Desulfhydrase reaction

Reaction 3 is the cysteine desulfhydrase reaction which by -elimination produces HjS, pyruvate, and NH3. This reaction is catalyzed by cystathionine-y-lyase, the B protein of tryptophan synthetase (E.C. 4.2.1.20), or crystalline tryptophanase (E.C. 4.1.99.1) (Meister, 1965). According to Meister cysteine desulfhydrase reactions are probably catalyzed by other enzymes. Cystathionine-y-lyase has not been found in higher plants (Giovanelli et al., this volume. Chapter 12). 

IV. THE DESULFHYDRASE REACTION Enzyme preparations from rat and dog liver are capable of liberating H2S from cysteine directly. The reaction proceeds anaerobically. Ammonia is liberated as well as H2S. The reaction that occurs appears to be represented by equation 4. 

Hydrogen sulfide is a well known general metabolite produced on sulfate reduction by certain bacteria. Moreover, organic forms of sulfur can give rise to HS , hence H2S in certain bacteria. Thus, cysteine desulfhydrase (EC 4.4.1.1, cystathionine y-lyase) converts L-cysteine to H2S, pyruvate, and NH3. This enzyme shows a requirement for pyridoxal phosphate and the unstable ami-noacrylic acid is an intermediate (Equation 1) in the reaction ... 

This reaction is readily reversible. Another means of metabolizing serine, which accounts for its glucogenic character, as well as that of glycine, is the conversion of serine to pyruvate, as indicated in Figure 20.12. This reaction is catalyzed by serine dehydratase. A similar enzyme, threonine dehydratase, converts threonine to a-ketobutyrate, and the latter is then converted to propionyl-CoA, as indicated in Figure 20.13. Another similar enzyme, cysteine desulfhydrase, con-... 

There is a general requirement for pyridoxal-5-phosphate (24, 25, 27, 44) although not all of the activity lost on dialysis is restored by adding the cofactor. This requirement explains the inhibition by hydroxylamine and hydrazine (24, 25). The reaction is a typical pyridoxal-5-phosphate catalyzed a,/ -elimination with a mechanism similar to serine dehydrase and cysteine desulfhydrase (45). The coenzyme is probably bound as a Schiff base with an amino group of the enzyme since there is an absorption maximum at 415 nm in solutions of the purified garlic enzyme (40). The inhibition by L-cysteine is presumably caused by formation of a thiazolidine with the coenzyme (46). Added pyridoxal-5-phosphate also combines directly with the substrate. The dissociation constant for the complex is about 5 X lO M. When this is taken into account, the dissociation constant of the holoenzyme can be shown to be about 5 X 10 M (47). The higher enzyme activity in pyrophosphate buflFer than in Tris or phosphate may be explained by pyrophosphate chelation of metal ions which otherwise form tighter complexes with the substrate and coenzyme (47). This decreases the availability of added coenzyme. 

In all reactions, the first stage is formation of SchifTs base a by condensation of PalP and the amino acid. Schiff s bases a and b represent part of transamination, but for the complete mechanism see Transamination. Racemization a- b, followed by b-ia-iamino acid-1-PalP, with addition of the proton in the opposite configuration. Amino acid decarboxylation a -> d- c - amine + PalP. Serine hydroxymethyltransferase (EC 2.1.2.1) X = OH L-serine + PalP a f- g glycine + PalP reversal of these reactions leads to L-serine synthesis from glycine the hydroxymethyl group is carried by te-trahydrofolic acid. Cysteine desulfhydrase (EC 4.4.1.1) X = SH cy eine + PalP a b-> c-y hydro-... 

Cysteine Desulfhydrase. Cysteine undergoes a desulfuration that is believed to be analogous to the dehydration of serine. The enzyme cysteine desulfhydrase requires pyridoxal phosphate and forms pyruvate, H2S, and NH3. This enzyme has been found in animal liver and presumably occurs in microorganisms that release H2S from cysteine. Evidence has been obtained with H2S that indicates some reversibility of the reaction, but no thermodynamic data are available. A similar enzyme has been reported to form a-ketobutyrate, NH3, and H2S from homocysteine. ... 

Kallio has partially purified the cysteine and homocysteine desulfhydrases from P. morganii and found that pyridoxal phosphate was active as cofactor. The possibility that threonine might be an intermediate in reaction 21 was ruled out by the finding that threonine was relatively inactive as a substrate. The role of pyridoxal phosphate as cofactor for animal desulfhydrases is suggested from the findings of Braunstein and Azarkh" that liver homogenates from pyridoxine-deficient animals had lower cysteine desulfhydrase activity than those from normal animals. Dietary supplementation with pyridoxine raised the activity to the normal level. 

As a more general pathway of metabolism in which the compound is completely degraded, we may take as a model the reaction of the cysteine desulfhydrases, which form pyruvic acid from the carbon chain. The subsequent metabolism of the pyruvic acid should then be along the well-Fromageot, C., Advances in Enzymol. 7, 369 (1947) Enzymes 1 (pt. 2), 1237 (1951). 

Corroborative evidence has been obtained from experiments with isolated tissue preparations. The in vitro formation of cystine by liver slices (and a saline extract) was observed in nuxtures containing dl-homocysteine and DL-serine. Neither substance was effective when incubated alone. The L-forms of the two amino acids were implicated in the reaction, as D-homocysteine and o-serine did not substitute for the DL-forms and the isolated cystine had the L-configuration. The reaction proceeded best under anaerobic conditions and in the present of O.OOlAf CN , as the latter inhibits cysteine desulfhydrase activity. Methionine substituted very poorly for homocysteine in vitro. 

There are apparently two distinct enzymic reactions which result in the anaerobic degradation of cysteine. The first is a direct desfilfhydration, the second is initiated by transamination with a-ketoglutarate to yield glutamate and 8-mercaptopyruvate. According to the original observations of Desnuelle and Fromageot, Smythe and others (71) the mechanism of cysteine desulfhydrase action is as follows ... 

Support for this enzymic desulfhydration came from the work of Metzler and Snell (72), who proposed a detailed chemical mechanism of desulfhydration in model experiments in the presence of pyridoxine and a metal ion. Similar enzyme models were proposed by others (73). The enzymic aspect of this reaction is not entirely satisfactory however, ce relatively little is known of the desulfhydrase enzyme, either derived from the liver (74) or from bacteria (75). In addition several pyridoxal phosphate-protein enzymes can catalyze cysteine desulfhydration (see Section IV). 

The action of cysteine desulfhydrase is inhibited by hydrogen cyanide. Binkley and du Vigneaud (15) have observed that the production of hydrogen sulfide, which appears in the comrse of the reaction due either to homocysteine desulfhydrase or to cysteine desulfhydrase, is arrested in the presence of hydrogen cyanide, without however diminishing appreciably the yield of cysteine. This observation shows that the mechanism in question does not come into play in the experiments of Binkley and du Vigneaud, but the re.sults do not show that this mechanism plays no role in the animal. The theory of Toennies (121) assumes the existence of a direct transsulfuration between methionine and serine. This theory is based on the ability of methionine sulfur to form sulfonium derivatives... 

Cysteine can be determined by a bacteria of the type Proteus morganii. The enzyme involved, cysteine desulfhydrase, catalyses the reaction ... 

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