Cooperative phenomena in polynuclear metalloproteins

Abstract

Polynuclear metalloproteins are rather numerous in nature, and these systems still now constitute a formidable puzzle for biochemists and chemists in the comprehension of their spectroscopic and catalytic properties. Probably most of the present interest in the field of coordination chemistry toward the synthesis and theoretical studies of simple inorganic polynuclear complexes arises from the necessity of finding reliable models for explaining the spectroscopic properties of the more complex, naturally occurring, polynuclear systems. The metals present in a metalloprotein can play different roles, and for this reason they have been generally distinguished as (a) structural, or (b) catalytic or functional. This obviously represents a rather naive classification, because the situation is more complex. For example, as will be shown later, each catalytic ion plays also a structural role and in many cases structural ions can also influence the catalytic efficiency of the enzyme. It is aim of this paper to illustrate with reference to some selected polynuclear metalloproteins the structure and the location of the metal binding sites, as well as to point out the role of the different metals and the mutual influences between the various centers. I have selected two systems to discuss here: bovine superoxide dismutase, and the blue copper oxidases. The first represents an example in which both functional and structural ions are present, whereas the blue copper oxidases represent systems in which different metal sites all concur in a cooperative fashion to the catalytic reaction.