2-carboxyarabinitol-l,5-bisphosphate that fast converts into two phosphoglyceric acid (PGA acid) units. Each 3-PGA is then reduced to S-PGAy g (phosphoglyceraldehyde), which is finally converted into a C-6. This type of complex process is reported in a very concise form in many textbooks as shown in Eq. 39.12 ... 

Calvin and his school believe that ribulose diphosphate is the immediate precursor of 3-phosphoglyceric add. Thus the photosynthetic cycle would begin by a carboxylation of Ru-PP with formation Cj + C = 2Ca) of two molecules of phosphoglyceric acid (PGA). Starting with PGA, all... 

Fig. 35. Experiment to ascertain the path of carbon in the secondary processes of photosynthesis. C 02 was supplied to algal suspensions. The algae were then killed at specified times and extracts from them were separated in two-dimensional paper chromatography. PGA = 3-phosphoglyceric acid, triose = 3-phosphoglycerinaldehyde + dihydroxyacetone phosphate. The blackened spots on the copies of the chromatograms shown indicate that is present in the respective compounds (modified after Baron 1967).

Fig. 36. The Calvin cycle (black lines) and pentose phosphate cycle (red lines). PGA = 3-Phosphoglyceric acid, PGAL = 3-phosphoglyceraldehyde, Rib. = ribose-5-phosphate, Xyl = xylulose-5-phosphate, Ru-diP = ribulose-1,5-diphosphate, C4 = erythrose-4-phosphate, FDP = fructose-1,6-diphosphate. A few of the enzymes participating are encoded, 1 = carboxydismutase, 2 = triose phosphate dehydrogenase, 3 = triose phosphate isomerase, 4 = aldolase, 5 = phosphatase, 6 = phosphoglucoisomerase. Details of the conversion of glucose-6-P into ribulose-5-P are given in Fig. 43. It should be pointed out that the pentose phosphate cycle presents only here and there a true reversal of the Calvin cycle. In many instances the mechanisms and enzymes are different.

Another important inhibitor of PFK activity is 3-phosphoglyceric acid (3-PGA). As the citrate concentration reflects the availability of fuels in the tricarboxylic acid cycle, so does the 3-PGA concentration reflect the level of metabolites at the terminal sequence of the glycolytic pathway. In this fashion, additional information is available to the cell to modulate the rate of glycolysis in accordance with its ever-changing needs. 

Most CO2 assimilation takes place at a- and 8-carbons of carbonyl groups of organic molecules to give a-hydroxy-, a-keto, and )8-keto acids 90, 91). There has been no report of a-hydroxyketo acid formation by artificial CO2 fixation with the intention to mimic CO2 assimilation in green plants, in which two moles of 3-phosphoglycerate (PGA) are produced hy CO2 fixation to ribulose-l,5-bisphosphate(RuDP) ... 

Abbreviations AAN, aminoacetonitrile PGA, 3-phosphoglycerate TCA, tricloracetic acid TP, triosphosphate... 

Fig. 8-6. Hypothetical site of action of thyroid hormones. GAP glyceraldehyde-3-phosphate PGA phosphoglyceric acid DHAP dihydroxy-acetone phosphate a-GP04. a-glycerophosphate...

The carbon flow from 3-phosphoglycerate, phosphoenolpyruvate, pyruvate and acetyl-CoA. Even if the synthesis of aromatic amino acids by shikimate pathway /28,29,30,31/ and also prenyl-PP synthesis via mevalonate /32,33,34/ has been established in chloroplasts by identification of respective plastidic enzymes, it is still a matter of discussion from where PEP origins to supply DAHP synthesis of the shikimate pathway and from where pyruvate is delivered to supply the plastidic pyruvate dehydrogenase complex (for isolation see Treede and Heise, this Conference). Because phosphoglycerate mutase (PGM) to form 2-PGA from 3-PGA could not be detected in chloroplasts /35/ and acetyl-CoA is preferably synthesized from added acetate by the actetyl-CoA synthetase /36/, particularly in spinach chloroplasts, it was argued that chloroplasts are dependent on import of these substrates from the external site. Evidence for PEP formation from 3-PGA within the chloroplast could be obtained by three different approaches (D. Schulze-Siebert, A. Heintze and G. Schultz, in preparation D. Schulze-Siebert and G. Schultz, in preparation, for plastidic isoenzyme of PGM in Ricinus see /37/ and in Brassica /38/). 

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