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Structure and Dynamics of Dioxygen Bound to Cobalt and Iron Heme

Ivan Degtyarenko ^*, Risto M. Nieminen ^* and Carme Rovira 

^* Laboratory of Physics, Helsinki University of Technology, FIN-02015 HUT, Finland; and Centre de Recerca en Química Teòrica, Parc Científic de Barcelona, 08028 Barcelona, Spain

Correspondence: Address reprint requests to Carme Rovira, Centre de Recerca en Química Teòrica, Parc Científic de Barcelona, Josep Samitier 1-5, 08028 Barcelona, Spain. Tel.: 34-93-4037112; Fax: 34-93-4037225; E-mail: crovira{at}pcb.ub.es.

In this study we use ab initio molecular dynamics simulations to analyze the structure and dynamics of the oxygen ligand in models of the oxymyoglobin active site and its cobalt-substituted analog. Our calculations are performed for iron-porphyrin and cobalt-porphyrin complexes with imidazole and oxygen as axial ligands, and we investigate the effect of the distal histidine in the structure and dynamics of the metal-oxygen unit (MeO₂, Me = Fe, Co). We find that the interaction between the distal histidine and the oxygen ligand is stronger for the cobalt complex than for the iron one, consistent with the superoxide ion character of the bound O₂. The dynamics of the O₂ ligand can be described as oscillations of the O-O axis projection on the porphyrin plane within a porphyrin quadrant combined with frequent jumps from one quadrant to another. However, the ligand motion is significantly faster for CoO₂ compared to FeO₂. As a result, the iron complex shows localized ligand sites, whereas for cobalt several configurations are possible. This gives support to the highly dynamic motion of the oxygen ligand found in several experiments on cobalt oxymyoglobin and model complexes and underlines the higher mobility of the CoO₂ fragment compared to FeO₂.