Alteration of the nucleotide-binding site asymmetry of chloroplast coupling factor 1 by catalysis

The structure of the chloroplast F1-ATPase at 3.2 A resolution

The structure of the F(1)-ATPase from spinach chloroplasts was determined to 3.2 A resolution by molecular replacement based on the homologous structure of the bovine mitochondrial enzyme. The crystallized complex contains four different subunits in a stoichiometry of alpha(3)beta(3)gammaepsilon. Subunit delta was removed before crystallization to improve the diffraction of the crystals. The overall structure of the noncatalytic alpha-subunits and the catalytic beta-subunits is highly similar to those of the mitochondrial and thermophilic subunits. However, in the crystal structure of the chloroplast enzyme, all alpha- and beta-subunits adopt a closed conformation and appear to contain no bound adenine nucleotides. The superimposed crystallographic symmetry in the space group R32 impaired an exact tracing of the gamma- and epsilon-subunits in the complex. However, clear electron density was present at the core of the alpha(3)beta(3)-subcomplex, which probably represents the C-terminal domain of the gamma-subunit. The structure of the spinach chloroplast F(1) has a potential binding site for the phytotoxin, tentoxin, at the alphabeta-interface near betaAsp(83) and an insertion from betaGly(56)-Asn(60) in the N-terminal beta-barrel domain probably increases the thermal stability of the complex. The structure probably represents an inactive latent state of the ATPase, which is unique to chloroplast and cyanobacterial enzymes.

THECHLOROPLASTATP SYNTHASE: A Rotary Enzyme?

The chloroplast adenosine triphosphate (ATP) synthase is located in the thylakoid membrane and synthesizes ATP from adenosine diphosphate and inorganic phosphate at the expense of the electrochemical proton gradient formed by light-dependent electron flow. The structure, activities, and mechanism of the chloroplast ATP synthase are discussed. Emphasis is given to the inherent structural asymmetry of the ATP synthase and to the implication of this asymmetry to the mechanism of ATP synthesis and hydrolysis. A critical evaluation of the evidence in support of and against the notion that one part of the enzyme rotates with respect to other parts during catalytic turnover is presented. It is concluded that although rotation can occur, whether it is required for activity of the ATP synthase has not been established unequivocally.

Nucleotide binding of an ADP analog to cooperating sites of chloroplast F1-ATPase (CF1)

Pre-steady state nucleotide binding to the chloroplast F1-ATPase (CF1) was measured in a stopped-flow apparatus by monitoring the change of fluorescence intensity of TNP-ADP upon binding. The analysis of the time courses led to the proposal of a mechanism of nucleotide binding with the following characteristics. (a) It involves three sites binding nucleotides in a concerted manner. (b) Each binding site is able to undergo a conformational change from a loose binding state into a state refraining from any direct release of the bound nucleotide into the medium. Only the reverse reaction via the loose binding state enables release out of the tight binding state. (c) Due to very strong negative cooperativity, a maximum of two of the three sites can be found in the state of tight binding. (d) Cooperativity between the three sites leads to a slower nucleotide binding of the second nucleotide compared to the first nucleotide. Furthermore, the conformational change from the loose binding state to the tight binding state is slowed down if one of the other sites already is in the tight binding state. Three-sites mechanisms in which rotation leads to an exchange of the properties of the binding sites failed to simulate the observed time courses of nucleotide binding. However, as the experimental set up was designed to prevent catalysis taking place, our results entirely agree with the current finding that rotation requires catalytic turnover of the enzyme.

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.

The chloroplast ATP synthase: structural changes during catalysis

This article summarizes some of the evidence for the existence of light-driven structural changes in the epsilon and gamma subunits of the chloroplast ATP synthase. Formation of a transmembrane proton gradient results in: (1) a changed in the position of the epsilon subunit such that it becomes exposed to polyclonal antibodies and to reagents which selectively modify epsilon Lys109; (2) enhanced solvent accessibility of several sulfhydryl residues on the gamma subunit; and (3) release/exchange of tightly bound ADP from the enzyme. Theses and related experimental observations can, at least partially, be explained in terms of two different bound conformational states of the epsilon subunit. Evidence for structural changes in the enzyme which are driven by light or nucleotide binding is discussed with special reference to the popular rotational model for catalysis.

In vitro assembly of the core catalytic complex of the chloroplast ATP synthase

The regulatory gamma subunit and an alpha beta complex were isolated from the catalytic F1 portion of the chloroplast ATP synthase. The isolated gamma subunit was devoid of catalytic activity, whereas the alpha beta complex exhibited a very low ATPase activity (approximately 200 nmol/min/mg of protein). The alpha beta complex migrated as a hexameric alpha 3 beta 3 complex during ultracentrifugation and gel filtration but reversibly dissociated into alpha and beta monomers after freezing and thawing in the presence of ethylenediamine tetraacetic acid and in the absence of nucleotides. Conditions are described in which the gamma and alpha beta preparations were combined to rapidly and efficiently reconstitute a fully functional catalytic core enzyme complex. The reconstituted enzyme exhibited normal tight binding and sensitivity to the inhibitory epsilon subunit and to the allosteric inhibitor tentoxin. However, neither the alpha beta complex nor the isolated gamma subunit alone could bind the epsilon subunit or tentoxin with high affinity. Similarly, high affinity binding sites for ATP and ADP, which are characteristic of the core alpha 3 beta 3 gamma enzyme, were absent from the alpha beta complex. The results indicate that when the gamma subunit binds to the alpha beta complex, it induces a three-dimensional conformation in the enzyme, which is necessary for tight binding of the inhibitors and for high-affinity, asymmetric nucleotide binding.

