A density functional theory study of conformers in the ferrous CO complex of horseradish peroxidase with distinct Fe-C-O configurations

DALOSTO, S.D., PRABHU, N.V. VANDERKOOI, J.M.SHOW AUTHOR DETAILS, SHARP, K.A.

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Año: 2003 vol. 107 p. 1884 - 1892

1520-6106

The reactive site structure and CO IR spectral signature of horseradish peroxidase complexed with CO were studied using a combination of density functional theory and molecular modeling. The quantum chemical model used included the proximal His170, the distal amino acids His42 and Arg38, and the crystallographic water molecule that interacts with CO, His42, and Arg38. The mutant H42L was simulated by removing distal histidine from the cluster and the mutant R38L by removing the arginine. In the models where the Arg38 is present a hydrogen bond interaction is found between the Hε of Arg and the oxygen of adduct-CO. When both His42 and Arg38 are present they interact with the CO moiety, but they also interact with each other so their effect is not simply additive. The effect of a change in pH was modeled by removing potentially titratable hydrogen atoms identified by molecular mechanics and Poisson-Boltzmann pKa calculations on the active site. Previously observed IR spectral features of CO as a function of pH and active site mutations could be interpreted primarily in terms of changes in CO and Fe-CO bond lengths produced by changes in active site composition and charge state. The effect of the charge relay due to the possible second deprotonation (assuming that the first deprotonation took place in the His42) from the Arg38 or Hδ in the proximal His170 gives a decrease of the CO stretching frequency.