Homocysteine desulfuration

To extend the scope of NCL, efforts have been made to introduce modifications so as not to depend on a Cys site. Cys residues can be selectively desulfurized to Ala [124], even in the presence of Cys(Acm) and Met [125]. Homocysteine and selenocysteine also serve as ligation sites, and can then be converted into Met [126] and into Ala or didehydroalanine, respectively [127]. Recently, ligation at Phe sites has also been accomplished [128]. The use of auxiliaries that are removed after ligation has been extensively studied [129-134]. NCL has also been applied to SPPS [135, 136], and to the synthesis of cyclic peptides [137-139], In another extension of the technique, proteins have been prepared by tandem ligation [135, 140], and expressed protein ligation [141]. Recently, NCL has been applied to the synthesis of glycoproteins [104, 142-144]. 

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. ... 

B. Desulfuration of Homocysteine with Liberation of Hydrogen Sulfide 379... 

As in the case of homocysteine, cysteine sulfur may be split off the organic molecule in the form of hydrogen sulfide. This desulfuration is... 

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