O2 reduction by photosystem I involves phylloquinone under steady-state illumination

Area of Science:

• Biochemistry
• Photosynthesis Research
• Cyanobacterial Biology

Background:

• Photosystem I (PSI) is a key protein complex in oxygenic photosynthesis.
• Quinones play vital roles in electron transport chains.
• Phylloquinone (Vitamin K1) is an essential cofactor in some photosynthetic organisms.

Purpose of the Study:

• To investigate the role of phylloquinone in oxygen (O2) reduction by PSI.
• To compare O2 reduction in wild-type (WT) and phylloquinone-deficient PSI complexes.

Main Methods:

• Isolation of PSI complexes from Synechocystis sp. PCC 6803 WT and menB mutant strains.
• Measurement of O2 reduction rates under varying light intensities.
• Analysis of quinone-binding site composition.

Main Results:

• PSI complexes from WT and menB mutant showed distinct light-intensity dependencies for O2 reduction.
• WT PSI O2 reduction rates did not saturate at high light intensities.
• MenB mutant PSI, lacking phylloquinone, exhibited saturation of O2 reduction rates.

Conclusions:

• The findings strongly suggest direct involvement of phylloquinone in light-induced O2 reduction by PSI.
• Phylloquinone is essential for efficient O2 reduction activity in cyanobacterial PSI.