Three-dimensional structure of CotA. (a) Three-dimensional structure of CotA with each cupredoxin domain coloured in a different colour; domain I coloured in blue, domain II coloured in gray and domain III coloured in violet. Copper atoms are represented by spheres coloured in yellow. These correspond to the mononuclear coppers 1 and 5, and the trinuclear centre, comprising coppers 2, 3 and 4. (b) Structural detail of the catalytic copper centres, the mononuclear type 1 copper centre (T1) where the copper atom is coordinated by a cysteine and two histidines, and the trinuclear centre which comprises a type 2 copper atoms (T2) and two type 3 (T3) copper atoms. The cysteine residue (C492) that coordinates the T1 copper atom is bound to two of the histidine residues (H491 and H493) that coordinate the two T3 coppers in the trinuclear centre. This motif has been proposed to constitute the path for transfer electrons from the T1 copper centre to the trinuclear centre. (c) Close view of the CotA trinuclear centre - Important acidic groups are labelled (E498 and D116) as well as the histidine ligands to the copper that establish hydrogen bonds with D116.