Proton slip of the chloroplast ATPase: its nucleotide dependence, energetic threshold, and relation to an alternating site mechanism of catalysis

The F-ATPase of chloroplasts couples proton flow to ATP synthesis, but is leaky to protons in the absence of nucleotides. This "proton slip" can be blocked by small concentrations of ADP or by inhibitors of the channel portion, CF0. We studied charge flow through the ATPase by flash spectrophotometry and analyzed the inhibition of proton slip by nucleotides, phosphate/arsenate, and insufficient proton motive force. The following inhibition constants (at given background concentrations) were observed: ADP, 0.2 microM (0.5 mM P(i)); ADP, 13.4 microM (no P(i)); P(i), 43 microM (1 microM ADP); GDP, 2.5 microM (0.5 mM P(i)); ATP, 2 microM. ADP and P(i) mutually lowered their respective inhibition constants. Phosphate could be replaced by arsenate. Proton slip occurred only if the proton motive force exceeded a certain threshold, similar to that for ATP synthesis. The inhibition of proton slip by ADP and GDP qualified the respective nucleotide binding sites as belonging to the subset of two (or three) potentially catalytic sites out of the total of six. We interpreted the ADP-induced transition between different conduction states of the ATPase from "slipping" to "closed" to "coupled" as a consequence of the alternating site mechanism of catalysis. Whereas the proton translocator idles in the absence of nucleotides, the high-affinity binding of the first ADP/P(i) couple to one site clutches proton flow to some (conformational) change that can only be executed after the binding of another ADP/P(i) couple to a second site. From there on these sites alternate in the catalytic cycle. An entropic machine is presented which likewise models proton slip, unisite, and multisite ATP synthesis and hydrolysis.

Structural mapping of cysteine-63 of the chloroplast ATP synthase beta subunit

The single sulfhydryl residue (cysteine-63) of the beta subunit of the chloroplast ATP synthase F1 (CF1) was accessible to labeling reagents only after removal of the beta subunit from the enzyme complex. This suggests that cysteine-63 may be located at an interface between the beta and the alpha subunits of CF1, although alternative explanations such as a conformational change in beta brought about by its release from CF1 cannot be ruled out. Cysteine-63 was specifically labeled with [(diethylamino)methylcoumarinyl]-maleimide, and the distance between this site and trinitrophenyl-ADP at the nucleotide binding site on beta was mapped using fluorescence resonance energy transfer. Cysteine-63 is located in a hydrophobic pocket, 42 A away from the nucleotide binding site on beta.

Effect of low growth temperature on coupling between electron transport and proton flux in Vicia faba thylakoids

Coupling between electron transport and proton flux has been compared in chloroplasts from Vicia faba (cv. Windsor) plants grown at 20 and 5°C. Proton uptake by warm-grown thylakoids was sensitive to external pH and stimulated by micromolar adenine nucleotide above pH 7.0. Electron transport was modulated by pH, adenine nucleotide and energy transfer inhibitors (triphenyltin and Hg(2+) ). By contrast, proton uptake by cold-grown thylakoids was generally lower and was insensitive to micromolar ATP. The rate of non-phosphorylating electron flow in cold-grown thylakoids was relatively insensitive to pH and Hg(2+) and was not modulated by adenine nucleotides or triphenyltin. Stimulation of electron transport by phosphorylating conditions in cold-grown thylakoids was generally lower and insensitive to pH. It is concluded that the control of proton efflux through CF(0) -CF(1) differs in thylakoids of V. faba grown at warm and cold temperatures.

Rotational dynamics of chloroplast ATP synthase in phospholipid vesicles

The rotational dynamics of the purified dicyclohexylcarbodiimide-sensitive H(+)-ATPase (DSA) reconstituted into phospholipid vesicles and of the DSA coreconstituted with the proton pump bacterio-rhodopsin were examined by using the technique of time-resolved phosphorescence emission anisotrophy. The phosphorescent probe erythrosin isothiocyanate was used to covalently label the gamma-polypeptide of DSA before reconstitution. Rotational correlation times were measured under a variety of conditions. The rotational correlation time was independent of the viscosity of the external medium but increased significantly as the microviscosity of the membrane increased. This indicates the rotational correlation times are a measure of the enzyme motion within the membrane. The activation energy associated with the rotational correlation time is 8-10 kcal/mol. At 4 degrees C, the correlation time, typically approximately 100-180 microseconds, was unaffected by the addition of substrates and the presence of a membrane pH gradient. Therefore, molecular rotation of the DSA does not appear to play an important role in enzyme catalysis or ion pumping.