Alternative Photoreduction Pathway Involving Electron Tunneling and Proton Transfer in a Bifunctional Cryptochrome

Area of Science:

• Biochemistry
• Photobiology
• Molecular Biology

Background:

• The tryptophan triad's photoreduction channel is crucial for cryptochrome (CRY) function.
• Some CRY species retain biological function despite impaired primary electron transfer (ET) pathways.
• This suggests the existence of alternative functional mechanisms in CRY.

Purpose of the Study:

• To identify and characterize a secondary electron transfer (ET) pathway in Chlamydomonas reinhardtii cryptochrome (CraCRY).
• To investigate the role of this secondary ET pathway in CRY's functional robustness.

Main Methods:

• Utilized ultrafast electron tunneling and proton transfer analysis.
• Performed sequence alignment across different CRY species.
• Investigated cryptochrome mutants with impaired primary ET pathways.

Main Results:

• Identified a novel, secondary ET pathway in CraCRY, operating in the opposite direction to the primary pathway.
• This secondary pathway involves ultrafast electron tunneling and proton transfer.
• A conserved tryptophan residue (W_1') involved in the secondary pathway was identified across CRY species.

Conclusions:

• The identified secondary ET pathway provides functional robustness to CRY, especially when the primary pathway is compromised.
• This pathway may be essential for CRY function and could be a key evolutionary adaptation.
• The conserved nature of the key tryptophan residue suggests its importance in cryptochrome evolution and function.