Low abundance does not mean less importance in cysteine metabolism

Abstract

The cysteine molecule plays an essential role in cells because it is part of proteins and because it functions as a reduced sulfur donor molecule. In addition, the cysteine molecule may also play a role in the redox signaling of different stress processes. Even though the synthesis of cysteine by the most abundant of the isoforms of O-acetylserine(thiol)lyase in the chloroplast, the mitochondria and the cytosol is relatively well-understood, the role of the other less common isoforms homologous to O-acetylserine(thiol)lyase is unknown. Several studies on two of these isoforms, one located in the cytosol and the other one in the chloroplast, have shown that while one isoform operates with a desulfhydrase activity and is essential to regulate the homeostasis of cysteine in the cytosol, the other, located in the chloroplast, synthesizes S-sulfocysteine. This metabolite appears to be essential for the redox regulation of the chloroplast under certain lighting conditions.