Bacteria cysteine biosynthesis

Grundy FJ, Henkin TM. The S box regulon a new global transcription termination control system for methionine and cysteine biosynthesis genes in gram-positive bacteria. Mol. Microbiol. 1998 30 737-749. 

Histidine alone, however, is not capable of reversing the growth effects of amitrole on green plants nucleotide bases are more effective. " Purine biosynthesis has been suggested as a more important site of action than IGP dehydratase.Since histidine is ultimately biosynthesized from a purine nucleotide, adenosine triphosphate, the inhibition of histidine formation may only be indirect. Amitrole causes the bleaching of newly formed plant tissue, and pigment biosynthesis has been suggested as an alternative site of herbicide action.Effects of 1,2,4-triazole on cysteine biosynthesis have been noted in bacteria, but no studies have been carried out on plants to assess the relevance of this to amitrole-phytotoxicity. ... 

In the biosynthesis of the thia2ole, cysteine is the common sulfur donor. In yeasts, the C-2 and N may be suppHed by glycine, and the remaining carbons byD-ribulose-5-phosphate [108321-99-9] (50). In anaerobic bacteria, the C-2 andN maybe recmited from tyrosine and the carbons from D-l-deoxyxylulose [16709-34-5] (51), whereas in aerobic bacteria the C-2 and N maybe derived from glycine, as in yeasts 7 (74—76,83—86) (see Fig. 9). 

In a series of papers, Cook et al.60-63 presented results of the 31P NMR studies of pyridoxal 5 -phosphate dependent enzyme. O-acetylserine sulf-hydrylase is the enzyme which catalyses the final step of biosynthesis of l-cysteine, the replacement of p-acetoxy group of O-acetyl-L-serine by thiol [30] in bacteria and plants. 

Pantothenic acid has a central role in energy-yielding metabolism as the functional moiety of coenzyme A (CoA), in the biosynthesis of fatty acids as the prosthetic group of acyl carrier protein, and through its role in CoA in the mitochondrial elongation of fatty acids the biosynthesis of steroids, porphyrins, and acetylcholine and other acyl transfer reactions, including postsynthetic acylation of proteins. Perhaps 4% of all known enzymes utilize CoA derivatives. CoA is also bound by disulfide links to protein cysteine residues in sporulating bacteria, where it may be involved with heat resistance of the spores, and in mitochondrial proteins, where it seems to be involved in the assembly of active cytochrome c oxidase and ATP synthetase complexes. 

Sulfate reduction. All plants, animals, and bacteria metabolize sulfur in order to synthesize amino acids such as cysteine and methionine. The sulfur may be assimilated as sulfate or as organic molecules containing sulfate. The reduction of sulfate in biosynthesis is termed assimilatory sulfate reduction and can take place in aerobic or anaerobic environments (cf. Goldhaber and Kaplan 1974 Rheinheimer 1981 Cullimore 1991). 

CGS catalyzes the 7-replacement reaction of an activated form of L-homoserine with L-cysteine, leading to cystathionine. 0-Succinyl-L-homoserine (l-OSHS), 0-acetyl-L-homoserine (OAHS), and 0-phospho-L-homoserine (OPHS) are substrates for CGS ftom bacteria, fungi, and plants, respectively. The plant enzyme is also able to convert the microbial substrates, albeit at much higher values. This reaction is the first step in the transsulfuration pathway that converts L-Cys into L-homocysteine, the immediate precursor of L-methionine. The 0-activated L-homoserine substrate is situated at a metabolic branch point between L-Met and L-Thr biosynthesis, and which substrate is used by CGS depends on the species. In analogy with TS, CGS is tightly regulated by SAM concentration in plants. ... 

FIGURE 22-13 Biosynthesis of cysteine from serine in bacteria and plants. The origin of reduced sulfur is shown in the pathway on the right. 

Cystathionine y-synthase is the best studied enzyme catalyzing both y-replacement and /3,y-elimination reactions. The enzyme is found in plants and bacteria and normally functions to catalyze the formation of cystathionine from 0-acylhomoserine and cysteine during the biosynthesis of methionine (66) [Eq. (57)] ... 

Cystathionine (made by cystathionine synthase from homocysteine and serine) plays a central role both in the biosynthesis of methionine in plants and bacteria and in the biosynthesis of cysteine in animals. In humans, deficiency of cystathionine synthase leads to a condition called homocystinuria, in which homocysteine overaccumulates. The condition results in severe mental retardation and dislocation of the lens of the eye. 

Cystathionine-y-synthase isolated from Salmonella typhimurium is a tetramer (molecular weight 160000) and catalyses, in vivo, the y-replacement of O-suc-cinylhomoserine with cysteine [79] to yield cystathionine. The latter, by way of homocysteine, is involved in the biosynthesis of methionine. In other species of bacteria and plants the succinyl moiety may be replaced by acetyl, phosphoryl, or malonyl moieties [80]. In the absence of cysteine the enzyme catalyses an abnormal reaction resulting in the formation of a-oxobutyrate. The latter reaction has been utilised for mechanistic investigations pertinent to the y-eUmination-deamination process (vide infra). 

Early studies of homocysteine biosynthesis in plants were oriented largely by what was known in microorganisms. Microorganisms were known to synthesize homocysteine by transsulfura-tion, the transfer of sulfur between cysteine and homocysteine via cystathionine. In bacteria, transsulfuration proceeds only from cysteine to homocysteine, whereas in fungi transsulfuration proceeds reversibly between cysteine and homocysteine (Flavin, 1975). Our early studies therefore asked whether transsulfuration occurs in plants, and if so, in what direction it proceeds. 

The proteins NodP (ATP sulfurylase) and NodQ (adenosine 5 -phosphosulfate (APS) kinase) are associated in a sulfate-activating complex that enables the synthesis of the sulfate donor (PAPS) for Nod factor sulfation [42]. PAPS is also the sulfate donor used for the synthesis of cysteine and it is produced in E. coli by the normal household proteins CysC, CysD, and CysN [43, 44]. The genes cysCDN are part of the cysteine regulon which is repressed in the presence of cysteine and other reduced sulfur compounds [45]. This means that the E. coli machinery of PAPS biosynthesis can be used for chitin oligosaccharide sulfation if the bacteria are cultivated with sulfate as the only sulfur source. The total PAPS demand for the synthesis of cysteine and methionine (87 and 146 pmol g dried cells, respectively) [46] largely exceeds the maximum demand that can be estimated for chitin oligosaccharide sulfation (around 20 pmol g dried cells). 

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