Evolutionary Dynamics of RuBisCO: Emergence of the Small Subunit and its Impact Through Time.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is an ancient protein critical for CO2-fixation and global biogeochemistry. Form-I RuBisCO complexes uniquely harbor small subunits that form a hexadecameric complex together with their large subunits. The small subunit protein is thought to have significantly contributed to RuBisCO's response to the atmospheric rise of O2 ∼2.5 billion years ago, marking a pivotal point in the enzyme's evolutionary history. Here, we performed a comprehensive evolutionary analysis of extant and ancestral RuBisCO sequences and structures to explore the impact of the small subunit's earliest integration on the molecular dynamics of the overall complex. Our simulations suggest that the small subunit restricted the conformational flexibility of the large subunit early in its history, impacting the evolutionary trajectory of the Form-I RuBisCO complex. Molecular dynamics investigations of CO2 and O2 gas distribution around predicted ancient RuBisCO complexes suggest that a proposed "CO2-reservoir" role for the small subunit is not conserved throughout the enzyme's evolutionary history. The evolutionary and biophysical response of RuBisCO to changing atmospheric conditions on ancient Earth showcase multi-level and trackable responses of enzymes to environmental shifts over long timescales.