In vivo molybdate inhibition of sulfate transfer to porphyridium capsular polysaccharide

Active transport of exogenous sulfate into log phase cells of Porphyridium aerueineum followed Michaelis-Menten kinetics, and the apparent Km for sulfate transport is approximately 2.5 × 10⁻⁶m. Molybdate, also a group VI anion, is a competitive inhibitor of sulfate transport, and the inhibition is freely reversible. Once in the cell, molybdate depresses the rate of sulfate pool utilization by blocking sulfate transfer to polysaccharides destined for secretion to the cell surface. Specifically, molybdate inhibits the formation of adenosine 5′-phosphosulfate and in turn the formation of adenosine 3′-phosphate 5′-phosphosulfate, the activated donor for sulfate transfer reactions. Combined with the previous identification of adenosine 3′-phosphate 5′-phosphosulfate, this is taken as evidence that the adenosine 5′-phosphosulfate/adenosine 3′-phosphate 5′-phosphosulfate enzymatic sequence for sulfate activation and sulfate donor reactions is operating in Porphyridium. Thiosulfate is utilized as effectively as sulfate as both a sulfur source for growth and polysaccharide synthesis.