Pyruvic acid biosynthesis

L Glutamic acid is not an essential ammo acid It need not be present m the diet because animals can biosynthesize it from sources of a ketoglutaric acid It is however a key intermediate m the biosynthesis of other ammo acids by a process known as transamination L Alanine for example is formed from pyruvic acid by transamination from L glutamic acid... 

Several additional points should be made. First, although oxygen esters usually have lower group-transfer potentials than thiol esters, the O—acyl bonds in acylcarnitines have high group-transfer potentials, and the transesterification reactions mediated by the acyl transferases have equilibrium constants close to 1. Second, note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis (Chapter 25), and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate, as well as some amino acids. 

The acetyl-CoA derived from amino acid degradation is normally insufficient for fatty acid biosynthesis, and the acetyl-CoA produced by pyruvate dehydrogenase and by fatty acid oxidation cannot cross the mitochondrial membrane to participate directly in fatty acid synthesis. Instead, acetyl-CoA is linked with oxaloacetate to form citrate, which is transported from the mitochondrial matrix to the cytosol (Figure 25.1). Here it can be converted back into acetyl-CoA and oxaloacetate by ATP-citrate lyase. In this manner, mitochondrial acetyl-CoA becomes the substrate for cytosolic fatty acid synthesis. (Oxaloacetate returns to the mitochondria in the form of either pyruvate or malate, which is then reconverted to acetyl-CoA and oxaloacetate, respectively.)... 

Step 1 of Figure 29.13 Carboxylation Gluconeogenesis begins with the carboxyl-afion of pyruvate to yield oxaloacetate. The reaction is catalyzed by pyruvate carboxylase and requires ATP, bicarbonate ion, and the coenzyme biotin, which acts as a carrier to transport CO2 to the enzyme active site. The mechanism is analogous to that of step 3 in fatty-acid biosynthesis (Figure 29.6), in which acetyl CoA is carboxylated to yield malonyl CoA. 

Some sugar residues in bacterial polysaccharides are etherified with lactic acid. The biosynthesis of these involves C)-alkylation, by reaction with enol-pyruvate phosphate, to an enol ether (34) of pyruvic acid, followed by reduction to the (R) or (5) form of the lactic acid ether (35). The enol ether may also react in a different manner, giving a cyclic acetal (36) of pyruvic acid. 

Cyclic acetals of pyruvic acid are common in extracellular polysaccharides (compare, for example, Ref. 6). They have also been found in some LPS, namely, those from Shigella dysenteriae type 6 and E. coli 0-149 (Ref. 139), and in the teichoic acid from Brevibacterium iodinum. The biosynthesis of these acetals has already been discussed. 

Amino Acid Biosynthesis Aromatic amino acid family Aspartate family Glutamate family Pyruvate family Serine family Histidine family Other... 

The tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

Branched Chain Amino Acid Biosynthesis. The branched chain amino acids, leucine, isoleucine and valine, are produced by similar biosynthetic pathways (Figure 2.11). In one pathway, acetolactate is produced from pyruvate and in the other acetohydroxybutyrate is produced from threonine. Both reactions are catalysed by the same enzyme that is known as both acetolactate synthase (ALS) and acetohy-droxy acid synthase (AHAS). 

Note that the C02 added to pyruvate in the pyruvate carboxylase step is the same molecule that is lost in the PEP carboxykinase reaction (Fig. 14-17). This carboxylation-decarboxylation sequence represents a way of activating pyruvate, in that the decarboxylation of oxaloacetate facilitates PEP formation. In Chapter 21 we shall see how a similar carboxylation-decarboxylation sequence is used to activate acetyl-CoA for fatty acid biosynthesis (see Fig. 21-1). 

In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway (see Fig. 14-21) and by malic enzyme (Fig. 21-9a). The NADP-linked malic enzyme that operates in the carbon-assimilation pathway of C4 plants (see Fig. 20-23) is unrelated in function. The pyruvate produced in the reaction shown in Figure 21-9a reenters the mitochondrion. In hepatocytes and in the mammary gland of lactating animals, the NADPH required for fatty acid biosynthesis is supplied primarily by the pentose phosphate pathway (Fig. 21-9b). 

Scheme IS Biosynthesis of the C5 unit of thiamin from [3-D3]pyruvic acid and l-[3-D2, 3- sO]gIyceroI in E. coli. Deuterium from [2-D3]acetate, L-[l-D2]glycerol and L-[2-Di]glycerol is not directly incorporated...

Beta replacement is catalyzed by such enzymes of amino acid biosynthesis as tryptophan synthase (Chapter 25),184 O-acetylserine sulfhydrylase (cysteine synthase),185 186a and cystathionine (3-synthase (Chapter 24).187 188c In both elimination and (3 replacement an unsaturated Schiff base, usually of aminoacrylate or aminocrotonate, is a probable intermediate (Eq. 14-29). Conversion to the final products is usually assumed to be via hydrolysis to free aminoacrylate, tautomerization to an imino acid, and hydrolysis of the latter, e.g., to pyruvate and ammonium ion (Eq. 14-29). However, the observed stereospecific addition of a... 

In the biosynthesis of the capsular polysaccharide from Xanthomonas campestris, the modification was shown265 to occur at the level of a polyprenyl pentasaccharide diphosphate intermediate prior to polymerization of the repeating units, and enolpyruvate phosphate was a precursor of the pyruvic acid residues. A similar observation was made during a study of the biosynthesis of Rhizobium meliloti exopolysaccharide.266... 

