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Kevin Redding (PI) and Raimund Fromme (Co –I) working with collaborators at The Pennsylvania State University have been investigating the PSI reaction center in Heliobacterium modesticaldum.
Photosynthetic reaction centers (RCs), the protein-pigment complexes that allow biological organisms to harvest light energy and convert it into a biologically useful form of chemical energy, first appeared on this planet over 3 billion years ago and have since diversified and are used by very different forms of life. The RCs have traditionally been divided into two groups based on their composition and types of electron acceptor: the type I RCs (like Photosystem I in plants, algae and cyanobacteria) contain iron-sulfur clusters and reduce soluble low-potential proteins, such as ferredoxins, while the type II RCs (like Photosystem II) contain pheophytin and reduce membrane-soluble quinones. Although the heterodimeric Photosystem I (PSI) is the exemplar of the Type I RCs, the other members of this group are all homodimeric. The team’s understanding of these homodimeric RCs is rudimentary, but what they have discovered indicates that they function quite differently from PSI, especially in terms of their internal secondary acceptors. The group’s DOE-funded work on the RC from heliobacteria (HbRC), an anaerobic Gram-positive bacterium, has shown that it lacks a peripheral subunit containing two additional iron-sulfur clusters (FA and FB) that are present in PSI. Instead it uses the inter-peptide FX cluster to reduce different low-potential acceptors. They have recently found that FX in the HbRC has a reduction potential of -0.50 V, a similar potential of the FA/FB clusters in PSI, allowing it to perform a similar function. They have confirmed previous findings that the embedded quinone is not essential for forward electron transfer in the HbRC, in stunning contrast to the process in PSI. Instead, Redding’s recent work indicates that the HbRC can reduce membrane-soluble quinones in the absence of soluble acceptors. These results begin to blur the distinction between Type I and Type II RCs.
A key limitation to the work has been the lack of a structural model of the HbRC. They have made great progress in this regard, having produced HbRC crystals that exhibit X-ray diffraction to a resolution of 0.25 nm. Moreover, in the last funding period the group made much progress in modification of the HbRC, despite great challenges. One of these is the oxygen sensitivity of the organism, which is exacerbated by the facile conversion of the unique bacteriochlorophyll (BChl) g pigments to a Chl a-like molecule. Redding has characterized this process in sufficient detail to use it to analyze the effects of pigment conversion upon RC photochemistry without requiring solvent extraction and reconstitution. Full conversion renders the RC incapable of carrying out long-lived charge separation, although it retains the ability to perform initial charge separation. Surprisingly, they have found that the heterodimeric BChl g'/Chl a'F special pair formed by partial conversion is photochemically active, albeit altered in its properties. They will use this system to study the functional properties of HbRC containing altered pigments. The lab has also succeeded in introducing foreign DNA (replicating plasmids) into this organism, never before accomplished with this family of bacteria. This will allow genetic modification of the HbRC in the near future, a technique that has proven very useful with the other RCs.
The project’s long-term goal is ultimately to bring knowledge of the HbRC to the same level of sophistication as that of the purple bacterial RC and of Photosystems I and II. This fundamental knowledge will provide insight into ways that nature has modified the photosynthetic RCs for different purposes over evolutionary time, which should facilitate the engineering of RCs for specific technological purposes.