Photochemistry on a budget: solving the Cryo-EM structure of a long-wavelength photosystem I

In the last decades, different photosynthetic organisms were identified which can exploit longer wavelength photons to drive photosynthesis; both academia and industry showed a renewed interest in trying to couple it with existing photosynthetic bioprocesses, with the objective of achieving higher productivities. These organisms benefit from an extended absorption range (up to 750 nm) that would account for a 19% nominal increase in available photons, and allow improving the theoretical maximum efficiency of photosynthesis. However, photosystems (PS) conduct extremely energy-insensitive redox chemistry, in which most of the energy absorbed from photons is consumed to handle the dangerous chlorophyll a photochemistry, to limit the damage incurring from side- and back-reactions, and at the same time maintain the yield of forward electron transfer as high as possible. Using lower-energy photons to perform the same reactions, as in the case of chlorophyll f containing photosystems, puts even more stringent bioenergetic requirements.

To characterize these huge protein complexes, research in the fields of bioenergetics and photosynthesis has always relied on spectroscopic and modelling techniques, and since the advent of structural biology, on high-resolution protein structures which represent a key element in elucidating the chemistry, mechanisms and energy fluxes of the molecular machineries of interest. With the recent advancement in the field of cryogenic electron microscopy single particle analysis (Cryo-EM SPA)—an ensemble of data processing techniques that enable the determination of protein electrostatic potential maps from transmission electron microscopy images of a monodispersed solution of the protein of interest—it is now possible to obtain high-resolution structures of very large complexes using considerably fewer samples, lower time and optimization than those required by protein crystallography. What a decade ago was thought to be impossible is now becoming a predominant approach in many fields. Cryo-EM Single Particle Analysis (SPA) is collected at liquid nitrogen temperature (77 K).

Consequently, the structure of multiple PSI and PSII supercomplexes from cyanobacteria, algae, and plants were determined within the last five years, showing the extreme diversity of evolution in the organization of the antenna around the highly conserved cores. The importance of these contributions not only lies in the identified structures; the possibility to understand the in vivo organization of complexes in the thylakoid membranes also represents a paramount aspect. A new era for structural biology has started, in which Cryo-EM will likely play a dominant role in determining the structures of large multi-subunit membrane complexes. In this project, we focused our effort in solving the structure of a far-red light adapted photosystem I, the second light driven protein complex in the photosynthetic electron transport chain. Harnessing the energy deriving from photons, photosystem I has the role of oxidizing plastocyanin, a soluble copper electron carrier on the luminal donor side reducing ferredoxin on the stromal acceptor side, generating one of the most negative redox potentials in nature.

The structural information obtained will help us assign the 8 sites containing the long-wavelength pigments, allowing us to mathematically model excitation energy transfer and charge separation in the complex, bringing us one step closer to understanding the functional penalties of long-wavelength photosystems, and recognizing their possible biotechnological applications.

by Giovanni Consoli

Figure 1. Main processing stages of the structure of far-red PSI obtained through Cryo-EM SPA. Left: the original images, the 2D classification of the particles selected, and the 3D volumes obtained. Right: the molecular model fitted to the experimental data.