Pyruvate carboxylase, which participates in gluconeogenesis and lipogenesis Acetyl-CoA carboxylase, which participates in fatty acid biosynthesis Propionyl-CoA carboxylase, which participates in isoleucine catabolism 3-Methylcrotonyl-CoA carboxylase, which participates in leucine catabolism... 

Eight enzyme-catalyzed reactions are involved in the conversion of acetyl-CoA into fatty acids. The first reaction is catalyzed by acetyl-CoA carboxylase and requires ATP. This is the reaction that supplies the energy that drives the biosynthesis of fatty acids. The properties of acetyl-CoA carboxylase are similar to those of pyruvate carboxylase, which is important in the gluconeogenesis pathway (see chapter 12). Both enzymes contain the coenzyme biotin covalently linked to a lysine residue of the protein via its e-amino group. In the last section of this chapter we show that the activity of acetyl-CoA carboxylase plays an important role in the control of fatty acid biosynthesis in animals. Regulation of the first enzyme in a biosynthetic pathway is a strategy widely used in metabolism. 

No experimental data are available concerning the biosynthesis of 2-desoxyhexoses. The naturally occurring 2-desoxyhexomethyloses (XXII) could be formed, possibly, by condensation of two molecules of acetaldehyde (which can arise from oxalacetic acid or pyruvic acid) and one molecule of glycol aldehyde—that is, from 4-desoxytetroses (XXI) and acetaldehyde. 

Brevicolline.—The /3-carboline part of the plant alkaloid brevicolline (114) has been shown to derive from tryptophan (94) and pyruvic acid.37 Putrescine (4) and related compounds provide the pyrrolidine ring.38 A key intermediate in brevicolline biosynthesis is likely to be (113), derived by oxidative decarboxylation of (111), which in turn is formed through the condensation of (94) with pyruvic acid condensation of (113) and (112) (formed from putrescine) would lead to (114). This has been supported by successfully mimicking the biogenetic sequence, starting with the chemical oxidative decarboxylation of (111).39... 

Overall, the biosynthesis of 160 is characterized by the dimerization of 168 to give the central structure of the molecule. This head-to-tail dimerization strategy is efficient, using the same substrate twice, and is a sensible route, given the existence of the shikimate pathway, which provides, in turn, a precursor to 168. An analogous dimerization route can be seen for the biosynthesis of K252c (1), described in Sect. 5, where two molecules of indole-3-pyruvic acid imine (125), derived in turn from L-tryptophan (123), are dimerized to give an intermediate that leads to chromopyrrolic acid (128). In both cases, the monomer precursors, either 168 or 125, serve as both nucleophiles and electrophiles, and are activated to react by the presence of the appropriate enzymes. 

These results confirmed that branched-chain amino acid catabolism via the BCDH reaction provides the fatty acid precursors for natural avermectin biosynthesis in S. avermitilis. In contrast, B. subtilis appears to possess two mechanisms for branched-chain precursor supply. The dual substrate pyruvate/branched-chain a-keto acid dehydrogenase (aceA) and an a-keto acid dehydrogenase (bfmB), which also has some ability to metabolize pyruvate, appears to be primarily involved in supplying the branched-chain initiators of long, branched-chain fatty acid biosynthesis [32,42], Two mutations are therefore required to generate the bkd phenotype in B. subtilis [31,42],... 

Fatty acid biosynthesis (and most biosynthetic reactions) requires NADPH to supply the reducing equivalents. Oxaloacetate is used to generate NADPH for biosynthesis in a two-step sequence. The first step is the malate dehydrogenase reaction found in the TCA cycle. This reaction results in the formation of NAD from NADH (the NADH primarily comes from glycolysis). The malate formed is a substrate for the malic enzyme reaction, which makes pyruvate, CO2, and NADPH. Pyruvate is transported into the mitochondria where pyruvate carboxylase uses ATP energy to regenerate oxaloacetate. 

N-acetylglucosamine (see Chapter 9) is a component of glycoproteins, connective tissue proteoglycans, and complex lipids. It may be synthesized in the human organism from fructose-6-phosphate, as indicated in Figure 18.17. N-acetylglucosamine is also a precursor of N-acetylmannosamine, which along with pyruvic acid participates in the biosynthesis of sialic acid. 

In nature, thiamine pyrophosphate also catalyses reactions of a-keto-acids other than pyruvic acid. One such sequence leads through some remarkable chemistry to the biosynthesis of the branched-chain amino acids valine and isoleucine. 

The metabolic flux distributions around the intermediate pyruvate for different strains and environmental conditions are summarised in Fig. 12. This part of the metabolism has been shown to be an important node for the interconversion between glycolytic C3 metabolites and C4 metabolites of the tricarboxylic acid (TCA) cycle. The different anaplerotic reactions are of special importance for the production of recombinant proteins as they provide precursors, such as oxaloace-tate, for amino acid biosynthesis. Due to that, the flux distribution is noticeably affected by both the cultivation conditions and the carbon source used which indicates flexible adaptation to the environmental situation. The flux from pyruvate to oxaloacetate through the reaction catalysed by pyruvate carboxylase was found to be the main anaplerotic pathway in B. megaterium. 

The RebD enzyme was characterized as the first member of a new subfamily of heme-containing oxidases [34,36]. The enzyme acted as both a catalase and a CPA synthase, apparently converting two molecules of 7-chloroindole-3-pyruvic acid imine into ll,ll -dichloro-CPA. Formation of CPA by StaD, an enzyme homolog to RebD, was also confirmed for STA biosynthesis [35],... 

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