Factor Analysis of the Near-ultraviolet Absorption Spectrum of Plastocyanin Using Bilinear, Trilinear, and Quadrilinear Models

Factor analysis was used to resolve the spectral components in the near-uv absorption spectrum of plastocyanin. The data set was absorption as a function of four variables: wavelength, species of plastocyanin, oxidation state of the copper center, and environmental pH. The data were fit with the traditional bilinear model, as well as with trilinear and quadrilinear models. Trilinear and quadrilinear models have the advantage that they uniquely define the components, avoiding the indeterminacy of bilinear models. Bilinear analysis using the absorption spectra of tyrosine and copper metallothionein as targets resulted in a two-component solution which was nearly identical to that obtained using trilinear and quadrilinear models, for which no targets are required. The two-component models separate the absorption into tyrosine and copper center components. The absorption of tyrosine is found to be pH dependent in reduced plastocyanin, and the absorption magnitude of the reduced copper center is the same in the four different plastocyanin species. Further resolution is provided by a three-component quadrilinear model. The results indicate that there are at least two different electronic transitions which cause the absorption of the reduced copper center and that one of them couples to a tyrosine residue. It is the absorption of this coupled tyrosine residue which is pH dependent. Correlation of the results with previous studies indicates that it is Tyr 83 which is the perturbed residue. The separation of the absorption of the copper center and Tyr 83 provides spectroscopic probes for the conformations of the north pole and east face reaction sites on the plastocyanin protein.