Influence of divalent cations on nucleotide exchange and ATPase activity of chloroplast coupling factor 1

Structure of F1-ATPase from the obligate anaerobe Fusobacterium nucleatum

The crystal structure of the F₁-catalytic domain of the adenosine triphosphate (ATP) synthase has been determined from the pathogenic anaerobic bacterium Fusobacterium nucleatum. The enzyme can hydrolyse ATP but is partially inhibited. The structure is similar to those of the F₁-ATPases from Caldalkalibacillus thermarum, which is more strongly inhibited in ATP hydrolysis, and in Mycobacterium smegmatis, which has a very low ATP hydrolytic activity. The β_E-subunits in all three enzymes are in the conventional ‘open’ state, and in the case of C. thermarum and M. smegmatis, they are occupied by an ADP and phosphate (or sulfate), but in F. nucleatum, the occupancy by ADP appears to be partial. It is likely that the hydrolytic activity of the F. nucleatum enzyme is regulated by the concentration of ADP, as in mitochondria.

Regulation of the ATPase activity of ABCE1 from Pyrococcus abyssi by Fe-S cluster status and Mg²⁺: implication for ribosomal function

Ribosomal function is dependent on multiple proteins. The ABCE1 ATPase, a unique ABC superfamily member that bears two Fe₄S₄ clusters, is crucial for ribosomal biogenesis and recycling. Here, the ATPase activity of the Pyrococcus abyssi ABCE1 (PabABCE1) was studied using both apo- (without reconstituted Fe-S clusters) and holo- (with full complement of Fe-S clusters reconstituted post-purification) forms, and is shown to be jointly regulated by the status of Fe-S clusters and Mg²⁺. Typically ATPases require Mg²⁺, as is true for PabABCE1, but Mg²⁺ also acts as a negative allosteric effector that modulates ATP affinity of PabABCE1. Physiological [Mg²⁺] inhibits the PabABCE1 ATPase (K(i) of ∼1 μM) for both apo- and holo-PabABCE1. Comparative kinetic analysis of Mg²⁺ inhibition shows differences in degree of allosteric regulation between the apo- and holo-PabABCE1 where the apparent ATP K(m) of apo-PabABCE1 increases >30-fold from ∼30 μM to over 1 mM with M²⁺. This effect would significantly convert the ATPase activity of PabABCE1 from being independent of cellular energy charge (φ) to being dependent on φ with cellular [Mg²⁺]. These findings uncover intricate overlapping effects by both [Mg²⁺] and the status of Fe-S clusters that regulate ABCE1's ATPase activity with implications to ribosomal function.

Vanadyl as a probe of the function of the F1-ATPase-Mg2+ cofactor

The Mg2+ dependent asymmetry of the F(1)-ATPase catalytic sites leads to the differences in affinity for nucleotides and is an essential component of the binding-change mechanism. Changes in metal ligands during the catalytic cycle responsible for this asymmetry were characterized by vanadyl (V(IV) + O)2+, a functional surrogate for Mg2+. The (51)V-hyperfine parameters derived from EPR spectra of VO2+ bound to specific sites on F(1) provide a direct probe of the metal ligands. Site-directed mutations of metal ligand residues cause measurable changes in the (51)V-hyperfine parameters of the bound VO2+, thereby providing a means to identification. Initial binding of the metal-nucleotide to the low-affinity catalytic site conformation results in metal coordination by hydroxyl groups from the P-loop threonine and catch-loop threonine. Upon conversion to the high-affinity conformation, carboxyl groups from the Walker homology B aspartate and MF(1)betaE197 become ligands in lieu of the hydroxyl groups.

Unique rotary ATP synthase and its biological diversity

F1F0 ATP synthases convert energy stored in an electrochemical gradient of H+ or Na+ across the membrane into mechanical rotation, which is subsequently converted into the chemical bond energy of ATP. The majority of cellular ATP is produced by the ATP synthase in organisms throughout the biological kingdom and therefore under diverse environmental conditions. The ATP synthase of each particular cell is confronted with specific challenges, imposed by the specific environment, and thus by necessity must adapt to these conditions for optimal operation. Examples of these adaptations include diverse mechanisms for regulating the ATP hydrolysis activity of the enzyme, the utilization of different coupling ions with distinct ion binding characteristics, different ion-to-ATP ratios reflected by variations in the size of the rotor c ring, the mode of ion delivery to the binding sites, and the different contributions of the electrical and chemical gradients to the driving force.

The structural basis for unidirectional rotation of thermoalkaliphilic F1-ATPase

The ATP synthase of the thermoalkaliphilic Bacillus sp. TA2.A1 operates exclusively in ATP synthesis direction. In the crystal structure of the nucleotide-free alpha(3)beta(3)gamma epsilon subcomplex (TA2F(1)) at 3.1 A resolution, all three beta subunits adopt the open beta(E) conformation. The structure shows salt bridges between the helix-turn-helix motif of the C-terminal domain of the beta(E) subunit (residues Asp372 and Asp375) and the N-terminal helix of the gamma subunit (residues Arg9 and Arg10). These electrostatic forces pull the gamma shaft out of the rotational center and impede rotation through steric interference with the beta(E) subunit. Replacement of Arg9 and Arg10 with glutamines eliminates the salt bridges and results in an activation of ATP hydrolysis activity, suggesting that these salt bridges prevent the native enzyme from rotating in ATP hydrolysis direction. A similar bending of the gamma shaft as in the TA2F(1) structure was observed by single-particle analysis of the TA2F(1)F(o) holoenzyme.

