Cysteine synthase

Cysteine contributes sulphur atoms to chelators, and therefore the synthesis of cysteine is a further important control point. Cysteine synthase (CSase) is the final enzyme in the biosynthetic pathway. Kawashima and colleagues (2004 and references therein) have produced tobacco plants with altered levels of this protein in the cytosol and/or chloroplasts. All transformants showed enhanced tolerance to Cd, Se and Ni, but not to Pb or Cu. In particular, the plants expressing CSase both in the cytosol and the chloroplasts had an even higher Cd tolerance, and possessed enhanced levels of Cys and GSH. The same plants also accumulated more Cd. 

Kawashima, C., G., Noji, M., Nakamura, M., Ogra, Y., Suzuki, K. T., and Saito, K., 2004, Heavy metal tolerance of transgenic tobacco plants over-expressing cysteine synthase, Biotechnol. Lett. 26 153-157. 

This enzyme [EC 4.2.99.8], also known as cysteine synthase and O-acetylserine sulfhydrylase, catalyzes the pyr-idoxal-phosphate-dependent reaction of H2S with O -acetylserine to produce cysteine and acetate. Some alkyl thiols, cyanide, pyrazole, and some other heterocyclic compounds can also act as acceptors. 

Cystathionine (3-lyase (cystathionase) O-Acetylserine sulfhydrylase (cysteine synthase)... 

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 E. coli L-cysteine is formed from L-serine and the sulfide ion S2 in a reaction that also requires acetyl-CoA and is catalyzed by the consecutive action of an acyl transferase and cysteine synthase. Outline the mechanism of this conversion indicating participation of any essential coenzymes. 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

Harada, E. et al., Transgenic tobacco plants expressing a rice cysteine synthase gene are tolerant to toxic levels of cadmium, J. Plant Physiol., 158, 655-661, 2001. 

Phosphopantothenic acid reacts with cysteine, forming 4 -phosphopant-othenyl cysteine, which is decarboxylated to 4 -phosphopantetheine in a flavin-dependent reaction. In most bacteria, phosphopantetheinyl cysteine synthase and decarboxylase occur as a single bifunctional enzyme, but the human enzymes occur as two separate proteins (Daugherty et al., 2002). 

In addition to compounds related to mimosine, other NPAAs are synthesized by transfer of an alanine residue catalysed by cysteine synthase, e.g. the formation of isoxazolin-5-on-2-yl-alanine in Lathyrus sativus (Ikegami et al, 1993). Moreover, partial purification of cysteine synthase from Allium tuberosum revealed that this enzyme also catalyses the formation of cysteine derivatives, such as S-allylcysteine (Ikegami et al, 1993). The various aspects of the formation of NPAAs by cysteine synthase are presented by Noji et al. (1993) and Ikegami and Murakoshi (1994). 

Ikegami, R, Itagaki, S. and Murakoshi, I. (1993) Purification and characterization of two forms of cysteine synthase from Allium tuberosum. Phytochemistry, 32, 31-4. 

Ikegami, R, Mizuno, M. and Murakoshi, I. (1990) Enzymatic synthesis of the thyrotoxic amino acid, mimosine, by cysteine synthase. Phytochemistry, 29,3461-6. 

Noji, M., Murakoshi, L. and Saito, K. (1993) Evidence for identity of p-pyrazolealanine synthase with cysteine synthase in watermelon formation of beta-pyrazolealanine by cloned cysteine synthase in vitro and in vivo. Biochem. Biophys. Res. Commun., 197,1111-7. 

PLP-dependent desulfhydrases necessarily show very similar mechanisms, but often come from independent evolutionary lineages. For example, although most bacterial L-cysteine desulfhydrases are fold-type I enzymes belonging to the same evolutionary branch as cystathionine f3- and 7-lyases, L-cysteine desulfhydrase from Fusobacterium nucleatum is a member of the fold-type II group and its closest sequence homologue is a cysteine synthase. D-cysteine desulfhydrase from E. coli is also a fold-type II enzyme not strictly related to other desulfhydrases but resembling instead an ACC deaminase. ... 

