Plants cysteine biosynthesis

Equation (1) is catalyzed by serine acetyltransferase (E.C. 2.3.1.30) and Eq. (2) by cysteine synthase (E.C. 4.2.99.8). Evidence that these reactions represent the major pathway for cysteine biosynthesis is as follows (a) Serine acetyltransferase has been demonstrated in a number of plants (see Section II,B,1), and OAS is a natural constituent of cultured tobacco cells, being present at a concentration of at least 120 nmoles/g fresh weight (Smith, 1977). (b) Cysteine synthase has been demonstrated in a wide variety of plants. All such enzyme preparations show activities with OAS far in excess of those with serine (Section II,B,2). (c) The physiological role of the two enzymes is well established in microorganisms (Siegel, 1975), lending credence to their role in plants. 

An important proviso of this scheme is that under normal conditions the rate of synthesis of cysteine is limited by the availability of its sulfide precursor. Then, changes in the rate of formation of sulfide will be closely coupled to changes in the rate of cysteine formation. Evidence in support of the limitation of the rate of cysteine biosynthesis by the availability of sulfide includes the following, (a) The concentration of acid-volatile sulfide in L. minor growing on sulfate is about 11 fxM (Brunold and Schmidt, 1978), while the of cysteine synthase for sulfide is at least 180 f/M (Table II). This comparison suggests that cysteine synthase may normally be operating below its for sulfide, (b) Administration of H2S (18 ppm in the gas phase) to L. minor caused about a 50% increase in the steady-state concentration of soluble cysteine in the plants within a few minutes (Brunold and Erismann,... 

In addition to the major elfectors (AdoMet, threonine, and lysine), cysteine and isoleucine may participate in the control of methionine biosynthesis, at least in some plants. Both isoleucine and cysteine would be expected to accumulate as a result of the diversion of O-phosphohomoserine toward threonine. Isoleucine is a potent competitive inhibitor of the homoserine kinase of pea seedlings (Thoenef aL, 1978), but not that of barley seedlings (Aarnes, 1976). Cysteine inhibits homoserine dehydrogenase (see Bryan, this volume. Chapter 11) and can inhibit the stimulation by AdoMet of some (Madison and Thompson, 1976) but not all (Aarnes, 1978 Thoen et al., 1978) preparations of threonine synthase. Any regulatory effect of cysteine may, however, be of short duration since the combined mechanisms described in Section II,D for regulation of cysteine biosynthesis would be expected to restore the normal concentration of this amino acid. Details of the control of methionine biosynthesis by the major effectors AdoMet, threonine, and lysine are presented below. 

Bonner, E.R. et al. (2005) Molecular basis of cysteine biosynthesis in plants - structural and functional analysis of O-acetylserine sulfhydry-lase from Arabidopsis thaliana. J. Biol Chem. 280,38803-38813... 

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. ... 

Humans can synthesize 12 of the 20 common amino acids from the amphiboHc intermediates of glycolysis and of the citric acid cycle (Table 28-1). While nutritionally nonessenrial, these 12 amino acids are not nonessential. AH 20 amino acids are biologically essential. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolic intermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionally essential amino acids. Identification of the twelve amino acids that humans can synthesize rested primarily on data derived from feeding diets in which purified amino acids replaced protein. This chapter considers only the biosynthesis of the twelve amino acids that are synthesized in human tissues, not the other eight that are synthesized by plants. 

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. 

In plants, little is known about the basic building blocks and the reactions involved in thiamine biosynthesis. An early study with chloroplasts of spinach indicated that 1-deoxy-D-xylulose 5-phosphate, tyrosine, and cysteine act as precursors of the thiazole moiety in analogy to the pathway in E. coli. More recently, it has been shown that a homolog of the THIC protein that converts 5-aminoimidazole ribotide into 38 is essential (25). These results suggest that the plant pathway is similar to the pathway in prokaryotes but not to that in yeast. 

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). 

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