Distal ligand

Distal ligand None closer than 4 A H2O at 4.22A None closer than 4 A... 

Recent work has resolved some of the issues that complicate direct electrochemistry of myoglobin, and, in fact, it has been demonstrated that Mb can interact effectively with a suitable electrode surface (103-113). This achievement has permitted the investigation of more complex aspects of Mb oxidation-reduction behavior (e.g., 106). In general, it appears that the primary difficulty in performing direct electrochemistry of myoglobin results from the change in coordination number that accompanies conversion of metMb (six-coordinate) to reduced (deoxy) Mb (five-coordinate) and the concomitant dissociation of the water molecule (or hydroxide at alkaline pH) that provides the distal ligand to the heme iron of metMb. 

Electrochemistry of Variants with an Altered Distal Ligand... 

The H64 distal ligand of wild-type myoglobin does not coordinate to the heme iron in either the reduced or the oxidized form of the native protein but stabilizes the coordination of a distally boimd water molecule of metMb. Replacement of H64 with other amino acid residues can, therefore, change the coordination environment of the heme iron in two ways. Such variants either may possess a distal residue that is able to coordinate to the heme iron or may possess a distal residue that is incapable of either coordinating to the iron or of forming a hydrogen bond with a coordinated water molecule. 

Distal ligand proposed as arising from ligation of His64. 

Miller, S. A., Metallocene-Mediated Olefin Polymerization The Effects of Distal Ligand Perturbations on Polymer Stereochemistry, PhD. Thesis, California Institute of Technology, Pasadena, 2000. 

B. Identity of a Distal Ligand Selective Modifications of THE Apoprotein... 

The distal ligand has also been proposed to be a histidine imidazole nitrogen based on MCD spectroscopy, as found in hemoglobin and myglobin. ... 

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