Contribution of electric field (DELTA psi) to steady-state transthylakoid proton motive force in vivo

摘要

Proton motive force (pmf), which drives the phosphorylation of ADP by the F_1F_O ATP synthase, is sustained in chloroplasts by photoelectron transport. As described by chemiosmotic mechanism, pmf is the sum of the chemical and electric potentials, or DELTA pH and DELTA psi, respectively (Mitchell,1966). Experimental observation suggests that a pmf of at least 120 to 180 mV (2 to 3 pH units) is required absolutely to sustain steady-state ATP synthesis (Hangarter and Good,1982). The generally accepted view contends that the electric field component does not contribute or at best contributes minimally to pmf (van Kooten, et al.,1986). However, if DELTA pH is the sole component of pmf, then the inferred pH of the thylakoid lumen during photosynthesis(5.8 or lower) is inconsistent with lumenal pH requirements (Kramer, et al.,1999). A significant contribution of DELTA psi to pmf (30 to 60 mV) would satisfy the energetic requirements for ATP synthesis within the observed pH limitations of the biochemical system. This possibility raised a number of questions: 1) Does DELTA psi contribute significantly to pmf, 2) If so, what factors influence the magnitude of DELTA psi, and 3) To what extent does it contribute to pmf, in vivo? These issues were addressed through experiments using partially uncoupled thylakoids and a mathematical computer model based on values obtained from the literature. It was concluded that in a relatively low ionic strength environment (<10 mM) and with a moderately large lumenal buffering capacity (25 to 50 mM), DELTA psi may exist as a significantly large component of pmf, during steady state photosynthesis (Cruz, et al.,2001). More recently, we have performed experiments using the freshwater alga, Chlamydomonas reinhardtii, toextend these observations.