Possible Mechanisms for the Enzymatic Condensation of Porphobilinogen

Abstract

THE problem of the mechanism of the enzymatic condensation of four moles of porphobilinogen (PBG) to form uroporphyrinogen III (urogen III) remains unsolved. Bogorad1–3 has been able to direct the course of this condensation in vitro by using one or two enzyme systems. Porphobilinogen deaminase (PBG-D) or uroporphyrin I synthetase from spinach catalyses the formation of uroporphyrin I (urogen I) from porphobilinogen. Uroporphyrinogen isomerase (U-Is) or uroporphyrinogen III co-synthetase from wheat germ and PBG-D direct the formation of urogen III from porphobilinogen. Kinetic investigations carried out by Bogorad suggest that the action of PBG-D on porphobilinogen is rate-determining and that uroporphyrinogen isomerase participates in a faster step for which porphobilinogen is also a substrate. Uroporphyrinogen isomerase does not act on porphobilinogen alone nor on urogen I (ref. 4). This suggests that some intermediate, the synthesis of which is catalysed by PBG-D, and porphobilinogen are substrates for uroporphyrinogen isomerase.