The combining power of myoglobin for alkyl isocyanides and the structure of the myoglobin molecule

Abstract

It was found by St. George and Pauling(1) that, although the combining powers of ferroheme with ethyl isocyanide, isopropyl isocyanide, and tertiary butyl isocyanide are essentially the same (the extreme values of the equilibrium constants differing by only a factor of 3), hemoglobin combines far more strongly with ethyl isocyanide than with t-butyl isocyanide, corresponding to a factor of 200 in the equilibrium constants, with isopropyl isocyanide having an intermediate value. These facts led St. George and Pauling to the conclusion that there is steric hindrance between the alkyl groups of the isocyanides and a part of the globin in the hemoglobin molecule. Inasmuch as it is known that the heme group is attached to globin by way of an imidazole ring of a histidine side chain of the globin on the side opposite that to which the alkyl isocyanide molecule is attached, it was concluded that the heme groups of hemoglobin are buried within the protein molecule.