The role of Mg2+ in the hydrolytic activity of the isolated chloroplast ATPase: study by high-performance liquid chromatography

Regulation of the type IV secretion ATPase TrwD by magnesium: implications for catalytic mechanism of the secretion ATPase superfamily

TrwD, the VirB11 homologue in conjugative plasmid R388, is a member of the large secretion ATPase superfamily, which includes ATPases from bacterial type II and type IV secretion systems, type IV pilus, and archaeal flagellae assembly. Based on structural studies of the VirB11 homologues in Helicobacter pylori and Brucella suis and the archaeal type II secretion ATPase GspE, a unified mechanism for the secretion ATPase superfamily has been proposed. Here, we have found that the ATP turnover of TrwD is down-regulated by physiological concentrations of magnesium. This regulation is exerted by increasing the affinity for ADP, hence delaying product release. Circular dichroism and limited proteolysis analysis indicate that magnesium induces conformational changes in the protein that promote a more rigid, but less active, form of the enzyme. The results shown here provide new insights into the catalytic mechanism of the secretion ATPase superfamily.

Comparison of different cations (Mn2+, Mg2+, Ca2+) on the hydrolytic activity of chloroplast ATPase

The influences of Mn2+, Mg2+, and Ca2+ on the enzymic activity of chloroplast ATPase have been compared, using an HPLC method for the separation of ADP. The dissociation constants of the divalent ion-ATP complexes have been determined by a spectrophotometric method, with the dye antipyrylazo III, and enzymic constants (dissociation constant of the ion-enzyme complexes, Michaelis constants, maximum rates) have been calculated. The comparison between the rates obtained, respectively, with Mn2+ and Ca2+ alone with that given by the mixture of these two ions, allows us to conclude that, as for Mg2+, Mn2+ is also an activator of chloroplast ATPase and that metal-free ATP is the true substrate.

Cooperativity between the enzymatic sites of F1-ATPase revisited by the use of HPLC methods

The fundamental question of the cooperativity between the enzymatic sites of F1-ATPase is examined in the light of new measurements of the enzymatic rate of ATP hydrolysis by CF1, the enzyme isolated from spinach chloroplasts. The experimental data, obtained with a chromatographic method, fit a model that involves two kinds of independent enzymatic sites working with metal-free ATP, with no need of cooperativity between the sites. Binding measurements between ADP or ATP and CF1 by the chromatographic method of Hummel and Dreyer (1962) also support this conclusion. The present data and interpretation are in agreement with those reported recently (Reynafarje and Pedersen, 1996) which show that the first order rate constant of ATP hydrolysis by MF1, the analogous enzyme from mitochondria, is virtually constant under experimental conditions involving either unisite or multisite hydrolysis of ATP. The present data and interpretation are discussed together with those reported previously, in particular with regard to the methods that were used to support the commonly accepted opposite viewpoint.

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.

Synthesis, structure, and properties of MeSer1-tentoxin, a new cyclic tetrapeptide which interacts specifically with chloroplast F1 H(+)-ATPase differentiation of inhibitory and stimulating effects

A new tentoxin analogue, in which the L-methyl alanine residue is substituted by L-methylserine, has been prepared following the synthetic pathway recently described for the synthesis of tentoxin [Cavelier, F., & Verducci, J. (1995) Tetrahedron Lett. 36, 4425-4428]. Using two-dimensional homonuclear proton nuclear magnetic resonance and structural analysis, we observed that MeSer1-tentoxin, like tentoxin, adopts several conformations in aqueous solution and presents self-aggregative properties. This analogue was found to be conformationally similar to the natural toxin. It showed the same efficiency as tentoxin in inhibition of ATPase activity of the isolated chloroplast F1 proton ATPase (CF1) as well as in inhibition of the ATP synthase activity of the membrane-bound enzyme (CF0CF1) in thylakoids and proteoliposomes. At concentrations above 10 microM, MeSer1-tentoxin did not reactivate CF1 to a high extent, contrary to tentoxin. It appeared, however, to bind in the same way, since the reactivating effect of tentoxin was inhibited by MeSer1-tentoxin. These results show that it is possible, using tentoxin analogues, to separate inhibitory and activating effects on the chloroplast ATPase, despite the limited chemical difference between the two toxins.

