Mutation induced modulation of hydrogen bonding to P700 studied using FTIR difference spectroscopy

Area of Science:

• Photosynthesis research
• Molecular biology
• Biophysics

Background:

• P700 is the primary electron donor in photosystem I (PS I).
• Hydrogen bonding plays a crucial role in modulating P700's electronic and physical organization.
• Understanding these interactions is key to deciphering photosynthetic energy transfer efficiency.

Purpose of the Study:

• To investigate the role of hydrogen bonding by ThrA739 in P700 organization.
• To analyze the impact of mutating ThrA739 on P700's electronic structure and charge distribution.
• To elucidate the specific hydrogen bond interactions involving P700 chlorophylls and histidine residues.

Main Methods:

• Site-directed mutagenesis was employed to create a ThrA739 to alanine mutant (TA(A739)) in Chlamydomonas reinhardtii.
• Fourier transform infrared (FTIR) difference spectroscopy was used to compare wild-type and mutant PS I particles.
• Analysis focused on (P700(+)-P700) FTIR difference spectra to identify spectral changes.

Main Results:

• Mutation of ThrA739 significantly altered the (P700(+)-P700) FTIR difference spectra.
• The 13(3) ester carbonyl modes of P(A)(+) and P(B)(+) showed opposite shifts (upshift/downshift), indicating charge redistribution from P(B) to P(A).
• The 13(1) keto carbonyl modes of P(B)/P(B)(+) and P(A)/P(A)(+) exhibited changes consistent with this charge redistribution, suggesting a retained hydrogen bond to P(A)'s keto carbonyl, possibly via water.

Conclusions:

• ThrA739 is critical for the electronic organization of P700, specifically influencing charge distribution in the oxidized state.
• The mutation redirects charge from P(B) to P(A) in the P700(+) state.
• A hydrogen bond to P(A)'s 13(1) keto carbonyl likely persists in the mutant, potentially mediated by a water molecule.