Influence of an aggregated multienzyme system on transient time: kinetic evidence for compartmentation by an aromatic-amino-acid synthesizing complex of Neurospora crassa.

The aromatic complex of Neurospora crassa is an aggregated multienzyme system which catalyzes five consecutive reactions in the central pathway leading to the biosynthesis of the aromatic amino acids. In an attempt to understand the physiological importance of this complex in particular, as well as the importance of cellular organization of enzyme systems in general, we have isolated the complex and have begun to characterize its catalytic properties. Optimum conditions for the assay of the overall 5-step reaction catalyzed by the partially purified complex have been determined. An analog computer was programmed to represent an unaggregated system of five enzymes with rate constants identical to those found for the constituent enzymes of the complex. By direct comparison, it was shown that the lags (transient times) obtained for the overall reaction were 10-15 times longer for the hypothetical unaggregated system than for the complex. We conclude from these data that the aggregated multienzyme system compartmentalizes intermediate substrates during the course of the overall reaction. We suggest that, in addition to "channeling" intermediates of competing pathways, reduction of the transient time may be an important consequence of the containment of intermediates within a physically associated enzyme sequence. The fact that the aromatic complex exhibits a second catalytic property unique to aggregated enzyme systems, "coordinate activation" [Welch, G.R. & Gaertner, F.H. (1975) Arch. Biochem. Biophys., in press] indicates that the physical association of these enzymes may have more than one physiological function.