Our group is interested in the synthesis of molecular hydrogen by Fe hydrogenases as well as electron transfer in a photosynthetic reaction centre (photosystem I) and hydrogenases. We use techniques such as heterologous expression and mutagenesis of algae hydrogenases, enzymology, spectroscopy. We also study redox processes involving ferredoxin-NADP+-reductase and other enzymes involved to gain an understanding of the regulation of the electron fluxes involved in order to manipulate them for hydrogen production. In a complementary approach, we are developing artificial photocatalysts for water photodissociation and hydrogen production. These molecules are synthesized from Ruthenium complexes as photosensors connected to the metal centers as catalytic sites. They are characterized by time resolved spectroscopic methods.
Team Leader
Winfried LEIBL
winfried.leibl@cea.fr
Photoproduction of hydrogen
Our research on photoproduction of hydrogen from water is based on two approaches: the functional characterisation of hydrogenases isolated from photosynthetic organisms (possibly genetically modified) and the development of artificial bio-inspired complexes with photocatalytic properties.
Some green algae synthesize iron-hydrogenases, enzymes which catalyse the reduction of protons to dihydrogen: 2H+ + 2e– --- H2. In this reaction the electrons come from the reaction center of photosystem I and are transferred one by one by ferredoxin. The catalytic mechanism of this apparently simple reaction is still badly known: the interaction with reduced ferredoxin, charge storage, catalytic role of the Fe atoms, role of the ligands. In particular, ferredoxin, the partner of the hydrogenase plays a key role in the energetic metabolism as it distributes electrons to numerous metabolic pathways, including to the Calvin cycle via the enzyme Ferredoxin-NADP+ Reductase (FNR).
In the context of exploring the possibility of hydrogen photoproduction by microalgaes, we are interested in two aspects in particular: understanding of the energetic metabolism linked to hydrogen production and development of methods for functional characterisation of hydrogenases. An important step towards these aims was recently achieved with the study of a complex electron transfer chain containing one of the most important partners of ferredoxin, FNR. This chain, reconstituted from the isolated partners, was characterised in detail (energetics, kinetics, catalytic efficiency) by cyclic voltammetry within an electrochemical cell and by kinetic flash absorption spectroscopy. At present this work is continued along two lines. First, we will apply the approach, successfully tested with FNR, to the study of hydrogenase and, second, we will characterize in vitro and in vivo different forms of FNR present in cyanobacteria to quantify the function of FNR in overall electron flow (collaboration G. Ajlani, LBMS).
[Fe-Fe] hydrogenases from algae are usually expressed in small amount and extremely sensitive towards oxygen. To increase the amount of enzyme and to simplify purification, we are using heterologous expression of [Fe-Fe] hydrogenases in a facultatively anaerobic bacterium. With this system it is possible to purify sufficient quantities of hydrogenases to study their detailed enzymatic properties. Heterologous expression also allows site-directed mutagenesis to study the role of certain amino acids with respect to the properties of the enzyme (interaction with ferredoxin, proton transfer, effect on catalysis).
Artificial photocatalytic complexes
The second approach of our research concerns the development of artificial photocatalytic complexes for production of hydrogen by water photolysis. The synthesized molecules (collaboration A. Aukauloo, University Paris-Sud) are composed of a Ruthenium complex as photoactive oxidation/reduction potential, and a redox active ligand able to catalyze either the oxidation of water or the reduction of protons. The design of the catalytic ligand with a cavity for binding of one or more metal ions follows a bio-inspired approach. The electron transfer between these two subunits, chromophore and catalytic site, is controlled by a rigid aromatic spacer which keeps the distance between the two parts fixed and well defined chromophore, converting light energy into redox potential.
Time-resolved spectroscopic methods in combination with DFT calculations are employed for functional characterization of the supramolecular complexes. The results obtained are the basis for the development of new generations of molecules. The bio-inspired approach for the design of the catalytic sites is based on the research going on in other groups working on biological catalytic systems (hydrogenase and water oxidation complex of photosystem II).
