Hydrogen bonding plastocyanins

Negative values for redox couple entropy have also been obtained for the Cu(II)/Cu(I) reduction, in aqueous medium, of the blue copper proteins stellacyanin, plastocyanin and azurin.14 The decrease in molecular disorder has been attributed in this case to the fact that the charge neutralization of the redox site (from + 1 to 0) favours the formation of hydrogen bonds between the solvent (water) and the copper centre.17... 

Two features of the NMR structure present relevant new information. Phe-82 and Phe-29 are tyrosine and tryptophan, respectively, in the algal protein, and are apparently now, according to NMR, hydrogen bonded (internally) to each other. The deletion of the two residues 57 and 58 leads to an alteration of the shape of the acidic patch, a step back to the absence of the patch in evolutionarily more primitive plastocyanin. 

The structure of amicyanin (99-106 residues) also more closely resembles that of plastocyanin than azurin as shown by both sequence comparisons and the crystal-stracture data. It has short connecting loops and no flap . Strand 5 is shorter and more regular than that in plastocyanin. Amicyanin, however, has an extension at the N-terminal end of the polypeptide chain (rather than the C-terminus as in pseudoazurin) the extra 20 amino acids form a ninth /3-strand which extends the /3-barrel by hydrogen bonding to the exposed edge of one of the two /3-sheets (i.e. to strand 6 in Figure 3). 

The conserved hydrogen bond network in cupredoxins is believed to play a role in maintaining the geometry of the copper center and in determining the relative constraints of the Cu and Cir sites, which in turn influences the relative stability of the two states of the protein and, ultimately, the reduction potential. " Indeed, mutations of the Asn in the extra-loop H-bond (i.e., the Asn next to the N-terminal His ligand) in both azurin and plastocyanin resulted in reduced stability and, in the case of azurin, an increase of the reduction potential from 286 mV for wild-type azurin to 396 mV for the variant protein. In rusticyanin, the Asn residue in the extra-loop H-bond is replaced with a Ser residue.Mutation of this Ser in rusticyanin resulted in reduced stability and a decrease of reduction potential by 110 mV. Based on these results, the authors... 

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