Structure-based Hamiltonian model for IsiA uncovers a highly robust pigment protein complex

TL;DR

This study models the IsiA pigment-protein complex to analyze its excitation dynamics and robustness at different temperatures, revealing high stability at room temperature and sensitivity to chlorophyll positioning.

Contribution

We developed a structure-based Hamiltonian model of IsiA that simulates excitation events and assesses temperature-dependent robustness and sensitivity to chlorophyll shifts.

Findings

IsiA energetics are highly robust at room temperature.

Chlorophyll position shifts significantly affect optical properties.

The model highlights functional differences between cryogenic and biological temperatures.

Abstract

The iron stress-induced protein A (IsiA) is a source of interest and debate in biological research. The IsiA super-complex, binding over 200 chlorophylls, assembles in multimeric rings around photosystem I (PSI). Recently, the IsiA-PSI structure was resolved to 3.48 {\AA}. Based on this structure, we created a model simulating a single excitation event in an IsiA monomer. This model enabled us to calculate the fluorescence and the localisation of the excitation in the IsiA structure. To further examine this system, noise was introduced to the model in two forms -- thermal and positional. Introducing noise highlights the functional differences in the system between cryogenic temperatures and biologically relevant temperatures. Our results show that the energetics of the IsiA pigment-protein complex are very robust at room temperature. Nevertheless, shifts in the position of specific chlorophylls lead to large changes in their optical and fluorescence properties. Based on these results we discuss the implication of highly robust structures, with potential for serving different roles in a context dependent manner, on our understanding of the function and evolution of photosynthetic processes.