myo-Inositol Catabolism in Bacillus subtilis

The iolABCDEFGHIJ operon of Bacillus subtilis is responsible for myo-inositol catabolism involving multiple and stepwise reactions. Previous studies demonstrated that IolG and IolE are the enzymes for the first and second reactions, namely dehydrogenation of myo-inositol to give 2-keto-myo-inositol and the subsequent dehydration to 3D-(3,5/4)-trihydroxycyclohexane-1,2-dione. In the present studies the third reaction was shown to be the hydrolysis of 3D-(3,5/4)-trihydroxycyclohexane-1,2-dione catalyzed by IolD to yield 5-deoxy-d-glucuronic acid. The fourth reaction was the isomerization of 5-deoxy-d-glucuronic acid by IolB to produce 2-deoxy-5-keto-d-gluconic acid. Next, in the fifth reaction 2-deoxy-5-keto-d-gluconic acid was phosphorylated by IolC kinase to yield 2-deoxy-5-keto-d-gluconic acid 6-phosphate. IolR is known as the repressor controlling transcription of the iol operon. In this reaction 2-deoxy-5-keto-d-gluconic acid 6-phosphate appeared to be the intermediate acting as inducer by antagonizing DNA binding of IolR. Finally, IolJ turned out to be the specific aldolase for the sixth reaction, the cleavage of 2-deoxy-5-keto-d-gluconic acid 6-phosphate into dihydroxyacetone phosphate and malonic semialdehyde. The former is a known glycolytic intermediate, and the latter was previously shown to be converted to acetyl-CoA and CO2 by a reaction catalyzed by IolA. The net result of the inositol catabolic pathway in B. subtilis is, thus, the conversion of myo-inositol to an equimolar mixture of dihydroxyacetone phosphate, acetyl-CoA, and CO2.