The enzyme uroporphyrinogen III syn-thase (UROS) (also known as uropor-phyrinogen III cosynthase) catalyzes a remarkable reaction in which preuropor-

phyrinogen is rearranged and cyclized to yield uroporphyrinogen III (Figure 4). Uroporphyrinogen III is the common precursor for hemes, chlorophylls, vitamin Bj2> and all other tetrapyrroles (for a review see Reference 28). The UROS substrate, preuroporphyrinogen, is generated by the preceding enzyme of the tetrapyr-role pathway, PBGD (see above section) by a reaction that involves the polymerization of 4 molecules of the monopyrrole precursor PBG. Preuroporphyrinogen has a halflife of less than 5 minutes at neutral pH values (15) cyclizing spontaneously to uro-

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