5 - Assimilation of Inorganic Sulfate into Cysteine

Abstract

This chapter describes assimilation of inorganic sulfate into cysteine. Inorganic sulfate is the form of sulfur most commonly available in the physical environment for use by organisms. Plants utilize sulfate as their sulfur source for the synthesis of the many sulfur-containing compounds essential for life. Most of these compounds, including cysteine, methionine, and GSH, contain sulfur in oxidation state −2. Plants must, therefore, effect the reduction of sulfate in an 8e- reduction. Reduction of sulfate by plants quantitatively accounts for most biological sulfate reduction and is important in the supply of sulfur-amino acids to the organisms that are unable to use sulfate as a sulfur source. The chapter discusses the reduction of sulfate to sulfide and its incorporation into cysteine in plants. Cysteine is the principle starting metabolite for the synthesis of other sulfur-containing metabolites. Sulfate assimilation is common to many organisms, including many which are non-photosynthetic. The reduction and assimilation of inorganic sulfate and nitrate in plants have several features in common. Both processes entail 8e- reductions to inorganic forms in energy-requiring reactions prior to incorporation into appropriate acceptor molecules. With the exception of the partial reduction of nitrate to nitrite in the cytoplasm, assimilation of sulfate and nitrite occurs in chloroplasts in reactions that are dependent on light for a supply of Fdred and adenosine triphosphate.