Multiplicity of 3-hexulosephosphate synthase from bacterium MB 58. 1. Product-induced transition in velocity

Purified 3-hexulosephosphate synthase (HPS) from the facultative methylotrophic Bacterium MB 58 exhibits a burst-like progress curve. The transition appears in an abrupt manner at physiological substrate concentrations. The following phase of lower activity again shows linear progress in the observation period. Effects of substrates, temperature or dilution on the enzyme can be ruled out as causes of this transition. However, it is obvious that it takes place after the same amount of product has accumulated. That the reaction product causes this diminution in velocity can be demonstrated by varying the enzyme concentration or the possibility for accumulating product or the product concentration itself. The substrates of HPS counteract this product-induced transition in that an increased amount of the former makes necessary an increased amount of the latter to trigger the transition. The shape of the progress curve and experiments with different amounts of in situ-generated or synthesized product show that no simple product inhibition is involved. For the explanation of all phenomena observed three enzyme forms showing different affinities for substrates and product are postulated. These forms may be interconverted into each other by substrates and product of HPS in a specific manner.