Crevice Structures in Hemoprotein Reactions.

The hemoproteins are such versatile oxidation-reduction catalysts and such remarkable oxygen-binding pigments that there has always been a lively interest in the structural factors that determine the reactivity of the heme iron atom. The high specificity which characterizes many of their reactions, which is shown in only a rudimentary manner by heme itself, would appear to rule out structures with no direct Fe-protein bonding; so, in general, the binding of the heme can be considered in terms of one or two Fe-protein bonds, with additional bonding through the vinyl and propionic acid side chains. Furthermore, the configuration of the heme is also an important factor. A hemoprotein with the heme bound flat on the surface of the protein would be expected to show differences in reactivity, especially in the case of complex formation with ligands, from one in which the heme is buried in a crevice or fold in the polypeptide chain. In the past the need to consider the influence of structural factors on reactivity, notably crevice structures, has arisen only in studies on cytochrome-c, hemoglobin, and myoglobin. However. in the present paper it will be shown that certain features of the formation of ferriperoxidase and ferricatalase complexes, which set them apart from the other three hemoproteins, can be explained by the simple hypothesis that a crevice structure with two Fe-protein bonds exists in the parent compounds, and, when a complex formation occurs, one of the bonds is broken. liberating a group with a high proton affinity.'