Overview: Inorganic sulfur and sulfate activation

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Publisher Summary This chapter presents an overview of inorganic sulfur and sulfate activation. The oxidized and reduced forms of sulfur can be interconverted by various organisms. The metabolism of the most highly oxidized naturally occurring form of sulfur, sulfate, begins with the entrance of sulfate into the cell, which is accomplished through active transport mediated by a carrier-enzyme system. The uptake of sulfate is followed by activation and by transfer and reduction reactions in appropriate organisms. Sulfate must be converted to an activated form before it can be utilized metabolically. There are two forms of activated sulfate, adenosine 5'- phosphosulfate (APS) and 3'-phosphoadenylylsulfate (adenosine 3'-phosphate 5'-phosphosulfate, PAPS). They are formed in two sequential enzyme-catalyzed reactions involving ATP-sulfurylase (sulfate adenylyltransferase) and APS kinase (adenylylsulfate kinase). The first step in the biosynthesis of PAPS, leading to the formation of APS, is catalyzed by the enzyme ATP-sulfurylase. This reaction is greatly favored energetically in the reverse direction as the free energy of hydrolysis of the sulfate group in APS is considerably higher than the free energy of the phosphate linkage of ATP.