Molecular Mechanisms of Extremophile Photosystems

Abstract

This thesis describes a spectroscopic study of two photosystems expressed by organisms which inhabit some of the most inhospitable environments to phototrophic life. The first is photosystem I (PSI) of Chroococcidiopsis thermalis, which has evolved functionality in the absence of light with sufficient energy to drive conventional photosynthesis. The second is photosystem II (PSII) of Cyanidioschyzon merolae, which retains unusually high functionality in acidic environments. The work presented here aims to elucidate the molecular mechanisms responsible for the two "extremophile" adaptions. We also exploit the acid tolerance of the PSII complex to probe the sequence of events in the late stages of biological water splitiing. Key results are summarised below.

Far-Red-Light Adapted (FRL) PSI: 1. Photochemical activity of FRL-PSI illuminated with >775 nm light is driven by a charge transfer (CT) band associated with a reaction centre chl pair. 2. The properties of this photochemically active CT band are broadly consistent with a canonical chl a to chl a CT transition of conventional (canonical) PSI complexes. The band has previously been shown to drive long-wavelength photochemical activity of canonical photosystems, but with negligible photochemical efficiency. 3. Photochemical activity of FRL-PSI is approximately four orders of magnitude more efficient than in canonical PSI when illuminated with >775 nm and at 2 K. We assign the increased functionality to enhanced photochemical activity of the FRL-PSI CT band. 4. The oscillator strength of the CT band is approximately ten times more intense in FRL-PSI relative to canonical PSI. 5. The cryogenic quantum efficiency of photochemical activity of the CT band increased from <1% to >10%. 6. Excitation energy is transferred from the lowest energy chl antenna pigments to the CT band at 1.8 K. The quantum efficiency of this EET is >95% at physiological temperatures. 7. A series of previously identified chl f pigments possess electrochromic shift features which preclude their assignment to redox active (reaction centre - RC) pigments. These results demonstrate that FRL adaptation of the RC is confined to the chl pair which gives rise to the CT band of canonical PSI. This, together with modification of the inner light harvesting chls accounts for the enhanced FRL activity. Importantly energy transfer between the antenna pigments and the CT band can account for FRL functionality without the need for a redox active FRL chl f.

Acidophilic PSII: 1. The electronic structure of the oxygen evolving complex (OEC) in EPR active states (S2 and S3) is independent of pH in the 4.1 to 6.1 range, and the cofactor remains intact. 2. The S2 to S3 state transition yield decreases with pH from pH 6.1 to 4.1. 3. A late catalytic intermediate (S3' state) of the (OEC) is identical to that of canonical PSII. These results demonstrate that the protective mechanism of the acidophilic PSII is not caused by modification of the metal cofactor. However, an overall pH resilience of the system allows us to measure the S3 state at low pH. These measurements demonstrate that the S3 state is not readily protonated, and that pH effects of Sr insertion are not responsible for isolation of a proposed water deficient S2 to S3 state intermediate (S3'). They further support a consensus that OEC deprotonation occurs before water insertion and cofactor oxidation in the S2 to S3 transition. Show more