Abstract

Photoreceptors and light-driven retinal-proteins share many common features. After a rapid light-induced isomerization around a specific double bond,the altered chromophore structure drives the protein on a much slower time-scale into an active state in which the biological function occurs (signalling , transport). Light may also be used to switch the system back to the inactive dark state. Transient release and uptake of protons appears to be another common feature of these chromoproteins. To understand their mechanism,the following questions are of importance:

• What is the nature of the coupling between the chromophoric cofactor and the protein binding pocket that switches the protein between two states with different structure and activity?
  
• How is the initial structural change in the binding pocket transmitted to the distant signal transduction domain?

To investigate these questions several biophysical methods will be introduced and illustrated with experimental results from the signal transducing photoreceptors rhodopsin,cyanobacterial phytochrome and photoactive yellow protein (PYP) as well as the proton transporting retinal-protein bacteriorhodopsin.