V-type ATPases are capable of establishing a steep pH gradient across the membra

Question

V-type ATPases are capable of establishing a steep pH gradient across the membranes in which they are present. They are so called ‘V-type’ because they were originally identified as the culprit in acidification of plant Vacuoles. Establishment of a steep pH gradient also establishes a steep electrical gradient across that membrane. In biological situations, V-type ATPases are therefore often found in the same membranes as ion channels.

Suppose you have a sample of a vacuole preparation from a plant source that is capable of establishing an intravacuolar (luminal) pH of 3.5 when incubated at 25oC in a pH 7.5 buffer system containing 3 mM MgATP and 50 mM KCl. However, when another sample of the same vacuole preparation is incubated in the same buffer system except that the KCl is replaced by 25 mM K2SO4, the intravacuolar pH established is much higher and not much different from that of the pH 7.5 buffer system. Provide an explanation for these results. Think bioenergetically.

Explanation / Answer

V-ATPases establish a steep pH gradient by pumping protons from cytosolic to exoplasmic face of vacuole membranes against a proton electrochemical gradient.

When 50 mM KCl is present in the buffer,chloride flux helps to form an electrical gradient across the membrane by causing hyperpolarisation(inside more negative).So a stable inflow of H+ ions persist,causing inside more acidic.

But if 25 mM K2SO4 is present instead of 50 mM KCl,same amount of anions will be generated but anion flow inside the vacuole will be less,so weak electrochemical gradient will be present leading to less H+ entry,hence pH will be almost similar as that of buffer(pH=7.5)

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