Biosynthesis Occurs by Sulfhydryl Transfer to Activated Serine

Cysteine Biosynthesis Occurs by Sulfhydryl Transfer to Activated Serine 

The biosynthesis of L-cysteine entails the sulfhydryl transfer to an activated form of serine. This pathway to L-cysteine has been most thoroughly studied in E. coli. In the first step an acetyl group is transferred from acetyl-CoA to serine to yield (9-acetylserine (fig. 21.8a). The reaction is catalyzed by serine transacetylase. The formation of cysteine itself is catalyzed by O-acetylserine sulfhydrylase. 

In some organisms sulfur incorporation involves homocysteine as an intermediate. In such cases cysteine formation occurs by a transsulfuration reaction, with the intermediate formation of L,L-cystathionine (fig. 21.86). Cystathionine is formed in a simple condensation reaction from serine and homocysteine by cystathionine-jS synthase. 

The biosynthesis of cysteine by direct sulfhydrylation and by a transsulfuration route in which the sulfur is derived from homocysteine. The direct sulfhydrylation pathway (a) is indicated as occurring with H2S as the source of sulfur. The transsulfuration 

Formation of 3 -phosphoadenosine-5 -phosphosulfate (PAPS), an active intermediate involved in sulfate reduction. The eight-electron reduction of S042 to HjS is poorly understood except for the initial steps in the activation of sulfate (shown in yellow). Reduction in yeast and plants involves the APS derivative shown. In E. coli a PAPS derivative is used. 

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