Myoglobin proximal histidine

Adachi, S., S. Nagano, Y. Watanabe, K. Ishimori, and 1. Morishima (1991). Alteration of human myoglobin proximal histidine to cysteine or tyrosine by site-directed mutagenesis Characterization and their catalytic activities. Biochem. Biophys. Res. Commun. 180, 138-144. 

The reasons for the difference between the spectra of ferryl myoglobin at acid and alkaline pH are not clear. However, it has been suggested that deprotonation of the proximal histidine ligand at alkaline pH may be responsible [175,176], Furthermore a third form of ferryl iron was detected in varying amounts in preparations of horseradish peroxidase compound II, ferryl myoglobin and cytochrome c peroxidase compound I [162], To account for these spectra it was proposed that the iron-histidine bond was broken, leaving a five-coordinate ferryl haem. 

Finally, the proximal histidines of myoglobin and heme oxygenase have also been mutated to tyrosine in order to convert these proteins to a catalase. As expected, replacement of His with Tyr in Mb resulted in a 250 mV decrease in reduction potential because of the negative charge of Tyr. Surprisingly, this mutation had little effect on myoglobin s ability to carry out heterolytic or homolytic 0-0 cleavage. 

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