At low substrate concentrations, the reaction is believed to follow an ordered route, with the sequential removal of CO2 from the D, A, B, and C rings, whereas at higher substrate/enzyme levels a random route seems to be operative. The enzyme functions as a dimer in solution, and both the enzymes from human and tobacco have been crystallized and solved at good resolutions.
The reaction catalyzed by UroD
UroD is regarded as an unusual decarboxylase, since it performs decarboxylations without the intervention of any cofactors, unlike the vast majority of decarboxylases. Its mechanism has recently been proposed to proceed through substrate protonation by an arginine residue. A 2008 report demonstrated that the uncatalyzed rate for UroD's reaction is 10−19 s−1, so at pH 10 the rate acceleration of UroD relative to the uncatalyzed rate, i.e. catalytic proficiency, is the largest for any enzyme known, 6 x 1024 M−1.
Proposed reaction mechanism of uroporphyrinogen III decarboxyklase
^Silva PJ, Ramos MJ. Density-functional study of mechanisms for the cofactor-free decarboxylation performed by uroporphyrinogen III decarboxylase. J Phys Chem B 2005;109:18195-200. doi:10.1021/jp051792s.
Elder GH, Lee GB, Tovey JA (1978). "Decreased activity of hepatic uroporphyrinogen decarboxylase in sporadic porphyria cutanea tarda.". N. Engl. J. Med. 299 (6): 274–8. doi:10.1056/NEJM197808102990603. PMID661926.
de Verneuil H, Bourgeois F, de Rooij F, et al. (1992). "Characterization of a new mutation (R292G) and a deletion at the human uroporphyrinogen decarboxylase locus in two patients with hepatoerythropoietic porphyria.". Hum. Genet. 89 (5): 548–52. doi:10.1007/bf00219182. PMID1634232.
Garey JR, Hansen JL, Harrison LM, et al. (1989). "A point mutation in the coding region of uroporphyrinogen decarboxylase associated with familial porphyria cutanea tarda.". Blood. 73 (4): 892–5. PMID2920211.
Roméo PH, Raich N, Dubart A, et al. (1986). "Molecular cloning and nucleotide sequence of a complete human uroporphyrinogen decarboxylase cDNA.". J. Biol. Chem. 261 (21): 9825–31. PMID3015909.
Dubart A, Mattei MG, Raich N, et al. (1986). "Assignment of human uroporphyrinogen decarboxylase (URO-D) to the p34 band of chromosome 1.". Hum. Genet. 73 (3): 277–9. doi:10.1007/BF00401245. PMID3460962.
de Verneuil H, Grandchamp B, Beaumont C, et al. (1986). "Uroporphyrinogen decarboxylase structural mutant (Gly281----Glu) in a case of porphyria.". Science. 234 (4777): 732–4. doi:10.1126/science.3775362. PMID3775362.
Roberts AG, Elder GH, De Salamanca RE, et al. (1995). "A mutation (G281E) of the human uroporphyrinogen decarboxylase gene causes both hepatoerythropoietic porphyria and overt familial porphyria cutanea tarda: biochemical and genetic studies on Spanish patients.". J. Invest. Dermatol. 104 (4): 500–2. doi:10.1111/1523-1747.ep12605953. PMID7706766.
Meguro K, Fujita H, Ishida N, et al. (1994). "Molecular defects of uroporphyrinogen decarboxylase in a patient with mild hepatoerythropoietic porphyria". J. Invest. Dermatol. 102 (5): 681–5. doi:10.1111/1523-1747.ep12374134. PMID8176248.
McManus JF, Begley CG, Sassa S, Ratnaike S (1996). "Five new mutations in the uroporphyrinogen decarboxylase gene identified in families with cutaneous porphyria". Blood. 88 (9): 3589–600. PMID8896428.
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, et al. (1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene. 200 (1–2): 149–56. doi:10.1016/S0378-1119(97)00411-3. PMID9373149.