Kinetic and spectroscopic studies of the redox centers of photosystem II

Abstract

The work conducted in this thesis aimed to elucidate the function of key components of the photosystem II (PS II) enzyme. Several experimental methodologies were employed, allowing a complete snap-shot of the enzyme in a particular meta-stable state. Particular emphasis was placed on resolving processes that occur within the PS II enzyme at redox cofactors involved in electron transfer. Topics addressed in this thesis include the following: Protein mediated electron transfer between the light active pigments (P680) and the metal catalytic site - the oxidation of the redox active tyrosine D116 (Yz) of the D1 polypeptide. It was shown that the Yz{u2022} weakly interacts (dipolar only) with the S{u2081} state Mn cluster of the OEC. A model for water molecule mediated H-bond bridging between the Mn cluster and Yz{u2022} is discussed. This model rationalizes both observed substrate water exchange rates and the patterns of 'split signal' ESR line-shapes seen in the various S states. The low temperature carotenoid donor of PS II - associated with photo-protection - is heterogeneous. A significant fraction exhibits weak magnetic interaction with its surrounding environment -possibly with the ferr-cytochrome b559, ~12 {u00C5} away. From the latest PS II structural data, this implies the major carotenoid donor is that closest to P680, CarD. Comparison with earlier results on ca{u207A} radicals in PS II and in vitro suggest that CarD{u207A} may have two conformational states, corresponding to a localized or de-localised charge distribution along the chain. The electronic structure of the primary plasto-quinone acceptor (QA) of PS II. The ESR spectrum of the native semi-quinone iron signal was simulated using the spin Hamiltonian formalism. By comparison with density functional theory calculations, the native exogenous ligand to the non-heme iron of PS II is suggested to be carbonate. Its role in proton mediation to QB is discussed. The nature of the primary electron acceptor( pheoD1) of PS II. The Q{u03B3} excitation of the pheosD1, was shown to be dominantly at 683 nm (in spinach). Its coupling to the neighboring accessory chlorophyll (chID1) is estimated to be small (< 10 cm{u207B}{u00B9}). The kinetics of photo-assembly of the inorganic core -the site of water oxidation. A Markov analysis was used to interpret the mechanism of photo-activation. By varying the turnover (photo-induced advancement) conditions used to photo-assemble the apo-WOC-PS II inorganic core, the complete kinetics of photo-assembly were deduced. Show more