SOLUTION STRUCTURE OF REDUCED PLASTOCYANIN FROM THE BLUE-GREEN-ALGA ANABAENA-VARIABILIS

The three-dimensional solution structure of plastocyanin from Anabaena variabilis (A.v. PCu) has been determined by nuclear magnetic resonan ce spectroscopy. Sixty structures were calculated by distance geometry from 1141 distance restraints and 46 dihedral angle restraints. The d istance geometry structures were optimized by simulated annealing and restrained energy minimization. The average rms deviation from the mea n structure for the 20 structures with the lowest total energy is 1.25 Angstrom for the backbone atoms and 1.75 Angstrom for all heavy atoms . Overall, the global tertiary fold of A. v. PCu resembles those of ot her plastocyanins which have been structurally characterized by X-ray diffraction and NMR methods. This holds even though A.v. PCu is longer than any other known plastocyanins, contains far less invariant amino acid residues, and has an overall charge that differs considerably fr om those of other plastocyanins (+1 vs -9 +/- 1 at pH greater than or equal to 7). The most striking feature of the A.v. PCu structure is th e absence of the beta-turn, formed at the remote site by residues (58) -(61) in most higher plant plastocyanins. The displacement caused by t he absence of this turn is compensated for by an extension of the smal l helix [from Ala53(51) to Ser60(58) in A.v. PCu] found in other plast ocyanins. Moreover, the extra residues of A.v. PCu from Pro77 to Asp79 form an appended loop. These two features allow A.v. PCu to retain al most the same global fold as observed in other plastocyanins. From a c omparison with the structures of other plastocyanins it is concluded t hat the lack of negatively charged residues at the remote site, rather than the specific structure of A.v. PCu, is the main reason for the f ailure of the remote site of this plastocyanin to function as a signif icant electron transfer site.