ATP synthesis catalyzed by a V-ATPase: An alternative pathway for energy conservation operating in plant vacuoles?

Abstract

The electrochemical H+ gradient generated in tonoplast vesicles isolated from maize seeds was found to be able to drive the reversal of the catalytic cycle of both vacuolar H+-pumps (Façanha and de Meis, 1998). Here we describe the reversibility of the vacuolar V-type H +-ATPase (V-ATPase) even in the absence of the H+ gradient in a water-Me2SO co-solvent mixture, resulting in net synthesis of [γ-32P]ATP from [32P]Pi and ADP. The water-Me2SO (5 to 20 %) media promoted inhibition of both PP i hydrolysis and synthesis reactions whereas it slightly affected the ATP hydrolysis and clearly stimulated the ATP synthesis, which was unaffected by uncoupling agents (FCCP, Triton X-100 or NH4+). This effect of Me2SO on the ATP⇔32P exchange reaction seems to be related to a decrease of the apparent K m of the V-ATPase for Pi. The results are in accordance to the concept that the energetics of ATP synthesis catalysis depends on the solvation energies interacting in the enzyme microenvironment. A possible physiological significance of this phenomenon for the metabolism of desiccation-tolerant plant cells is discussed. © 2008 Prof. H.S. Srivastava Foundation for Science and Society.