Evolution of proteins formed by β-sheets. I. Plastocyanin and azurin

Plastocyanin and azurin form a family of small copper-containing proteins, active in the electron transport systems of plants and bacteria, respectively. The crystal structures of two members of this family have been determined: poplar leaf plastocyanin and Pseudomonas aeruginosa azurin. Both proteins contain two β-sheets, packed face-to-face. Using computed superpositions of the structures, we have aligned the sequences, identified homologous positions, and studied how the structures have changed as a result of mutations. The residues in the vicinity of the copper-binding site show minimal amino acid substitution and form almost identical structures. Other portions of these proteins are more variable in sequence and in structure. Buried residues tend to maintain their hydrophobic character, but mutations change their volume. The mean variation in volume of homologous buried residues is 54 ų. The differences in size and shape of these buried residues are accommodated by a 3.8 Å shift in relative position of the packed β-sheets. This shift does not affect the copper binding site, because the residues that form this site are in, or adjacent to, just one of the β-sheets.