Plastocyanin conformation. An analysis of its near ultraviolet absorption and circular dichroic spectra

Abstract

The near-ultraviolet absorption and circular dichroic spectra of plastocyanin are dependent upon the redox state, solution pH, and ammonium sulfate concentration. This dependency was observed in plastocyanin isolated from spinach, poplar, and lettuce. Removal of the copper atom also perturbed the near-ultraviolet spectra. Upon reduction there are increases in both extinction and ellipticity at 252 nm. Further increases at 252 nm were observed upon formation of apo plastocyanin eliminating charge transfer transitions as the cause. The spectral changes in the near-ultraviolet imply a flexible tertiary conformation for plastocyanin. There are at least two charge transfer transitions at approximately 295-340 nm. One of these transitions is sensitive to low pH's and is attributed to the His 87 copper ligand. The redox state dependent changes observed in the near-ultraviolet spectra of plastocyanin are attenuated either by decreasing the pH to 5 or by increasing the ammonium sulfate concentration to 2.7 M. This attenuation cannot be easily explained by simple charge screening. Hydrophobic interactions probably play an important role in this phenomenon. The pH and redox state dependent conformational changes may play an important role in regulating electron transport.

pH dependent conformational changes and electrostatic effects in plastocyanin

“…Lowering the pH also affects these two regions. The changes are most dramatic for reduced PC which is consistent with the observation that lowering the pH affects the activity of reduced PC to a much greater extent than oxidized PC (18,28). The changes ca.…”

“…The pH dependence of PC conformation is important both because PC is located within the intrathylakoid space and must experience large changes in pH during light-dark cycles and also because pH has been shown to affect the activity of reduced PC (18,28). Near-UV circular dichroism (CD) was used to study the effect of pH on PC conformation (Fig.…”

“…That between 270 and 285 nm is attributed to tyrosine residues. The region above 285 nm may be due to charge transfer bands possibly associated with histidine residues (28). Changing the oxidation state of PC primarily affects the phenylalanine and charge tranfer regions (22,29).…”

“…m Chemical modification of the "east face" also alters the pH dependence of the reduction potential. Control PC shows the positive shift in reduction potential upon lowering the pH associated with inactivation of reduced PC (18,28). In contrast, the reduction potential of modified PC is independent of pH to below pH 5.…”

Degradation of Plastocyanin in Copper-deficient Chlamydomonas reinhardtii

“…Nevertheless, in vitro folding studies revealed that although apoplastocyanin was capable of folding in the absence of copper, the folded structure was not stable in solution in the absence of high concentrations of salt (Koide et al, 1993). Our preparation of apoplastocyanin was therefore characterized by CD spectroscopy for the diagnostic ␤-strand spectrum (Draheim et al, 1986) (Fig. 2).…”