The enzymatic catalysis of many biological processes of life is supported by the presence of cofactors and prosthetic groups originating from the common tetrapyrrole precursor uroporphyrinogen-III. Uroporphyrinogen-III decarboxylase catalyzes its conversion into coproporphyrinogen-III, leading in plants to chlorophyll and heme biosynthesis. Here we report the first crystal structure of a plant (Nicotiana tabacum) uroporphyrinogen-III decarboxylase, together with the molecular modeling of substrate binding in tobacco and human enzymes. Its structural comparison with the homologous human protein reveals a similar catalytic cleft with six invariant polar residues, Arg(32), Arg(36), Asp(82), Ser(214) (Thr in Escherichia coli), Tyr(159), and His(329) (tobacco numbering). The functional relationships obtained from the structural and modeling analyses of both enzymes allowed the proposal for a refined catalytic mechanism. Asp(82) and Tyr(159) seem to be the catalytic functional groups, whereas the other residues may serve in substrate recognition and binding, with Arg(32) steering its insertion. The crystallographic dimer appears to represent the protein dimer under physiological conditions. The dimeric arrangement offers a plausible mechanism at least for the first two (out of four) decarboxylation steps.