A Density Functional Study on Reaction Center Models of Horse Heart Carbonmonoxy Myoglobin
- Effect of Distal Histidine to the Electronic States -

Kouji CHIBAa, Toshiyuki HIRANOb, Fumitoshi SATOb* and Masahiro OKAMOTOa

aGraduate School of Systems Life Science, Kyushu University
6-10-1 Hakozaki, Fukuoka, 812-8581, Japan
bInstitute of Industrial Science, the University of Tokyo
4-6-1 Komaba, Meguro, Tokyo, 153-8505, Japan

(Received: July 1, 2008; Accepted for publication: August 18, 2008; Advance publication: September 24, 2008)

Density functional calculations were carried out on some reaction center models of horse heart carbonmonoxy myoglobin (MbCO) to investigate, from a quantum chemical perspective, the relationship between the heme-ligand moiety and the imidazole hydrogen in the distal histidine. Compared to the d positioned model, the e positioned one, in which the hydrogen is close to the carbonyl (CO) ligand, produced positive electrostatic potential around the heme Fe-CO moiety, and stabilized the energy levels of the highest occupied states originated in heme Fe. The difference in the hydrogen position also influenced the CO, and the amount of the difference in Mulliken charge on the CO carbon was 0.016. The interaction between the distal histidine and heme-ligand in MbCO was remarkable, and it was suggested that quantum chemical study to take distal histidine into consideration is indispensable in the representation of the electronic structure of the MbCO reaction center.

Keywords: Carbonmonoxy myoglobin, Density functional calculations, Distal HIS, Hydrogenation sites

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