Proton flux through the chloroplast ATP synthase is altered by cleavage of its gamma subunit

Electron transport, the proton gradient and ATP synthesis were determined in thylakoids that had been briefly exposed to a low concentration of trypsin during illumination. This treatment cleaves the gamma subunit of the ATP synthase into two large fragments that remain associated with the enzyme. Higher rates of electron transport are required to generate a given value of the proton gradient in the trypsin-treated membranes than in control membranes, indicating that the treated membranes are proton leaky. Since venturicidin restores electron transport and the proton gradient to control levels, the proton leak is through the ATP synthase. Remarkably, the synthesis of ATP by the trypsin-treated membranes at saturating light intensities is only slightly inhibited even though the proton gradient is significantly lower in the treated thylakoids. ATP synthesis and the proton gradient were determined as a function of light intensity in control and trypsin-treated thylakoids. The trypsin-treated membranes synthesized ATP at lower values of the proton gradient than the control membranes. Cleavage of the gamma subunit abrogates inhibition of the activity of the chloroplast ATP synthase by the epsilon subunit. Our results suggest that overcoming inhibition by the epsilon subunit costs energy.

Inhibition of ATP hydrolysis by thermoalkaliphilic F1Fo-ATP synthase is controlled by the C terminus of the epsilon subunit

The F(1)F(o)-ATP synthases of alkaliphilic bacteria exhibit latent ATPase activity, and for the thermoalkaliphile Bacillus sp. strain TA2.A1, this activity is intrinsic to the F(1) moiety. To study the mechanism of ATPase inhibition, we developed a heterologous expression system in Escherichia coli to produce TA2F(1) complexes from this thermoalkaliphile. Like the native F(1)F(o)-ATP synthase, the recombinant TA2F(1) was blocked in ATP hydrolysis activity, and this activity was stimulated by the detergent lauryldimethylamine oxide. To determine if the C-terminal domain of the epsilon subunit acts as an inhibitor of ATPase activity and if an electrostatic interaction plays a role, a TA2F(1) mutant with either a truncated epsilon subunit [i.e., TA2F(1)(epsilon(DeltaC))] or substitution of basic residues in the second alpha-helix of epsilon with nonpolar alanines [i.e., TA2F(1)(epsilon(6A))] was constructed. Both mutants showed ATP hydrolysis activity at low and high concentrations of ATP. Treatment of the purified F(1)F(o)-ATP synthase and TA2F(1)(epsilon(WT)) complex with proteases revealed that the epsilon subunit was resistant to proteolytic digestion. In contrast, the epsilon subunit of TA2F(1)(epsilon(6A)) was completely degraded by trypsin, indicating that the C-terminal arm was in a conformation where it was no longer protected from proteolytic digestion. In addition, ATPase activity was not further activated by protease treatment when compared to the untreated control, supporting the observation that epsilon was responsible for inhibition of ATPase activity. To study the effect of the alanine substitutions in the epsilon subunit in the entire holoenzyme, we reconstituted recombinant TA2F(1) complexes with F(1)-stripped native membranes of strain TA2.A1. The reconstituted TA2F(o)F(1)(epsilon(WT)) was blocked in ATP hydrolysis and exhibited low levels of ATP-driven proton pumping consistent with the F(1)F(o)-ATP synthase in native membranes. Reconstituted TA2F(o)F(1)(epsilon(6A)) exhibited ATPase activity that correlated with increased ATP-driven proton pumping, confirming that the epsilon subunit also inhibits ATPase activity of TA2F(o)F(1).

Interaction of oxyanions with thioredoxin-activated chloroplast coupling factor 1

Interaction between F(1)-ATPase activity stimulating oxyanions and noncatalytic sites of coupling factor CF(1) was studied. Carbonate, borate and sulfite anions were shown to inhibit tight binding of [14C]ATP and [14C]ADP to CF(1) noncatalytic sites. The demonstrated change of their inhibitory efficiency in carbonate-borate-sulfite order coincides with the previously found change in efficiency of these anions as stimulators of CF(1)-ATPase activity [Biochemistry (Mosc.) 43 (1978) 1206-1211]. Inhibition of tight nucleotide binding to noncatalytic sites was accompanied by stimulation of nucleotide binding to catalytic sites. This suggests that stimulation of CF(1)-ATPase activity is caused by interaction between oxyanions and noncatalytic sites. A most efficient stimulator of CF(1)-ATPase activity, sulfite oxyanion, appeared to be a competitive inhibitor with respect to ATP and a partial noncompetitive inhibitor with respect to ADP. The inhibition weakened with increasing time of CF(1) incubation with sulfite and nucleotides. Sulfite is believed to inhibit fast reversible interaction between nucleotides and noncatalytic sites and to produce no effect on subsequent tight binding of nucleotides. A possible mechanism of the oxyanion-stimulating effect is discussed.

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.