Tyrosine B10 Inhibits Stabilization of Bound Carbon Monoxide and Oxygen in Soybean Leghemoglobin

Authors

Suman Kundu, Iowa State UniVersity, Ames, Iowa
George C. Blouin, Rice UniVersity, Houston, Texas
Scott A. Premer
Gautam Sarath, University of Nebraska-LincolnFollow
John S. Olson, Rice UniversityFollow
Mark S. Hargrove, Iowa State UniversityFollow

Document Type

Article

Date of this Version

5-12-2004

Comments

Published in Biochemistry 2004, 43, 6241-6252.

Abstract

Detailed comparisons of the carbon monoxide FTIR spectra and ligand-binding properties of a library of E7, E11, and B10 mutants indicate significant differences in the role of electrostatic interactions in the distal pockets of wild-type sperm whale myoglobin and soybean leghemoglobin. In myoglobin, strong hydrogen bonds from several closely related conformations of the distal histidine (His^E7) side chain preferentially stabilize bound oxygen. In leghemoglobin, the imidazole side chain of His^E7 is confined to a single conformation, which only weakly hydrogen bonds to bound ligands. The phenol side chain of TyrB10 appears to “fix” the position of His^E7, probably by donating a hydrogen bond to the Nä atom of the imidazole side chain. The proximal pocket of leghemoglobin is designed to favor strong coordination bonds between the heme iron and axial ligands. Thus, high oxygen affinity in leghemoglobin is established by a favorable staggered geometry of the proximal histidine. The interaction between His^E7 and Tyr^B10 prevents overstabilization of bound oxygen. If hydrogen bonding from His^E7 were as strong as it is in mammalian myoglobin, the resultant ultrahigh affinity of leghemoglobin would prevent oxygen transport in root nodules.