Oxidation-reduction coupled phosphorylation in the dark with isolated spinach chloroplasts

Area of Science:

• Plant biochemistry
• Chloroplast function
• Bioenergetics

Background:

• Chloroplasts are vital for photosynthesis, converting light energy into chemical energy.
• ATP synthesis in chloroplasts is typically light-dependent, coupled to electron transport.
• Understanding alternative ATP production pathways is crucial for plant physiology.

Purpose of the Study:

• To investigate the mechanism of ATP synthesis in spinach chloroplasts in the absence of light (dark phosphorylation).
• To identify the role of lipophilic mediators in facilitating dark ATP production.
• To explore the relationship between dark phosphorylation and the chemiosmotic hypothesis.

Main Methods:

• Incubation of spinach chloroplasts with ferricyanide and a reductant.
• Addition of lipophilic mediators, such as 2,3,5,6-tetramethyl-p-phenylenediamine (DAD), to facilitate proton movement.
• Testing the effects of electron transport inhibitors (DCMU, DBMIB), uncouplers (NH4Cl, CCCP), and energy transfer inhibitors (Dio-9).

Main Results:

• Spinach chloroplasts, pre-loaded with ferricyanide, produced ATP in the dark when supplied with a reductant and a proton-releasing lipophilic mediator (DAD).
• Dark phosphorylation was unaffected by electron transport inhibitors but was inhibited by uncouplers and Dio-9.
• Catalytic amounts of DAD indicated its role in shuttling reducing equivalents across the membrane.

Conclusions:

• Dark ATP synthesis in spinach chloroplasts is mediated by lipophilic compounds that facilitate proton movement.
• The results support the chemiosmotic hypothesis, demonstrating ATP synthesis coupled to proton gradients independent of light-driven electron transport.
• DAD acts as a shuttle for reducing equivalents, linking external reductants to internal oxidants for ATP production.