We study structure-function relationships in several groups of proteins, of which many are involved in signal transduction or energy conversion. In our work on biocatalytic splitting of molecular hydrogen into protons and electrons, we analyse in particular the membrane bound [NiFe]hydrogenase, a protein of the remarkable small subgroup of hydrogenases which is active under aerobic conditions. We study the signaling mechanisms of phytochromes by focusing on the structures of photocycle intermediates and analysing domain movements. We analyse the structures of various G-protein coupled receptors (GPCRs; e.g. melanocortin-4 receptor, Ghrelin receptor), either alone or in complex with their cognate G-proteins or selected effector proteins (arrestins), to understand their signaling mechanisms. Other areas of our research include various microbial rhodopsins, cytomegalovirus proteins and their complexes, and DNA photolyases, in particular bacterial (6-4) photolyases.