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Download fileHeme Binding to Porphobilinogen Deaminase from Vibrio cholerae Decelerates the Formation of 1‑Hydroxymethylbilane
journal contribution
posted on 2018-01-23, 00:00 authored by Takeshi Uchida, Takumi Funamizu, Minghao Chen, Yoshikazu Tanaka, Koichiro IshimoriPorphobilinogen
deaminase (PBGD) is an enzyme that catalyzes the
formation of hydroxymethylbilane, a tetrapyrrole intermediate, during
heme biosynthesis through the stepwise polymerization of four molecules
of porphobilinogen. PBGD from Vibrio cholerae was
expressed in Escherichia coli and characterized in
this study. Unexpectedly, spectroscopic measurements revealed that
PBGD bound one equivalent of heme with a dissociation constant of
0.33 ± 0.01 μM. The absorption and resonance Raman spectra
suggested that heme is a mixture of the 5-coordinate and 6-coordinate
hemes. Mutational studies indicated that the 5-coordinate heme possessed
Cys105 as a heme axial ligand, and His227 was coordinated to form
the 6-coordinate heme. Upon heme binding, the deamination activity
decreased by approximately 15%. The crystal structure of PBGD revealed
that His227 was located near Cys105, but the side chain of His227
did not point toward Cys105. The addition of the cyanide ion to heme–PBGD
abolished the effect of heme binding on the enzymatic activity. Therefore,
coordination of His227 to heme appeared to induce reorientation of
the domains containing Cys105, leading to a decrease in the enzymatic
activity. This is the first report indicating that the PBGD activity
is controlled by heme, the final product of heme biosynthesis. This
finding improves our understanding of the mechanism by which heme
biosynthesis is regulated.