The enzyme cysteine synthase (EC 4.2.99.8) catalyzes the last step in the biosynthesis of cysteine, converting serine acetate 97 to cysteine 134. Floss et al. (84) studied this -replacement reaction using stereospecifically tritiated serine acetates prepared from the labeled serines 60 synthesized as in Scheme 18 (Section IV). Assessment of chirality of the cysteine produced by degradation to serine and use of tryptophan synthase showed that the j3-replacement reaction 97a134 had proceeded with retention of configuration (84) (Scheme 45),... 

Fig. 2. Summary of the free and bound pathways of sulfate assimilation in plants. Some related reactions and points of entry of several forms of inorganic sulfur are also shown. The reaction sequence catalyzed by (1) ATP sulfurylase, (2) APS sulfotransferase, (3) thiosulfonate reductase, and (4) cysteine synthase constitutes the bound sulfate assimilation pathway. The synthesis of OAS is catalyzed by (5) serine transacetylase. The reaction sequence (I), (6)-(9)or (1), (2), (10), (8), (9) constitutes the free pathway reactims (7) and (10) are nonenzymatic, (6) is catalyzed by APS sulfotransferase, (8) by sulfite reductase, and (9) by cysteine synthase. APS and PAPS are interrelated via (11) APS kinase and (12) NDP phophohydrolase. APS can be hydrolyzed via (13) APS sulfohydrolase or (14) APS cyclase.

Plants possess several mechanisms for the incorporation of sulfide into sulfur amino acids (Giovanellief al., this volume, Chapter 12). However, it is agreed that the most important of these is the incorporation of sulfide into cysteine in a reaction catalyzed by cysteine synthase (E.C. 4.2.99.8) using OAS as sulfide acceptor ... 

Cysteine synthase has been purified from various plants and its properties examined in some detail (Giovanelli et al., this volume. Chapter 12). Several features of the enzyme are relevant to the problem of sulfide assimilation. The first of these concerns the subcellular localizaticxi of the enzyme in leaf tissue. In wheat, kidney bean and rape the enzyme is reported to be associated with the soluble fraction (Ascano and Nicholas, 1976 Masada ef al., 1975 Smith, 1972). However, in spinach, pea, and clover leaf tissue the enzyme is reported to be associated with intact chloroplasts (Fankhauser et al., 1976 Ng and Anderson, 1978a, 1979). Some possible explanaticms for these differences have been discussed by Ng and Anderson (1978a). For pea chloroplasts, however, the data in Fig. 1 show that in the absence of OAS, sulfite is reduced to sulfide in a light-dependent reaction but addition of OAS causes an immediate consumption of sulfide with the concomitant formatim of cysteine. 

The cysteine synthase of chloroplasts also catalyzes the incorporation of selenide into selenocysteine in the presence of OAS (Ng and Anderson, 1978b). Since selenocysteine is presumed to be the precursor of the various selenoamino acids found in selenium-accumulator and nonaccumulator plants (Shrift, 1973), this affords an explanation for the incorporation of inorganic selenium into these compounds. In the presence of OAS, intact chloroplasts incorporate selenite into selenocysteine in a light-dependent reaction but experiments with inhibitors suggest that selenite is not reduced via sulfite reductase (Ng and Anderson, 1979). 

Our current understanding of the assimilation of inorganic sulfur into cysteine is summarized in Fig. 2. In spinach the enzymes ATP sulfurylase, APS kinase, APS sulfotransferase, thiosulfonate reductase, sulfite reductase, and cysteine synthase are known to be associated with chloroplasts. The subcel-lular localization of the other enzymes shown in Fig. 2 is either uncertain or unknown. The association of the enzymes of the bound pathway with mitochondria in Euglena (Brunold and Schifif, 1976) appears to be a special case. 

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. 

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