Production of uroporphyrin III requires two enzymes.
The substrates are four molecules of porphobilinogen.
The first reaction is catalyzed by uroporphyrinogen I synthase.
The second reaction 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.
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Last modified 1/5/95