Calcium gating of H+ fluxes in chloroplasts affects acid-base-driven ATP formation

pH-dependent Ca2+ binding to the F0 c-subunit affects proton translocation of the ATP synthase from Synechocystis 6803

It was shown before (Wooten, D. C., and Dilley, R. A. (1993) J. Bioenerg. Biomembr. 25, 557-567; Zakharov, S. D., Li, X., Red'ko, T. P., and Dilley, R. A. (1996) J. Bioenerg. Biomembr. 28, 483-493) that pH dependent reversible Ca2+ binding near the N- and C-terminal end of the 8 kDa subunit c modulates ATP synthesis driven by an applied pH jump in chloroplast and E. coli ATP synthase due to closing a "proton gate" proposed to exist in the F0 H+ channel of the F0F1 ATP synthase. This mechanism has further been investigated with the use of membrane vesicles from mutants of the cyanobacterium Synechocystis 6803. Vesicles from a mutant with serine at position 37 in the hydrophilic loop of the c-subunit replaced by the charged glutamic acid (strain plc 37) has a higher H+/ATP ratio than the wild type and therefore shows ATP synthesis at low values of deltamuH+. The presence of 1 mM CaCl2 during the preparation and storage of these vesicles blocked acid-base jump ATP formation when the pH of the acid side (inside) was between pH 5.6 and 7.1, even though the deltapH of the acid-base jump was thermodynamically in excess of the necessary energy to drive ATP formation at an external pH above 8.28. That is, in the absence of added CaCl2, ATP formation did occur under those conditions. However, when the base stage pH was 7.16 and the acid stage below pH 5.2, ATP was formed when Ca2+ was present. This is consistent with Ca2+ being displaced by H+ ions from the F0 on the inside of the thylakoid membrane at pH values below about 5.5. Vesicles from a mutant with the serine of position 3 replaced by a cysteine apparently already contain some bound Ca2+ to F0. Addition of 1 mM EGTA during preparation and storage of those vesicles shifted the otherwise already low internal pH needed for onset of ATP synthesis to higher values when the external pH was above 8. With both strains it was shown that the Ca2+ binding effect on acid-base induced ATP synthesis occurs above an internal pH of about 5.5. These results were corroborated by 45Ca2+-ligand blot assays on organic solvent soluble preparations containing the 8 kDa F0 subunit c from the S-3-C mutant ATP synthase, which showed 5Ca2+ binding as occurs with the pea chloroplast subunit III. The phosphorylation efficiency (P/2e), at strong light intensity, of Ca2+ and EGTA treated vesicles from both strains were almost equal showing that Ca2+ or EGTA have no other effect on the ATP synthase such as a change in the proton to ATP ratio. The results indicate that the Ca2+ binding to the F0 H+ channel can block H+ flux through the channel at pH values above about 5.5, but below that pH protons apparently displace the bound Ca2+, opening the CF0 H+ channel between the thylakoid lumen and H+ conductive channel.

Distinguishing between luminal and localized proton buffering pools in thylakoid membranes

The dual gradient energy coupling hypothesis posits that chloroplast thylakoid membranes are energized for ATP formation by either a delocalized or a localized proton gradient geometry. Localized energy coupling is characterized by sequestered domains with a buffering capacity of approximately 150 nmol H(+) mg(-1) chlorophyll (Chl). A total of 30 to 40 nmol mg(-1) Chl of the total sequestered domain buffering capacity is contributed by lysines with anomolously low pK(a)s, which can be covalently derivatized with acetic anhydride. We report that in thylakoid membranes treated with acetic anhydride, luminal acidification by a photosystem I (duraquinol [DQH(2)] to methyl viologen [MV]) proton pumping partial reaction was nearly completely inhibited, as measured by three separate assays, yet surprisingly, H(+) accumulation still occurred to the significant level of more than 100 nmol H(+) mg Chl(-1), presumably into the sequestered domains. The treatment did not increase the observed rate constant of dark H(+) efflux, nor was electron transport significantly inhibited. These data provide support for the existence of a sequestered proton translocating pathway linking the redox reaction H(+) ion sources with the CF(0) H(+) channel. The sequestered, low-pK(a) Lys groups appear to have a role in the H(+) diffusion process and chemically modifying them blocks the putative H(+) relay system.

ATP synthase--past and future

This paper gives an overview of a lecture scheduled for the opening of the 10th European Bioenergetics Congress. In this lecture I plan to first reflect on the accomplishments of some of the individuals who were involved in research on the ATP synthase during the past 50 years. Then I will give a brief view of the present information about rotational catalysis by the ATP synthase. This will be followed by a discussion of some results from my laboratory that call for additional experimentation. Finally I will direct attention to other questions about the ATP synthase that should be addressed in future studies.

The ATP synthase--a splendid molecular machine

An X-ray structure of the F1 portion of the mitochondrial ATP synthase shows asymmetry and differences in nucleotide binding of the catalytic beta subunits that support the binding change mechanism with an internal rotation of the gamma subunit. Other structural and mutational probes of the F1 and F0 portions of the ATP synthase are reviewed, together with kinetic and other evaluations of catalytic site occupancy and behavior during hydrolysis or synthesis of ATP. Subunit function as related to proton translocation and rotational catalysis is considered. Physical demonstrations of the gamma subunit rotation have been achieved. The findings have implications for other enzymatic catalyses.

Calcium binding to the subunit c of E. coli ATP-synthase and possible functional implications in energy coupling

The 8-kDa subunit c of the E. coli F0 ATP-synthase proton channel was tested for Ca++ binding activity using a 45Ca++ ligand blot assay after transferring the protein from SDS-PAGE gels onto polyvinyl difluoride membranes. The purified subunit c binds 45Ca++ strongly with Ca++ binding properties very similar to those of the 8-kDa CF0 subunit III of choloroplast thylakoid membranes. The N-terminal f-Met carbonyl group seems necessary for Ca++ binding capacity, shown by loss of Ca++ binding following removal of the formyl group by mild acid treatment. The dicyclohexylcarbodiimide-reactive Asp-61 is not involved in the Ca++ binding, shown by Ca++ binding being retained in two E. coli mutants, Asp61-->Asn and Asp61-->Gly. The Ca++ binding is pH dependent in both the E. coli and thylakoid 8-kDa proteins, being absent at pH 5.0 and rising to a maximum near pH 9.0. A treatment predicted to increase the Ca++ binding affinity to its F0 binding site (chlorpromazine photoaffinity attachment) caused an inhibition of ATP formation driven by a base-to-acid pH jump in whole cells. Inhibition was not observed when the Ca++ chelator EGTA was present with the cells during the chlorpromazine photoaffinity treatment. An apparent Ca++ binding constant on the site responsible for the UV plus chlorpromazine effect of near 80-100 nM was obtained using an EGTA-Ca++ buffer system to control free Ca++ concentration during the UV plus chlorpromazine treatment. The data are consistent with the notion that Ca++ bound to the periplasimic side of the E. coli F0 proton channel can block H+ entry into the channel. A similar effect occurs in thylakoid membranes, but the Ca++ binding site is on the lumen side of the thylakoid, where Ca+2 binding can modulate acid-base jump ATP formation. The Ca+2 binding to the F0 and CF0 complexes is consistent with a pH-dependent gating mechanism for control of H+ ion flux across the opening of the H+ channel.