MUTANT MET121ALA OF PSEUDOMONAS-AERUGINOSA AZURIN AND ITS AZIDE DERIVATIVE - CRYSTAL-STRUCTURES AND SPECTRAL PROPERTIES

The crystal structures of the azurin mutant Met121Ala and its azide de rivative Met121Ala-azide from Pseudomonas aeruginosa have been determi ned. The final crystallographic R values are 21.3 and 19.4% for the tw o structures, respectively. In the Met121Ala mutant, the distance betw een the copper ion and His117 increases by 0.34 Angstrom compared with the wild-type structure. The removal of the methionine in the apical position induces a shortening of the distance from the copper ion to t he carbonyl O atom of Gly45 from 2.97 to 2.74 Angstrom. In the Met121A la-azide structure, the azide anion occupies the cavity created by rep lacing the Met121 side chain with the smaller methyl group of Ala. The azide anion binds with a terminal N atom to the copper ion at a dista nce of about 2.04 Angstrom. In addition, the copper ion has moved out of the trigonal plane by about 0.26 Angstrom towards the azide anion. Thus, the copper site in this structure has a distorted tetrahedral ar rangement. The spectroscopic characteristics show, in addition, that t he copper sites in the two structures are distinctively different. The Met121Ala mutant still maintains the properties of an ordinary type 1 copper site while the Met121Ala-azide derivative has an absorption ma ximum at about 409 nm and the copper hyperfine coupling has increased to a value intermediate between those of type 2 copper and the wild-ty pe azurin.