Environment effects on chemical reactivity of heme proteins

Heme proteins are involved in a variety of physiological processes, such as O2 transport, electron transfer, sensing of O2 or CO, and catalysis of redox reactions. Despite the differences in biologic function, all these proteins have iron protoporphyrin IX (heme b) as the active site. The amino acids surrounding the active site are responsible for the specific reactivity of each protein. We analyzed the environment effects on binding of small ligands such as O2 and NO to several heme proteins using density functional theory (DFT) calculations of model systems including selected amino acid residues, and also DFT calculations of the active site coupled to an electrostatic representation of the rest of the protein. Specifically, we considered the following problems: (1) the mechanisms underlying inactivation by nitric oxide of cytochrome P450; (2) O2 affinity of human and Ascaris hemoglobin and the role of oxygen hydrogen bonding to the distal amino acids; (3) the influence of the amino acid residues surrounding the proximal histidine in the Fehistidine bond cleavage upon binding of NO in FixL, horseradish peroxidase, and human hemoglobin. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002