Metal binding sites of H(+)-ATPase from chloroplast and Bacillus PS3 studied by EPR and pulsed EPR spectroscopy of bound manganese(II)

The metal binding sites of isolated F1 ATPase from spinach chloroplasts (CF1) and from the thermophilic bacterium Bacillus PS3 (TF1) have been studied by EPR and pulsed EPR spectroscopy using Mn(II) as a paramagnetic probe. After dialysis in the presence of EDTA, purified CF1 retains 0.14 +/- 0.07 Mg(II) and approximately 0.75 +/- 0.25 ADP. TF1 retains 0.31 +/- 0.03 Mg(II) and 0.08 +/- 0.01 nucleotide (ADP + ATP) after the same treatment. Supplementing known quantities of Mn(II) to metal-depleted CF1 allowed a spectroscopic characterization of the bound Mn(II) cations, for which the EPR spectra at X- and Q-band are reported. The zero field splitting parameters of Mn(II) are derived from the simulation of the EPR signal recorded at Q-band for a sample supplemented with 0.3 Mn/CF1. The values, magnitude of D approximately 200 x 10(-4) cm-1 and magnitude of E approximately 40 x 10(-4) cm-1 suggest that the Mn(II) binds to CF1 in a slightly distorted environment. The ESEEM spectra of complexes of Mn(II) with CF1 were also recorded for different Mn/CF1 ratios. For a complex with 0.8 Mn/CF1, the ESEEM spectrum shows two frequencies at 3.7 and 8.6 MHz that are attributed to the magnetic coupling with 31P with a hyperfine coupling constant of magnitude of A approximately 5.3 MHz, reflecting the interaction with a phosphate group from the endogenous ADP molecule. This demonstrates close proximity of the strong affinity metal site M1 and the endogenous ADP binding site N1, and binding of the ADP beta-phosphate to the divalent metal cation. For Mn(II) complexes with higher Mn/CF1 ratios, new frequency components below approximately 5 MHz are resolved in the spectra in addition to the peaks from 31P. From a comparison of the CF1 spectra and their magnetic field dependence across the Mn(II) EPR line shape with those of Mn(II) complexes with imidazole, glycine, poly-L-lysine, and nucleotide ligands, it is concluded that additional metal binding sites are filled at higher Mn contents and that these involve 14N donors. It is suggested that the most probable set of ligands of the divalent metal(s) for these additional metal sites in CF1 includes a lysine residue, in line with a previous proposal [Houseman, A. L. P., Morgan, L., LoBrutto, R., & Frasch, W. D. (1994) Biochemistry 33, 4910-4917]. Similar experiments for a Mn(II) complex with TF1 (0.4 Mn/TF1) showed no interaction with 31P; instead modulations are detected in the ESEEM below approximately 5 MHz that are attributed to a 14N ligand. This is tentatively attributed to the deprotonated amine of Lys-162 from a beta subunit, on the basis of the structural data available for the mitochondrial F1 complex. Addition of the substrate ATP to this Mn.TF1 complex leads to the formation of a ternary Mn.TF1.ATP complex with coordination of the Mn(II) by a phosphate group from the ATP as judged from the ESEEM results (magnitude of A(31P) approximately 4.5 MHz). An increase in the hyperfine coupling constant of 31P of the phosphate bound to Mn(II) to magnitude of A(31P) approximately 5.1 MHz is observed after incubation of the ternary complex at room temperature. This is interpreted as a significant rearrangement of the coordination sphere of the Mn(II) in the M1 site of the Mn.TF1.ATP complex and may reflect conformational changes of catalytic significance that occur in the nucleotide binding site during unisite hydrolysis of ATP to ADP by this complex.