Biology subjects

Polivka, T.

Publications and source records attributed to Polivka, T..

3 recordsLinked to original sources

bioRxiv2021

Structures of the Cyanobacterial Phycobilisome

The phycobilisome is an elaborate antenna that is responsible for light-harvesting in cyanobacteria and red-algae. This large macromolecular complex captures incident sunlight and transfers the energy via a network of pigment molecules called bilins to the photosynthetic reaction centers. The phycobilisome of the model organism Synechocystis PCC 6803 consists of a core to which six rods are attached but its detailed molecular architecture and regulation in response to environmental conditions is not well understood. Here we present cryo-electron microscopy structures of the 6.2 MDa phycobilisome from Synechocystis PCC 6803 resolved at 2.1 [A] (rods) to 2.7 [A] (core), revealing three distinct conformations, two previously unknown. We found that two of the rods are mobile and can switch conformation within the complex, revealing a layer of regulation not described previously. In addition, we found a novel linker protein in the structure, that may represent a long-sought subunit that tethers the phycobilisome to the thylakoid membrane. Finally, we show how excitation energy is transferred within the phycobilisome and correlate our structures with known spectroscopic properties. Together, our results provide detailed insights into the biophysical underpinnings of cyanobacterial light harvesting and lay the foundation for bioengineering of future phycobilisome variants and artificial light harvesting systems.

biochemistry↗

bioRxiv2021

Structure of the Quenched Cyanobacterial OCP-Phycobilisome Complex

Photoprotection is an essential mechanism in photosynthetic organisms to balance the harvesting of light energy against the risks of photodamage. In cyanobacteria, photoprotective non-photochemical quenching relies on the interaction between a photoreceptor, the Orange Carotenoid Protein (OCP), and the antenna, the phycobilisome (PBS). Here we report the first structure of the OCP-PBS complex at 2.7 [A] overall resolution obtained by cryo-electron microscopy. The structure shows that the 6.2 MDa PBS is quenched by four 34 kDa OCP organized as two dimers. The complex also reveals that the structure of the active form of the OCP is drastically different than its resting, non-quenching form, with an [~]60 [A] displacement of its regulatory domain. These results provide a high-resolution blueprint of the structural basis of the protective quenching of excess excitation energy that enables cyanobacteria to harvest light energy and fix CO2 across environmentally diverse and dynamic surface of our planet.

biochemistry↗

bioRxiv2021

Trivial Excitation Energy Transfer to Carotenoids is an Unlikely Mechanism for Non-Photochemical Quenching in LHCII

Higher plants defend themselves from bursts of intense light via the mechanism of Non-Photochemical Quenching (NPQ). It involves the Photosystem II (PSII) antenna protein (LHCII) adopting a conformation that favours excitation quenching. In recent years several structural models have suggested that quenching proceeds via energy transfer to the optically forbidden and short-lived S1 states of a carotenoid. It was proposed that this pathway was controlled by subtle changes in the relative orientation of a small number of pigments. However, quantum chemical calculations of S1 properties are not trivial and therefore its energy, oscillator strength and lifetime are treated as rather loose parameters. Moreover, the models were based either on a single LHCII crystal structure or Molecular Dynamics (MD trajectories) about a single minimum. Here we try and address these limitations by parameterizing the vibronic structure and relaxation dynamics of lutein in terms of observable quantities, namely linear absorption (LA) transient absorption (TA) and two-photon excitation (TPE) spectra. We also analyze a number of minima taken from an exhaustive meta-dynamical search of the LHCII potential energy surface. We show that trivial, Coulomb-mediated energy transfer to S1 is an unlikely quenching mechanism. Pigment movements are insufficient to switch the system between quenched and unquenched states. Modulation of S1 energy level as a quenching switch is similarly unlikely. Moreover, the quenching predicted by previous models is likely an artefact of quantum chemical over-estimation of S1 oscillator strength and the real mechanism likely involves non-trivial inter-molecular states.

biophysics↗