K. Ishikawa et al., IDENTIFICATION OF HISTIDINE-45 AS THE AXIAL HEME IRON LIGAND OF HEME OXYGENASE-2, The Journal of biological chemistry, 273(8), 1998, pp. 4317-4322
A truncated, soluble, and enzymatically active form of human heme oxyg
enase-2 (Delta HHO2) was expressed in Escherichia coli, To identify th
e axial heme ligand of HO-2, His-45 to Ala (Delta H45A) and His-152 to
Ala (Delta H152A) mutants have been prepared using this expression sy
stem. Delta H45A could form a 1:1 complex with hemin but was completel
y devoid of the heme degradation activity, A 5-coordinate-type ferrous
NO EPR spectrum was ob served for the heme-Delta H45A complex, The De
lta H152A mutant was expressed as an inclusion body and was recovered
from the lysis pellet by dissolution in urea followed by dialysis. The
solubilized fraction obtained, however, was composed of a mixture of
a functional enzyme and an inactive fraction. The inactive fraction wa
s removed by Sephadex G-75 column chromatography since it eluted out o
f the column at the void volume, The gel filtration-purified Delta H15
2A exhibited spectroscopic and enzymatic properties identical to those
of wild-type. We conclude, in contrast to the previous reports (McCou
brey and Maines (1993) Arch. Biochem. Biophys. 302, 402-408; McCoubrey
, W, K., Jr., Huang, T, J., and Maines, M. (1997) J. Biol, Chem. 272,
12568-12574), that His 45, but not His-152, in heme oxygenase isoform-
a is the proximal ligand of the heme and is essential for the heme deg
radation activity of the enzyme, His-152 appears to play a structural
role in stabilization of the heme oxygenase protein.