The substrates are four molecules of porphobilinogen.
The first reaction is catalyzed by uroporphyrinogen I synthase.
- The porphobilinogen molecules lose their amino groups.
- A linear tetrapyrrole with alternating acetic acid and propionic acid groups is produced. This linear molecule cyclizes slowly (nonenzymatically) to yield uroporphyrinogen I. Without the second reaction (below), the heme synthesis pathway would end with porphyrinogens of the I series, which have no known function.
The 2nd is catalyzed by uroporphyrinogen III cosynthase.
- This enzyme rapidly converts the alternating linear tetrapyrrole to the cyclic uroporphyrinogen III (pronounce), which has the substituents of its IV ring reversed: AP AP AP PA. This is the physiologically useful product.
