Molecular Insight into Intrinsic Heme Distortion in Ligand Binding in Hemoprotein

Abstract

A pair of myoglobins containing inherently distorted alpha-ethyl-2,4-dimethyldeuteroheme or undistorted 2,4-dimethyldeuteroheme were prepared, and the functional consequence of intrinsic heme deformation was investigated. The visible absorption peaks of the myoglobin bearing the distorted heme exhibited a bathochromic shift, indicating that the heme was deformed in the protein pocket. Ligand affinities for the ferric myoglobin with the distorted heme were found to be higher than those of the myoglobin bearing the undistorted heme. The observation suggested that the iron atom was more displaced toward the proximal histidine to weaken the coordination of the water molecule. In the paramagnetic proton NMR spectrum of ferrous deoxy protein, the deformed heme caused a 3.2 ppm lower-field shift of the proximal histidine signal, supporting an enhanced iron-histidine interaction. The deformed heme in ferrous myoglobin lowered the oxygen and carbon monoxide affinities by 25- and 480-fold, respectively, and caused the cleavage of the iron-histidine bond in a fractional population of the nitric oxide derivative. These results demonstrate a distinctive controlling mechanism for ligand binding by the deformed heme. Upon the heme distortion, the iron atom is more attracted by the proximal histidine to reduce the affinity of exogenous ligands for the ferrous heme.

Involvement of Propionate Side Chains of the Heme in Circular Dichroism of Myoglobin: Experimental and Theoretical Analyses

“…The reconstituted Mb with distorted heme 4 showed a very low oxygen affinity. 16 Although the absorption peaks of Mb4 were red-shifted in both the Soret and visible regions compared with Mb3 ( Figure 2 and Table 1), the shape of absorption spectrum was similar to those of Mb3. As shown in the inset of Figure 5, the structure of heme 4 is different from heme 3 only in the ethyl attachment to the α-meso position from heme 3.…”

“…As can be seen, it showed bell-shaped curve. CD intensity of COMb's was obtained in the present work and oxygen affinities of Mb's were from references; Mb1, 19 Mb2, 27 Mb3, 15 Mb4, 16 and Mb6. 18 Mb1 (native, protoheme) shows the highest intensity of the Soret CD band, whereas Mb2 (2,4-diethyl) and Mb3 (2,4-dimethyl) with high oxygen affinity exhibit low CD intensity.…”

“…2,4-Dimethyldeuteroheme (3) was prepared as reported previously. 15 α-ethyl-2,4-dimethyldeuteroheme (4), 16 6,7-diacetateheme (5), 17 6,7-dicarboxyheme (6), 18 etioheme I (7), 19 and meso-tetramethylheme (8) 20 were prepared as described. We expressed each individual heme by number as defined in Figure 1.…”

Modulation of Ligand-Field Parameters by Heme Ruffling in Cytochromes c Revealed by EPR Spectroscopy

“…Heme distortion is proposed to increase the Fe(III)-His bond strength in that work. 36 In contrast, DFT calculations on a Met-His-coordinated heme predict a negligible effect of heme ruffling on Fe(III)-His bond length with a bond length change of less than 0.001 Å upon a change of ruffling of 0.7 Å (see supporting information of ref. 16).…”

Steric bulkiness of pyrrole substituents and the out-of-plane deformations of porphyrins: nickel(II) octaisopropylporphyrin and its meso-nitro derivative

“…During the last two decades a large body of work has Steric bulkiness of pyrrole substituents and the out-of-plane deformations of porphyrins: nickel(II) octaisopropylporphyrin and its meso-nitro derivative shown that non-planar distortions and conformational flexibility of porphyrin chromophores in part determine protein function [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. One way of producing prototypical non-planar porphyrins is by substitution of sterically bulky groups at the periphery of the macrocycle.…”