Metabolism of Cysteine in Plants and Phototrophic Bacteria

Abstract

The sulfur amino acids cysteine and methionine function in many basic and essential processes of life. For cysteine this includes structural, catalytic, regulatory and metabolic functions. The special redox chemistry of sulfur and the thiol group in particular proved to be a versatile tool during evolution, not the least in electron transport processes in association with iron. Plants are primary producers and carry out assimilatory sulfate reduction to first synthesize cysteine that subsequently forms the backbone for methionine formation. This reaction sequence seems to be conserved in all phototrophic organisms. The position of cysteine biosynthesis between assimilation of inorganic sulfate and metabolization of organic sulfide makes it a prime target for coordination of both complex processes. It is thus a mediator between supply and demand in sulfur metabolism of a cell. Much attention has been paid to cysteine biosynthesis in plants, while less is known about the pathway in algae, cyanobacteria and purple bacteria. Recent evidence indicates that the two enzymes of cysteine synthesis, serine acetyltransferase and O-acetylserine-(thiol)-lyase are highly conserved between these groups and, at least in plants, form a reversible protein complex. This so-called cysteine synthase complex has been suggested to act as sensor for sulfide in cells and to be part of a regulatory loop that maintains cysteine homeostasis between sulfate reduction and cysteine consumption. Kinetic studies of the properties of the enzymes together with structural modelling of the proteins in the cysteine synthase complex as well studies using transgenic plants strongly support this unique regulatory system in plants. The degradation of cysteine is still an under-investigated subject. Possible alternative routes of thiol transfer and sulfide release are compiled here and discussed with respect to their putative functions in S-transfer reactions, cysteine degradation, detoxification reactions and iron-sulfur cluster biosynthesis.