# Three rate-determining protein roles in photosynthetic O2-evolution addressed by time-resolved experiments on genetically modified photosystems

## TL;DR

The paper investigates how proteins influence the rate of oxygen production in photosynthesis using genetically modified systems and advanced techniques.

## Contribution

The study identifies three specific protein roles that determine the rate of oxygen evolution in photosystem II.

## Key findings

- Protein environment accelerates proton-coupled electron transfer.
- Genetic modifications slow substrate-water insertion, making it detectable in experiments.
- Proteins balance enthalpic and entropic contributions to the reaction rate.

## Abstract

Light-driven water splitting by plants, algae and cyanobacteria is pivotal for global bioenergetics and biomass formation. A manganese cluster bound to the photosystem II proteins catalyzes the complex reaction at high rate, but the rate-determining factors are insufficiently understood. Here we trace the oxygen-evolution transition by time-resolved polarography and infrared spectroscopy for cyanobacterial photosystems genetically modified at two strategic sites, complemented by computational chemistry. Our results highlight three rate-determining roles of the protein environment of the metal cluster: acceleration of proton-coupled electron transfer, acceleration of substrate-water insertion after O2-formation, and balancing of rate-determining enthalpic and entropic contributions. Whereas in general the substrate-water insertion step may be unresolvable in time-resolved experiments, here it likely becomes traceable because of deceleration by genetic modification. Our results may stimulate new time-resolved experiments on substrate-water insertion in photosynthesis, clarification of enthalpy-entropy compensation in enzyme catalysis, and knowledge-guided development of inorganic catalyst materials.

Combining time-resolved IR spectroscopy, activation energy analyses, and computations, authors provide mechanistic insight into genetically altered reaction kinetics of light-driven oxygen evolution in photosystem II.