States of amino acid residues in proteins. XXII. Effect of cyanide on the reactivities of histidine and tyrosine residues in ferrihemoglobin and ferrimyoglobin.

Abstract

The reactivities of histidine and tyrosine residues in native structures of horse ferrihemogolobin and ferrimyoglobin and those in cyano derivatives and apoproteins of these hemoproteins were studied with diazonium-1-H-tetrazole (DHT) as the reagent, and the following facts were revealed. Approximately 28 out of the total 38 histidine residues and about 8 out of the total 12 tyrosine residues in the horse ferrihemoglobin molecule were bisazotized with this reagent, and addition of cyanide reduced the reactivities of some of these residues. The reduction was clearly observed at medium DHT concentrations, where the difference found in the degree of biscoupling between ferrihemoglobin and cyanoferrihemoglobin was 7.4 moles for histidine and 2.6 moles for tyrosine per mole of protein. In the case of ferrimyoglobin, 8—9 out of the total 11 histidine residues and both of the total two tyrosine residues were bisazotized, and addition of cyanide made one of these 8—9 histidine less reactive, but did not affect the reactivity of the two tyrosine residues. The reactivities of the histidine and tyrosine residues in apohemoglobin and in apomyoglobin were not much different from those of the residues in native hemoproteins, and cyanide did not affect their reactivities. This bears evidence that cyanide changes the reactivities through interaction with the heme group. A greater conformational change of ferrihemoglobin by the effect of cyanide as compared with that of ferrimyoglobin was deduced from these results.