Bis-histidyl hexacoordination in hemoglobins facilitates heme reduction kinetics.
Hexacoordinate hemoglobins are a class of proteins that exhibit reversible bis-histidyl coordination of the heme iron while retaining the ability to bind exogenous ligands. One hypothesis for their physiological function is that they scavenge nitric oxide, a reaction that oxidizes the protein and requires reduction of the heme iron to continue. Reduction kinetics of hexacoordinate hemoglobins, including human neuroglobin and cytoglobin, and those from Synechocystis and rice, are compared to myoglobin, soybean leghemoglobin, and several relevant mutant proteins. In all cases, bis-histidyl coordination greatly increases the rate of reduction by sodium dithionite when compared to pentacoordinate hemoglobins. In myoglobin and leghemoglobin, reduction is limited by the rate constant for electron transfer, whereas in the hexacoordinate hemoglobins reduction is limited only by bimolecular binding of the reductant. These results can be explained by differences in the reorganization energy for reduction between hexacoordinate and pentacoordinate hemoglobins.
Weiland
Theodore R
TR
Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011, USA.
Kundu
Suman
S
Trent
James T
JT
Hoy
Julie A
JA
Hargrove
Mark S
MS
D000818:Animals; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D000458:Cyanobacteria; D006418:Heme / Q000737:chemistry* / Q000378:metabolism*; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism*; D006639:Histidine / Q000737:chemistry / Q000378:metabolism; D006736:Horses; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D013329:Structure-Activity Relationship; D014907:Whales
D001426:Bacterial Proteins; D006454:Hemoglobins; D008024:Ligands; D010940:Plant Proteins; D006418:Heme; D006639:Histidine