Structural photoactivation of light-active proteins studied by femtosecond time-resolved crystallography

Photoactive proteins act in photosynthesis, light sensing, and in DNA repair. Upon absorption of light, the chromophore and surrounding protein undergo a series of structural changes on multiple time and length scales to become activated. We use femtosecond time-resolved crystallography to characterize these structural changes and I will discuss two examples of this work: 

Plants, fungi and bacteria use red-light active phytochrome proteins to collect information about ambient light conditions. The structures of the resting and light-activated states of bacteriophytochromes are known, but the structural mechanism of the protein remains elusive. I will present crystallographic snapshots of two bacterial phytochromes, time-resolved from femto- to milliseconds at room-temperature by serial X-ray crystallography at the Japanese X-ray free electron laser (SACLA). A photochemical mechanism emerges, and I will discuss implications for the primary photoresponse of phytochrome proteins.1,2,3 

The second example will be on photoactivation of a DNA photolyase. In these enzymes a FAD co-factor is photoreduced by electron transfer along a conserved chain of tryptophans. Our time-resolved structures follow thís electron transfer reaction in real time. They reveal how protein motions guide the charge transfer actively through small, but notable changes. We newly imply a conserved methionine residue in the charge transfer reaction.4 The results provide a basis for understanding of how protein dynamics control electron transfer, which is relevant for all charge transfer reactions in proteins, such as in photosynthesis and cellular respiration. 
 
1) Claesson et al., eLife, 2020
2) Carillo et al., Structure 2021
3) unpublished
4) unpublished

Faculty host is Brian Crane.