Phosphorylation in isolated chloroplasts coupled to dichlorophenyldimethylurea-insensitive silicomolybdate reduction

Area of Science:

• Photosynthesis research
• Chloroplast electron transport
• Bioenergetics

Background:

• Isolated chloroplasts are crucial for studying photosynthetic electron transport.
• Silicomolybdate serves as an artificial electron acceptor to probe these pathways.
• Understanding energy conservation mechanisms in Photosystem II is fundamental to photosynthesis.

Purpose of the Study:

• To reinvestigate electron transport in isolated chloroplasts using silicomolybdate as an electron acceptor.
• To elucidate the sites of silicomolybdate reduction and their relationship to Photosystem II and Photosystem I.
• To determine the coupling of silicomolybdate-dependent oxygen evolution to ATP synthesis.

Main Methods:

• Direct measurement of silicomolybdate reduction via deltaA750.
• Indirect measurement of oxygen evolution, with and without ferricyanide.
• Inhibition studies using 3-(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU) and dibromothymoquinone (DBMIB).
• Assessment of ATP synthesis coupled to electron transport.

Main Results:

• Silicomolybdate reduction occurs at two distinct sites, before the DCMU block (Photosystem II) and after the DBMIB block (Photosystem I).
• Silicomolybdate-dependent oxygen evolution is coupled to ATP synthesis with an ATP/2e- ratio of 1.0-1.1, inhibited by ferricyanide.
• In the presence of DCMU, ATP synthesis shows an ATP/2e- ratio of 0.6-0.8, characteristic of Site II, unaffected by uncouplers.

Conclusions:

• The data provide further evidence that the water-splitting reaction at Photosystem II is responsible for energy conservation.
• Silicomolybdate reduction sites correlate with known electron transport chain components.
• The study clarifies the bioenergetic coupling in isolated chloroplasts under specific experimental conditions.