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Figure 1.
Figure 1 Reaction catalyzed by URO-D and the two preceding steps
in the heme biosynthesis pathway. Acetate, propionate and methyl
side-chains are denoted A, P and M, respectively.
Porphobilinogen deaminase (PBG-D), uroporphyrinogen III synthase
(U3S). In the absence of U3S, hydroxymethylbilane cyclizes
without inversion of the D-ring, to form uroporphyrinogen I,
which differs from the III-isomer shown here by having an
identical arrangement of A/P substituents on all four pyrrole
rings. Figures 1 and 4 were generated using ChemDraw Pro
(CambridgeSoft Corp., Cambridge, MA).
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