Regulation of Enzymes for Erythrose 4-Phosphate Synthesis in Brevibacterium flavum

Abstract

Partially purified FBPase of B. flavum exhibited Michaelis-Menten kinetics, with Km of 22μm for F1, 6P₂, but was inhibited by its substrate, F1, 6P₂, at more than 30μm. The inhibition reached 76% at 1.0mm. FBPase was also competitively inhibited by AMP, SI, 7P₂, and S7P, the inhibitor constants being 32, 23, and 170 μm, respectively. The presence of S1, 7P₂ in B. flavum was suggested by the presence of ATP-dependent S7P kinase and SI, 7P₂ aldolase activities, which corresponded to 27 and 89% of the 6PFK and FI, 6P₂ aldolase activities, respectively. Transaldolase activity involved in S7P formation was also detected in cell-free extracts. Partially purified PG1 showed Kms of 1.4 and 0.54mm in the forward and reverse directions, respectively and was inhibited by E4P. The inhibition was of the mixed type with respect to G6P, with Ki and Ki′ of 32 and 260 μm, respectively. None of the metabolites tested inhibited TK. Whereas the specific activities of PGI and 6PFK were not affected by carbon sources in the culture medium, those of FBPase, 1PFK, and TK were elevated 2.6, 2.6, and 1.7-fold on growth on acetate, fructose, and citrate, respectively.