Cysteine desulfuration

A PLP cofactor attached to the lysine in an His-Lys-X-X-X-Pro-X-Gly-X-Gly motif is a crucial feature of these proteins. Additionally crucial is a conserved cysteinyl residue, which serves as the persulfide site. These proteins belong to fold-type 1 of PLP-dependent enzymes and are homodimers. Each monomer is subdivided into a large domain with one molecule of PLP in aldimine linkage with a Lys residue and a small domain, where the critical cysteinyl residue is located in the middle of a loop. An extended lohe in CDS contains the conserved Cys and constitutes one side of the entrance to the active site. This lohe in CSD may he responsible for the ability of the enzyme to discriminate between selenocysteine and cysteine. NifS binds and transforms the cysteine substrate in a manner usual for PLP-containing enzymes up to the stage of the central quinonoid intermediate. Cysteine desulfuration is initiated by the formation of a Schifif base between cysteine and PLP, followed by the abstraction of sulfur from the substrate and formation of an enzyme-bound cysteine persulfide and alanine via a ketimine intermediate. The cysteine residue acts as a nucleophile and attacks the sulfhydryl... 

Cys desulfurases were also reported to catalyze the decomposition of selenocysteine to L-alanine and elemental selenium with varying efficiency. The catalytic mechanism of both cysteine desulfuration and selenocysteine deselenation is similar. The PLP-binding Lys is the base that accepts the Ca proton of the substrate and reprotonates the intermediate to form a ketimine species. The selenohydryl group of L-seleno-cysteine is probably deprotonated and present in an anionic form. The deprotonation of the selenohydryl group may be facilitated by a His residue, located in the active site. The Cys residue is not essential for deselenation process. Selenium is, then, released spontaneously from the ketimine intermediate. ... 

The second important issue related to commercial use of desulfurization biocatalysts is their inhibition by sulfate. The sulfur repression mechanism in most Rhodococcus species limits their use or activity in presence of sulfate- and sulfur-containing amino-acids such as cysteine, methionine, etc. To alleviate this problem, expression of the dsz genes under the control of alternate promoters has been investigated. 

Another report describing an approach to achieve alleviation of sulfur repression came from the Matsui research group. The dsz genes were cloned into a strain Rhodococcus sp. strain T09 under the promoter rrn of the strain T09 using a Rhodococcus-E. coli shuttle vector [214,215], This resulted in a strain which desulfurized DBT to 2-HBP in presence of sulfate, cysteine, or methionine. Similar approach was also used by Kurane to construct a gene expressing dszA-D enzymes, which eliminate the sulfate inhibition effects [216],... 

Zheng L, White RH, Cash VL, Dean DR. 1994. A mechanism for the desulfurization of L-cysteine catalyzed by the nifS gene prodnct. Biochemistry 33 4714-20. 

The sulfur extrusion method 21 has been extensively investigated for the synthesis of threo-3-methyllanthionine 25 however, in this case desulfurization was not used to generate the amino acid (either in free or protected form), but rather to form different sulfide-bridged cyclic peptides as precursors for the total synthesis of nisin (3). Development of synthetic methods in this area was particularly important in respect to the synthesis of nisin, since this lantibiotic contains four // eo-3-methyllanthionine residues. The synthesis proceeded from a protected f/zreo-3-methyl-D-cysteine in the N-terminal position and L-cysteine in the C-terminal position. 

Cystine is reduced to cysteine, using NADH + H+ as a reductant. Cysteine undergoes desulfuration to yield pyruvate. 

The high chemoselectivity of NCL relies upon the distinct reactivity of an N-terminal cysteine. The requirement for cysteine at the ligation, however, restricts the applicability of NCL. It is possible to subject the ligation product to desulfurization, resulting in the net formation of a more commonly found X-Ala bond [29]. The presence of cysteines other than that needed for ligation is not tolerated, however, since desulfurization would occur at both protected and unprotected cysteine thiols. 

L-Cysteine was transformed into optically active a-alanine derivative as S-benzyl-A-phthaloyl derivative by desulfurization with Raney Ni (eq. 13.75).152... 

Yan, L. Z. and Dawson, P. E. (2001) Synthesis of peptides and proteins without cysteine residues by native chemical ligation combined with desulfurization. J. Am. Chem. Soc. 123, 526-533. 

Cysteine. In animals, cysteine is converted to pyruvate by several pathways. In the principal pathway, the conversion occurs in three steps. Initially, cysteine is oxidized to cysteine sulfate. Pyruvate is produced after a transamination and a desulfuration reaction. 

Pentelute BL, Kent SBH (2007) Selective desulfurization of cysteine in the presence of Cys (Acm) in polypeptides obtained by native chemical ligation. Org Lett 9 687-690... 

Wan Q, Danishefsky SJ (2007) Free-radical-based, specific desulfurization of cysteine a powerful advance in the synthesis of polypeptides and glycopolypeptides. Angew Chem Int Ed 46 9248-9252... 

The direct attachment of a thiol to a side-chain requires a post-ligation desulfurization step for its removal. This approach was first reported by Yan and Dawson, who extended hydrogenolytic desulfurization of cysteine to the chemical synthesis of proteins by NCL, utilizing Ala as a new ligation junction (Scheme 7) [134]. In the same study, they also noted that not only Raney nickel but also palladium could be used as the catalyst. In addition, the application was expanded to other residues such as Leu, Val, and Be. A subsequent refinement of this method... 

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