The structures are aligned using the software SSM at EBI http://www.ebi.ac.uk/msd-srv/ssm. The values calculated considering only the aligned amino acid residues are given in parenthesis. To quantify the sequence structure compatibility between the structures, two more parameters are computed over the aligned residues. (Rs)azu/pcy = 0.97 and (Rs)pcy/azu = 0.89. Each term contributing to the former corresponds to (ASAazu - <ASA>pcy)/<ASA>pcy, i.e., in Eq. (3) the observed value at a given position in the structure of azurin is compared to the average value corresponding to the aligned residue type at the same position in the sequence of plastocyanin. The opposite is done in the calculation of (Rs)pcy/azu.