Myoglobin histidine mutants

Roncone R, Monzani E, Murtas M et al (2004) Engineering peroxidase activity in myoglobin the haem cavity structure and peroxide activation in the T67R/S92D mutant and its derivative reconstituted with protohaemin-l-histidine. Biochem J 377 717-724... 

In this context, the control of O2 affinity by the distal environment of the heme group in hemoproteins can be probed by comparison of wild-type compounds with mutants which do not contain distal histidine [105]. Replacement of distal histidine by glycine results in a 10-fold decrease in O2 affinity. Within the hemoproteins themselves, the lack of distal histidine in aplysia myoblobin compared to horse myoglobin increases the dissociation rate constant by a factor of six, whereas the association rate constants are identical for both systems. The data are again consistent with the stabilization of bound O2 via hydrogen-bonding in horse myoglobin. 

In addition, this mutant myoglobin autooxidizes rapidly compared to the native one. On the other hand, the affinity for CO is greatly increased, leading to a value of M for the mutant of 1300, compared to 16 for the native. Thus the distal histidine stabilizes a coordinated O2 ligand by hydrogen bonding and destabilizes a coordinated CO ligand by steric clash. 

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