Proton pumping pyrophosphatase from higher plant mitochondria

Abstract

In higher plant cells, there are some enzymes capable of utilizing pyrophosphate (PPi) as an energy donor. Among these, membrane‐bound proton pumping pyrophosphatases (H+‐PPiase) have been identified. In addition to the well‐known vacuolar H+‐PPiase (V‐PPiase), there is evidence for the presence of a mitochondrial H+‐PPiase. This enzyme is localized on the inner surface of the inner membrane and catalyzes the specific hydrolysis of PPi, coupled to proton transport, with a H+/PPi stoichiometry of ca 2. This activity is Mg2+‐requiring, is stimulated by monovalent cations, and is inhibited by Ca2+, F− and diphosphonates. The H+‐PPiase contains a catalytic head which is constituted by a 35‐kDa protein which is loosely bound to the inner membrane. This protein exhibits a PPiase activity, stimulated by phospholipids, with characteristics very similar to the membrane‐bound enzyme. The mitochondrial PPiase is distinct from the V‐PPiase, because an antibody raised against the 35‐kDa protein does not react with tonoplast membranes. The mitochondrial H+‐PPiase seems to have an F‐type structure, similar to the F‐ATP synthase and the membrane‐bound PPiases from mammalian and yeast mitochondria. It is suggested that, beside synthesizing PPi, this enzyme may act as a buffer for the electrochemical proton gradient, by hydrolyzing PPi, during conditions of oxygen deprivation.