Biosynthetic Pathways from Serine

Glutamate Dehydrogenase and Glutamate 1397 1. Biosynthetic Pathways from Serine... 

Inspection of the amino acid biosynthetic pathways shows that all amino acids arise from a few intermediates in the central metabolic pathways (see fig. 21.1). Amino acids de-rived from a common intermediate are said to be in the same family. For example, the serine family of amino acids, which includes serine, glycine, and cysteine, all arise from glycerate-3-phosphate (see fig. 21.1). The carbon flow from the central metabolic pathways to amino acids is a regulated... 

The type II pneumocytes synthesize phosphatidylcholine in a fashion quite different from that in other cells. Most other cells synthesize phosphatidylserine from cytidine diphosphate diacylglycerol and serine. The phosphatidylserine is then decarboxylated to yield phosphatidyl ethanoiamine. The final step is the successive donation of three methyl groups via S-adenosylmethionine to form phosphatidylcholine. The pulmonary biosynthetic pathway is shown in Figure 54-2. The enzyme choline phosphotransferase forms PC directly from cytidine diphosphocholine and diacylglycerol. Phosphatidylinositol formation peaks at about 35 weeks. As PI decreases in concentration, PG begins to increase. 

Finally, we briefly consider the way in which Sec is generated and co-translationally incorporated in selenoproteins. There are 25 selenoprotein genes in humans, and Sec has been found in the active site of those to which a function has been attributed. Sec does not occur as the free amino acid, and the biosynthetic pathway of Sec from serine on tRNA in eukaryotes requires four enzymes, as illustrated in Figure 18.8. The specific tRNA is aminoacylated with serine by the conventional Seryl-tRNA synthetase (SerRS) and the... 

Figure 24.18 Structure of 3-ipho5phoglycerate dehydrogenase This enzyme, which catalyzes the committed step in the serine biosynthetic pathway, is inhibited by serine. Notice the two serine-binding dimeric regulatory domains—one at the top and the other at the bottom of the structure. [Drawn from IPSD.pdb.]...

Extensive studies have been performed to disclose the biosynthetic pathway of microcystin and their lower mass analogs, the nodularins (53,85). One of the major questions was the origin of the Adda residue. The methyl substimtion pattern was indicative of incorporation of either propionate or acetate followed by methylation via S-adenosylmethionine. Although both propionate and methionine were found to be incorporated, the pattern of labelled metabolites was clearly indicative of an acetate-plus-methionine sequence for Cl through C8. The remainder of Adda presumably derives from phenylalanine via phenylacetic acid. The other subunits are for the most part derived from predictable pathways. According to the biosynthetic intermediates isolated the assembly of the linear penta- and octapeptides occurs with the Adda unit as N-terminal residue and Arg as C-terminus. Cyclization apparently represents the last step, since conversion of N-methyl-serine and -threonine to Mdha and Mdhb, respectively, occurs in earlier steps. 

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