Electrochemical proton gradient in Micrococcus lysodeikticus cells and membrane vesicles

Area of Science:

• Microbiology
• Biochemistry
• Cellular Physiology

Background:

• The proton electrochemical gradient (delta mu- H+) is crucial for cellular energy generation.
• Understanding delta mu- H+ in bacteria like Micrococcus lysodeikticus provides insights into fundamental bioenergetics.
• Previous studies have characterized delta mu- H+ using various methods, but comparisons between intact cells and isolated vesicles are essential.

Purpose of the Study:

• To investigate and compare the proton electrochemical gradient (delta mu- H+) across the membrane of intact Micrococcus lysodeikticus cells and their isolated plasma membrane vesicles.
• To determine the influence of external pH on the components of delta mu- H+, specifically the pH gradient (delta pH) and membrane potential (delta psi).
• To elucidate the differences in delta mu- H+ regulation between intact cells and membrane vesicles.

Main Methods:

• Utilized weak acid distribution to measure the pH gradient (delta pH; interior alkaline).
• Employed the lipophilic cation [3H]tetraphenylphosphonium+ distribution to monitor membrane potential (delta psi; interior negative).
• Studied Micrococcus lysodeikticus cells and plasma membrane vesicles under varying external pH conditions (5.5 to 8.5) with reduced phenazine methosulfate as an electron donor.

Main Results:

• Intact cells maintained a relatively constant delta mu- H+ (-193 mV to -223 mV) across a broad pH range (5.5-8.5).
• In contrast, membrane vesicles showed a decrease in delta mu- H+ from -166 mV at pH 5.5 to -107 mV at pH 8.0 and above.
• The differences were attributed to pH-dependent alterations: intact cells compensated for decreasing delta pH with increasing delta psi, while vesicles exhibited a less pronounced increase in delta psi.

Conclusions:

• The proton electrochemical gradient is significantly more stable in intact Micrococcus lysodeikticus cells compared to isolated plasma membrane vesicles.
• Intact cells demonstrate a robust regulatory mechanism involving reciprocal changes in delta pH and delta psi to maintain a stable delta mu- H+.
• Isolated membrane vesicles display a diminished capacity to maintain delta mu- H+ at higher external pH values, highlighting the importance of cellular integrity for bioenergetic homeostasis.