Cystathionine y synthase

L-cystathionine L-homoserine + L-cysteine cystathionine-y- synthase Streptomyces phaeochromogenes ... 

This pyridoxal-phosphate-dependent enzyme [EC 4.2.99.9], also known as cystathionine y-synthase, catalyzes the reaction of O-succinyl-L-homoserine with L-cysteine to produce cystathionine and succinate. The enzyme can also use hydrogen sulfide and methanethiol as substrates, producing homocysteine and methionine, respectively. In the absence of a thiol, the enzyme can also catalyze a /3,y-elimination reaction to form 2-oxobu-tanoate, succinate, and ammonia. 

Aspartate y-decarboxylase Selenocysteine lyase NifS protein of nitrogenase Gamma elimination and replacement Cystathionine y-synthase Cystathionine y-lyase Threonine synthase... 

Cyclohexanedione, reaction with guanidinium groups, 126 Cyclophilin 488 human 488s D-Cycloserine 739s Cyclosporin 488, 488s p Cylinders 65, 66, 686 Cystathionine, 746s formation 746 Cystathionine p lyase 742 Cystathionine p-synthase 744 Cystathionine y-synthase 743, 746 Cystatins 622, 629... 

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. 

As summarized in Scheme II, PLP enzymes can catalyze replacements at the y-carbon of amino acids and eliminations of HY between C-fi and C-y. In mechanistic similarity to the aspartate-/3-decarboxylase reaction, in these processes the quinoid intermediate 1 loses a proton from C-/3, followed by elimination of an anionic group (Y ) from C-y, to generate the central intermediate PLP-vinylglycine, 4 (Scheme II). This species, the vinylogue of 1, can undergo a number of reactions. Addition of a new anionic group (Y ) and reversal of the reaction sequence constitutes the y-replacement reaction, as in cystathionine-y-synthase. On the other hand, in analogy to the protonation of 1 at C-a, 4 can be protonated at C-y,... 

A /3,y-elimination of succinylhomoserine to a-ketobutyrate is also catalyzed by cystathionine-y-synthase from Salmonella in the absence of L-cysteine. 

Studies on the / ,-/-elimination reaction catalyzed by y-cystathionase showed that in the conversion of homoserine to a-ketobutyrate one atom of deuterium from the solvent is incorporated at C-/ , where it occupies the pro-S position [156]. The stereochemistry of protonation at C-/ in this reaction is thus the same as in the / -/-elimination catalyzed by cystathionine-y-synthase. 

Trapping of the aminoacrylate intermediate in the reactions catalyzed by cystathionine-y-synthase and y-cystathionase produced the same diastereomer of KEDB which was different from the one formed with bacterial L-threonine dehydratase. Unfortunately, this experiment has apparently not been done with threonine synthetase. 

Cystathionine y-synthase (CGS) is a rather unique PLP-enzyme that catalyzes a transsulfuration reaction important in microbial methionine biosynthesis. It is the only known enzyme whose function is the catalysis of a PLP-dependent replacement reaction at the y-carbon of the amino acid substrate the succinyl moiety of O-succinyl-L-homoserine is replaced by i-Cys to give the thioether linkage of L,/.-cystathionine (scheme II). In the absence of L-Cys, the enzyme catalyzes a net y-elimination reaction from OSHS (scheme II). Because both reactions require the elimination of succinate, the catalytic pathways must diverge from a common reaction intermediate. It was originally hypothesized that a vinylglycine quinonoidal intermediate (structure 11)... 

Scheme II. Reaction mechanism for the PLP dependent cystathionine y-synthase y-elimination and y-replacement reactions proposed by Brzovic etal. (107). [Redrawn from (107) with permission.]...

S cystathionine y-synthase cystathionine /3-lyase methionine synthase ) dihydropicolinate synthase... 

Several PLP-dependent enzymes catalyze elimination and replacement reactions at the y-carbon of substrates, an unusual process which provides novel routes for mechanism-based inactivation. An example of this class of enzymes is cystathionine y-synthase [0-succinylhomoserine (thiol)-lyase], which converts (7-succinyl-L-homoserine and L-cysteine to cystathionine and succinate as part of the bacterial methionine biosynthetic pathway (Walsh, 1979, p. 823). Formation of a PLP-stabilized o-carbanion intermediate activates the )8-hydrogen for abstraction, yielding j8-carbanion equivalents and allowing elimination of the y-substituent. The resulting j8,y-unsaturated intermediate serves as an electrophilic acceptor for the replacement nucleophile. Suitable manipulation of the j8-carbanion intermediate allows strategies for the design of inactivators which do not affect enzymes which abstract only the a-hydrogen. 

Fio. 10. Mechanism proposed for inactivation of cystathionine y-synthase by propargylgly-cine (10). 

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)] ... 

In order to determine the stereochemical course at during the normal replacement reaction, Chang and Walsh converted (Z)-DL-[4- H)vinylglycine and ( )-DL-[3,4- H2]vinylglycine to cystathionine in presence of cystathionine y-synthase Salmonella) and cysteine 280). Stereospecific chemical and enzymic degradation of the resultant samples of cystathionine to homoserine followed by comparison of the NMR spectra with those of authentic (4/ )- and (4S)-l-[4- H]homoserine established that cysteine must add to the si face of the vinyl-... 

Autotrophic organisms synthesize methionine from asparfafe as shovm in the lower right side of Fig. 24-13. This involves fransfer of a sulfur atom from cysfeine info homocysfeine, using the carbon skeleton of homoserine, the intermediate cystathionine, and two PLP-dependent enzymes, cystathionine y-synthase and cystathionine p-lyase. This transsulfuration sequence (Fig. 24-13, Eq. 14-33) is essentially irreversible because of the cleavage to pyruvate and NH4+ by the P-lyase. Nevertheless, this transsulfuration pathway operates in reverse in the animal body, which uses two different PLP enzymes, cystathionine P s3mthase (which also contains a bound heme) and cystathionine y-lyase (Figs. 24-13,24-16, steps h and i), in a pathway that metabolizes excess methionine. 

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). 

The steric course at during the normal y-replacement and abnormal y-ehmina-tion-deamination reactions catalysed by cystathionine-y-synthase has been deduced in an elegant series of experiments described by Walsh and coworkers. An important aid in this stereochemical investigation was the observation that L-vinylglycine [88]... 

C. with deuterium and (ii) vinylglycine containing deuterium at only one of the vinyhc positions. Experiments based on this approach were performed [89] and it was shown that in the reaction catalysed by cystathionine-y-synthase, both (4S)-[4- H]-0-succinyl homoserine and (Z)-(4- H]vinylglycine were converted into (45 )-[4- H]cystathionine (Fig. 38). This result is only possible if both transformations proceed through the same intermediate (Fig. 38, 3). These experiments thus show that in the enzymic conversion of (4S)-[4- H]-0-succinyl homoserine (Fig. 38, 1) to products, the terminal double bond of the complex (Fig. 38, 3) has the (Z) configuration. This observation when considered in conjunction with the earlier demonstration that in the conversion the atom is removed [84-86], allows... 

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