Quantifying and Controlling the Efficiency of Photosynthetic Water Oxidation

Abstract

Photosystem II (PSII), the oxygen-evolving reaction center of natural photosynthesis, uses light energy to split water into chemical products that power the planet. This dissertation focuses on revealing the determinants of PSII efficiency at the protein structural level, developing models for quantifying it, and creating unnatural variants that outperform natural ones. First, recent advances in our understanding of the PSII Mn4CaO5 water-oxidizing complex (WOC) are discussed and used to propose a chemical mechanism for water oxidation. Next, a novel mathematical algorithm is introduced for modeling the catalytic cycle of water oxidation as measured by flash induced changes of oxygen yield. This method provides a general mathematical framework for determining the populations of intermediates, and the number and types of inefficiencies responsible for the flash periodicity of these yields for any cycle period. The next chapter discusses the effects of two natural variations in the sequence of the PSII D1 subunit on PSII-WOC efficiency and organismal fitness and growth. The D1 protein is present as at least two isoforms in most cyanobacteria; D1:1 is expressed under low light intensities, while D1:2 is preferentially expressed in high light or stress conditions. D1:2-PSII has a faster WOC turnover frequency than D1:1-PSII at saturating light intensities, but exhibits lower WOC cycling efficiency at low light intensities due to a 40% faster charge recombination rate. These phenotypes translate to actual growth advantages for cells containing D1:1 or D1:2 at low or high light, respectively. Finally, substitutions among the 25 individual amino acids that distinguish D1:1 and D1:2 were made in order to determine how they contribute to the observed phenotypes. A comparative analysis of nine point mutants studied shows a systematic linear inverse correlation between WOC cycling efficiency (charge recombination rate) and PSII quantum yield (charge separation). Additionally, the upper turnover frequency of the WOC and tolerance to photoinactivation both improve linearly with PSII quantum yield. These insights provide fundamental design principles for engineering PSII reaction centers with optimal photochemical efficiencies at low or high light intensities.