Porphobilinogen deaminase and uroporphyrinogen III synthase: structure, molecular biology, and mechanism.

Porphobilinogen deaminase (hydroxymethylbilane synthase) and uroporphyrinogen III synthase (uroporphyrinogen III cosynthase) catalyze the transformation of four molecules of porphobilinogen, via the 1-hydroxymethylbilane, preuroporphyrinogen, into uroporphyrinogen III. A combination of studies involving protein chemistry, molecular biology, site-directed mutagenesis, and the use of chemically synthesized substrate analogs and inhibitors is helping to unravel the complex mechanisms by which the two enzymes function. The determination of the X-ray structure of E. coli porphobilinogen deaminase at 1.76 A resolution has provided the springboard for the design of further experiments to elucidate the precise mechanism for the assembly of both the dipyrromethane cofactor and the tetrapyrrole chain. The human deaminase structure has been modeled from the E. coli structure and has led to a molecular explanation for the disease acute intermittent porphyria. Molecular modeling has also been employed to stimulate the spiro-mechanism of uroporphyrinogen III synthase.