3-Phosphoglycerate, from serine

FIGURE 22-12 Biosynthesis of serine from 3-phosphoglycerate and of glycine from serine in all organisms. Glycine is also made from C02 and NH( by the action of glycine synthase, with N5,N10-methy-lenetetrahydrofolate as methyl group donor (see text). 

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. 

Glycine is the major inhibitory neurotransmitter in the spinal cord. Most of the glycine in neurons is synthesized de novo within the nerve terminal from serine by the enzyme serine hydroxymethyltransferase, which requires folic acid. Serine, in turn, is synthesized from the intermediate 3-phosphoglycerate in the glycolytic pathway. The action of glycine is probably terminated via uptake by a high-affinity transporter. 

As shown on p. 168j, serine can be converted to pyruvate (elimination of water forms the unsaturated amino acid which goes via the imino acid to the a-keto acid) as well as hydroxypyruvate or phosphoserine. The phosphorylated form, which could also come from 3-phosphoglycerate, occurs in a few proteins and phospha-tides (Chapt. XIII). Other bases present in the phosphatides, i.e. ethanolamine and choline, also originate from serine. 

One excellent example of a Emax-type allosteric enzyme is Escherichia coli phosphoglycerate dehydrogenase (PGDH), a tetramer of identical subunits that catalyzes the formation of D-3-phosphohydroxypyruvate from D-3-phosphoglycerate in a reaction that uses NAD+ as a redox cofactor. This regulatory enzyme is allosteri-cally controlled by serine. All available information suggests that the effects on the for substrate are minor... 

Serine, Glycine, and Cysteine Are Derived from 3-Phosphoglycerate... 

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. 

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

Serine is formed from 3-phosphoglycerate (Fig. 15-3). Serine is also synthesized from glycine in a reaction catalyzed by serine hydroxymethyltransferase ... 

Serine is synthesized from 3-phosphoglycerate, an intermediate in glycolysis. The first step is an oxidation to 3-phosphohydroxypyruvate. This a-ketoacid is transaminated to 3-phosphoserine, which is then hydrolyzed to serine. 

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. 

Serine is synthesized in a direct pathway from glycerate-3-phosphate that involves dehydrogenation, transamination, and hydrolysis by a phosphatase (Figure 14.6). Cellular serine concentration controls the pathway through feedback inhibition of phosphoglycerate dehydrogenase and phosphoserine phosphatase. The latter enzyme catalyzes the only irreversible step in the pathway. 

One advantage of this pathway over serine dehydratase is that in transamination free ammonia is not released and the potential difficulty of ammonia toxicity is minimized. Serine in people is not an essential amino acid and can be synthesized in the liver from the glycolytic intermediate 3-phosphoglycerate, which can be oxidized to phosphohydroxypyruvate and then transaminated with glutamate to form phosphoserine. The... 

To some extent this may be subjective, imposing a human interpretation on the observations, but it also reflects an objective reality some reactions carry a greater flux of metabohtes than others, some are active in a wider range of cell types than others, and so on. To understand the entire chart, therefore, it is useful to collect the reactions into groups of transformation sequences known as metabolic pathways. The number of steps considered to be one pathway can be very small if very few steps are needed to convert one important metabolite into another. For example, serine biosynthesis is a three-step pathway, in which the aminoacid serine is synthesized from 3-phosphoglycerate. At the other extreme, beta-oxidation, the process that converts fatty acids from the form in which they are stored in fat cells into the form in which they are metabolically active, involves seven repetitions of the same four types of step, making an unbranched pathway of nearly 30 reactions. 

Phosphoglyceric acid, an oxidation product of serine, can be withdrawn from the pathway much as C4 and C5 acids are withdrawn from the TCA cycle (Jahnke et al. 1999). The acetyl-CoA product can reenter the cycle and lead to the production of succinate for use in biosyntheses (White 1995, p. 265). 

---