Light as a substrate: migration of LHCII antennas in extended Michaelis-Menten model for PSI kinetics.

We extended, for the first time, the Michaelis-Menten (M-M) model to describe the kinetics of photosystem I (PSI) complexes using light as a substrate. Our work is novel as it can be useful for studying the phenomenon of "state transitions" because it quantifies the affinity of light to PSI reaction centers depending on the associated light harvesting complex II (LHCII) antennas. We verified our models by measuring the PSI activity as a function of light intensity using an oxygen electrode for chloroplast from plants grown in low light conditions and treated with far red light. We determined the kinetics constant KM for: PSI-LHCI, PSI-LHCI-LHCII and PSI-PSII megacomplexes and have shown that KM for PSI located in the megacomplexes was smaller in magnitude than PSI-LHCI, thus demonstrating that LHCII antennas are functionally associated with PSI. The parameter [S]1/2used in our models is the equivalent of M-M constant. Far red light increases [S]1/2, which indicates that transition from state 1 to state 2 leads to an energy gain while reaching the PSI reaction centers. We also observed that redistribution of the absorbed excitation energy is realized not only by LHCII migration but also by association of the photosystems in the megacomplexes.