Catalytic Facilitation in Vitro by Two Multienyzme Complexes from Neurospora crassa

Abstract

Abstract The conversions of 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) to 3-enolpyruvylshikimate 5-phosphate (ES-5-P) and of N-(5'-phosphoribosyl)anthranilate (PR-anthranilate) to indole-3-glycerol phosphate (InGP) are catalyzed in vitro by two multienzyme complexes from Neurospora crassa. The rates of the over-all sequential reactions catalyzed by the two complexes were found to be greater than the rates of reactions initiated later in the sequence with known intermediates. Under conditions of saturation with the substrate, the rate of formation of ES-5-P from DAHP was 10 times the rate of formation of ES-5-P from shikimate, and the rate of formation of InGP from PR-anthranilate was twice the rate of formation of InGP from 1-(o-carboxyphenylamino)-1-deoxyribulose 5-phosphate. With both complexes, addition of the intermediate did not increase or inhibit the more rapid over-all reaction rates. We conclude that for the over-all reaction sequences the catalytic efficiencies of the enzymic components are increased in these multienzyme complexes either by a mechanism of intermediate substrate localization or by an activation of enzymic activities. Although these cases of catalytic facilitation in vitro have not as yet been correlated directly with the phenomenon of metabolic compartmentalization or channeling as observed in vivo, the activation of the components of a specific multienzyme complex or the localization of intermediate substrates within a specific complex are both mechanisms by which the cell could effect a compartmentalization or channeling of biochemical intermediates.