Sulfide is incorporated into cysteine, catalyzed by O-acetylserine (thiol)lyase, with O-acetylserine as substrate. The synthesis of O-acetylserine is catalyzed by serine acetyltransferase and together with O-acetylserine (thiol)lyase it is associated as enzyme complex named cysteine synthase.
The formation of cysteine is the direct coupling step between sulfur (sulfur metabolism) and nitrogen assimilation in plants. This differs from the process in yeast, where sulfide must be incorporated first in homocysteine then converted in two steps to cysteine.
Cysteine is sulfur donor for the synthesis of methionine, the major other sulfur-containing amino acid present in plants. This happens through the transsulfuration pathway and the methylation of homocysteine.
Both cysteine and methionine are sulfur-containing amino acids and are of great significance in the structure, conformation and function of proteins and enzymes, but high levels of these amino acids may also be present in seed storage proteins. The thiol groups of the cysteine residues in proteins can be oxidized resulting in disulfide bridges with other cysteine side chains (and form cystine) and/or linkage of polypeptides.
Disulfide bridges (disulfide bonds) make an important contribution to the structure of proteins. The thiol groups are also of great importance in substrate binding of enzymes, in metal-sulfur clusters in proteins (e.g. ferredoxins) and in regulatory proteins (e.g. thioredoxins).