Cysteine desulfhydrase and

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.1] (also referred to as homoserine deaminase, homoserine dehydratase, y-cystathionase, cystine desulfhy-drase, cysteine desulfhydrase, and cystathionase) catalyzes the hydrolysis of cystathionine to produce cysteine, ammonia, and a-ketobutanoate (or, 2-oxobutanoate). 

Other examples of PLP-requiring enzymes are the amino acid decarboxylases that lead to formation of amines, including several that are functional in nervous tissue (e.g., epinephrine, norepinephrine, serotonin, and y-aminobutyrate) cysteine desulfhydrase and serine hydroxymethyltransferase, which use PLP to effect the loss or transfer of amino acid side chains phosphorylase, which catalyzes phosphorolysis of the a-1,4-linkages of glycogen and cystathione beta-synthase in the transsulfiiration pathway of homocysteine. Additionally the biosynthesis of heme depends on the early... 

The enantiomers of this drug differ in their efficacy and activity, with (D)-penicilla-mine being the enantiomer required for pharmaceutical preparations. The (l)-enantiomer is toxic, and its absorption by the human body is more than the (D)-enantiomer. While both enantiomers of penicillamine are desulfhydrated by (r.)-cysteine desulfhydrase, only the (l)-isomer inhibits the action of this enzyme [2], The reported optical rotation values for (D)-penicillamine are ... 

Peak concentrations in the blood are obtained between 1 and 3 h after administration. Unlike cysteine (its nonmethylated parent compound), penicillamine is somewhat resistant to attack by cysteine desulfhydrase or L-amino acid oxidase. As a result, penicillamine is relatively stable in vivo [7,2],... 

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 ... 

L-/D-cvsteine. Hydrogen sulfide is produced from L-cysteine in a light-independent process that can be inhibited in vivo and in vitro by aminooxy acetic acid, an inhibitor of pyridoxal phosphate-dependent enzymes the hydrogen sulfide emitted in response to L-cysteine is directly derived from die L-cysteine fed ( 2 ). Therefore, hydrogen sulfide appears to be produced from L-cysteine by a pyridoxal phosphate-dependent, L-cysteine specific cysteine desulfhydrase. This conclusion is supported by the finding that in cucurbit... 

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. 

Wang, C. L., Lum, A. M., Ozuna, S. C., Clark, D. S., and Keasling, J. D. (2001). Aerobic sulfide production and cadmium precipitation by Escherichia coli expressing the Treponema denticola cysteine desulfhydrase gene. Appl. Microbiol. Biotechnol. 56, 425-430. 

Kredich, N.M., L.J. Foote, and B.S. Keenan. 1973. The stoichiometry and kinetics of the inducible cysteine desulfhydrase from Salmonella typhimurium.. Biol. Chem. 248 6187-6196. 

Degradation of L-cysteine by cysteine desulfhydrase or other PLP enzymes present in the cells was successfully prevented by addition of hydroxylamine or semi-carbazide to the incubation mixture. A mutant strain of Ps. thiazolinophilum lacking cysteine desulfhydrase was isolated and used to produce L-cysteine from d,l-ATC in a molar yield of 95% and at a product concentration of 31.4 g L 1[12S. Pseudomonas desmolytica A] 3872, one of the L-cysteine producers isolated was found to lack the ability to convert d-ATC into L-cysteine it is an ATC racemase-deficient strain 129l. However, little is known about the enzymological properties and function of the racemase. 

Cysteine has a number of potential metabolic pathways. With low cysteine intake, cysteine desulfhydrase may be of major importance. Cysteine desulfhydrase requires pyridoxal phosphate as a coenzyme and catalyzes the loss of H2S, similar to the loss of H20 with serine dehydratase. Thus, the final products are pyruvate, ammonia, and H2S. This pathway will produce pyruvate, a glucogenic precursor. However, the pathway, particularly with excess cysteine, must be limited, owing to the production of H2S, which is extremely toxic (Fig. 18.3). 

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. ... 

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). 

O-acetyl-L-serine (thiol)-lyase A (CysK), O-acetyl-L-serine (thiol)-lyase B (CysM) and MalY. Thus in order to engineer a potent L-cysteine over-producer excessive degradation of the amino acid must be prevented by inactivation of the genes encoding the major L-cysteine desulfhydrase activities. 

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. 

Partially purified cysteine desulfhydrase from rat liver is inhibited by cyanide, arsenous oxide, and the carbonyl reagents phenylhydrazine, semicarbazide, hydroxylamine, and sodium bisulfite. 

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. 

Binkley and Okeson purified the enzyme system that cleaves cystathionine and found that neither phosphate nor ATP was required for activity, thus correcting the previous report that ATP was required. In addition to splitting cystathionine, this enzyme preparation also produced H2S from cysteine. The authors suggest that their enzyme may be identical with cysteine desulfhydrase. Binkley also reported that he had been able to synthesize cystathionine enzymatically from homocysteine and serine by a fractionated liver preparation which had been freed from the cystathionine cleavage enzyme, serine dehydrase and homoserine deaminase. The activity of the enzyme synthesizing cystathionine was either inhibited or unaffected by ATP, DPN, AMP, and various metal ions. 

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 ... 

Fig. 4. The effects of dietary protein on the activities of cysteine dioxygenase, cysteine desulfhydrase in liver and the urinary taurine excretion of intact rat. Plotted are cysteine dioxygenase activity(0). Cysteine desulfhydrase activity (O )> urinary taurine contents (I j). Results are expressed as the mean S.D.(represented by vertical line) of six and three animals fed on basal diet (20% protein diet) and other diets, respectively. Animals were fed on experimental diets for 2 days prior to sacrifice.

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