The charge transfer band in horseradish peroxidase correlates with heme in-plane distortions induced by calcium removal

Horseradish peroxidase C (HRPC) is a class III peroxidase whose structure is stabilized by the presence of two endogenous calcium atoms. Calcium removal has been shown to decrease the enzymatic activity of the enzyme. The spin state of the iron, a mixture of high spin (HS) and mixed quantum spin state (QS) consisting of intermediate spin (IS) 3/2 + (HS) 5/2, is also significantly affected by calcium removal, going from a predominant QS component to a predominant HS component upon removal of one calcium. Removal of both calcium ions, however, results in the appearance of a significant LS contribution, easily monitored in the charge transfer (CT) band region by low-T absorption. Normal structural decomposition (NSD) calculations of the in-plane (ip) modes of the heme extracted from HRPC native and Ca^2+-depleted models show that removal of the proximal calcium is associated with perturbed Eu and increased A1g ip distortions of the heme. The effect of complete or distal calcium removal on the heme also results in increased A1g ip distortions, but in significantly decreased Eu distortions. The overall effect is to decrease the nonplanarity of the heme: the total ip distortion of the native HRPC heme is 0.200 and 0.134 ? for the Ca^2+-depleted species. Our NSD results corroborate the role proposed for the protein matrix, namely to fine-tune the active site by inducing subtle changes in heme planarity and spin state of the iron.