3-Phosphoglyceric acid intermediate

D. Both serine and threonine contain a hydroxyl group. Serine dehydratase produces pyruvate and ammonia from serine. Serine can be produced from glucose via the phosphoglyceric acid intermediates of glycolysis thus it is nonessential. Tetrahydrofolate (FH4) reacts with serine to form glycine and TV6, TV10-methylene-FH4. 

The net result of photosynthesis is reduction of carbon dioxide to form carbohydrates. A key intermediate is phosphoglyceric acid, from which various simple sugars are produced and disproportionated to form other carbohydrates. 

Figure 20.11 Conversion of 3-phosphoglyceric acid to serine. 3-Phosphoglyceric acid originates from dihydroxyacetone phosphate, a glycolysis intermediate.

Some plants employ a photosynthetic pathway creating at first a three-carbon phosphoglyceric acid (C3 or Calvin-Benson photosynthesis). These plants fractionate isotopes more intensely, and so have more negative values (-33%o to —22%o PDB) than plants which use a photosynthetic pathway creating at first a four-carbon malic and aspartic acid (C4 or Hatch-Slack photosynthesis -16%o to -9%o PDB). Crassulacean acid metabolism (CAM) is yet another photosynthetic pathway, which creates organic matter of intermediate isotopic composition (-35%o to -ll%o PDB). Methanogenic microbes are even more extreme in their fractionation of the light isotope (5 C down to -110%o and typically -60%o PDB ... 

B. In the synthesis of these three amino acids from glucose, serine is produced from the glycolytic intermediate phosphoglyceric acid. Arginine is produced from the TCA cycle intermediate cc-ketoglutarate, and aspartate by transamination of oxaloacetate. Therefore, glyceraldehyde 3-phosphate is the only common intermediate. 

Let us return to the so-called dark reaction of photosynthesis. In it, both the NADPH2 and the ATP formed in the light reactions are consumed in the fixation of C02. The fixation reactions were charted by Melvin Calvin and his co-workers in Berkeley, California (for which Calvin received the Nobel Prize for 1961), with the use of radioactive carbon dioxide. During these reactions, C02 is made to combine with a pentose (5-carbon) sugar, ribulose diphosphate, to give an unstable 6-carbon intermediate which breaks down to two molecules of the 3-carbon phosphoglyceric acid. 

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. 

Ca plants Plants that employ the Calvin pathway during photosynthesis. C3 plants are so named because they employ an intermediate compound, phosphoglyceric acid that contains three carbon atoms. Most higher plants and algae use the C3 pathway. These plants strongly discriminate (20%e) against during carbon incorporation. 

The starting compounds for the biosynthesis of vanillic acid in genetically modified Escherichia coli are erythrose 4-phosphate and phosphoenol pyruvate. The erythrose is an intermediate product in carbohydrate metabolism (Calvin cycle, dark reaction of photosynthesis). [154, 155] Phosphoenol pyruvate is produced in several steps from 3-phosphoglyceric acid, or from a technical point of view, from succinic acid via the citric acid cycle. [156]... 

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 ... 

Glyceric acid is formed as an intermediate of glycolate pathway. It is also obtained by dephosphorylation of 3-phosphoglyceric acid which is an intermediate of carbohydrate metabolism, the initial product of photosynthetic CO2 fixation via the Calvin cycle, or an oxygenation product of ribulose bis-phosphate in photorespiration. 

Serine is biosynthesized by oxidizing 3-phosphoglycerate (an intermediate in glycolysis), transaminating the product with glutamate and then hydrolyzing off the phosphate group. In Section 26.9, we will see how proteins are biosynthesized from amino acids. 

CO2, present in the form of HCO3—, is fixed in the acceptor, ribulose-1,5-diphosphate, by means of the enzyme carboxydismutase. An intermediate with 6 C atoms is formed, the identity of which is still unknown. This substance is unstable. It decomposes into two molecules of 3-phosphoglyceric acid. The latter is then reduced to 3-phosphoglyceraldehyde by means of the ATP and NADPH + H+ formed in the primary processes. 3-Phosphoglyceraldehyde exists in equilibrium with its isomer, dihydroxy acetone phosphate. The equilibrium is controlled by the enzyme triose phosphate isomerase. 3-Phosphoglyceralde-hyde and dihydroxy acetone phosphate ar referred to collectively as triose phosphate. 

Figure 1. Dynamic utilization of amino acids in pigs. Degradation of essential amino acids via interorgan cooperation results in synthesis of nonessential amino acids. BCAA, branched-chain amino acids D3PG, D-3-phosphoglycerate (cm intermediate of glucose metabolism) HYP, hydroxyproline. Synthesis of serine from its carbon skeleton (D3PG) requires amino acids (e.g. aspartate and glutamate) as donors of the amino group.

Within less than 1 min the radioactivity of carbon dioxide can be picked up in sugar phosphates, phosphoglyceraldehyde, phospho-pyruvic acid, phosphoglyceric acid (PGA), amino acids (particularly alanine and aspartic acid) and organic acids (particularly malic acid) (Fig. 5.5). By reducing the period of exposure to CO to a few seconds it was possible to show that the first stable intermediate product of photosynthesis was PGA (Fig. 5.6) and that virtually all the C was located in the carboxyl carbon (marked ) of this compound. 

By contrast, the Calvin cycle in photosynthesis illustrates a positive feed-back mechanism. The concentration of Calvin cycle intermediates persisting in photosynthetic cells in the dark is very low. When photosynthesis commences the availability of ribulose diphosphate (RuDP) may limit the rate of COj assimilation. However, the formation of phosphoglyceric acid (PGA) immediately raises the level of other intermediates in the cycle, including that of RuDP, and this in turn speeds up the rate of CO assimilation. Similarly, when cells... 

Serine. Oxidation of the a-hydroxyl group of the glycolytic intermediate 3-phosphoglycerate converts it to an 0x0 acid, whose subsequent transamination and dephosphorylation leads to serine (Figure 28—5). 

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