The structural role of the copper-coordinating and surface-exposed histidine residue in the blue copper protein azurin

Copper K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and N-15 NMR relaxation studies were performed on samples of a variant azu rin in which the surface-exposed histidine ligand of the copper atom (His11 7) has been replaced by glycine. The experiments were performed to probe th e structure of the active site and the protein dynamics. The cavity in the protein structure created by the His --> Gly replacement could be filled by external ligands, which can either restore the spectroscopic properties of the original type-1 copper site or create a new type-2 copper site. The bi nding of external ligands occurs only when the copper atom is in its oxidis ed state. III the reduced form, the binding is abolished. From the EXAFS ex periments, it is concluded that for the oxidised type-1 copper sites the pr otein plus external ligand (L) provide an NSS*L donor set deriving from His 46, Cys112, Met121 and the external ligand. The type-2 copper site features an S(N/O)(3) donor set in which the S-donor derives from Cys112, one N-don or from His46 and the remaining two N or O donors from one or more external ligands. Upon reduction of the type-1 as well as the type-2 site, the exte rnal ligand drops out of the copper site and the coordination reduces to 3- fold with an SS*N donor set deriving from His46, Cys112 and Met121. The Cu- S-delta(Met) distance is reduced from about 3.2 to 2.3 Angstrom. Analysis o f the NMR data shows that the hydrophobic patch around His117 has gained fl uxionality when compared to wild-type azurin, which may explain why the His 117Gly variant is able to accommodate a variety of external ligands of diff erent sizes and with different chelating properties. On the other hand, the structure and dynamics of the beta-sandwich, which comprises the main body of the protein, is only slightly affected by the mutation. The unusually h igh reduction potential of the His117Gly azurin is discussed in light of th e present results. (C) 2000 Academic Press.