Metal interactions with beef heart mitochondrial ATPase

High-affinity metal-binding site in beef heart mitochondrial F1ATPase: an EPR spectroscopy study

The high-affinity metal-binding site of isolated F(1)-ATPase from beef heart mitochondria was studied by high-field (HF) continuous wave electron paramagnetic resonance (CW-EPR) and pulsed EPR spectroscopy, using Mn(II) as a paramagnetic probe. The protein F(1) was fully depleted of endogenous Mg(II) and nucleotides [stripped F(1) or MF1(0,0)] and loaded with stoichiometric Mn(II) and stoichiometric or excess amounts of ADP or adenosine 5'-(beta,gamma-imido)-triphosphate (AMPPNP). Mn(II) and nucleotides were added to MF1(0,0) either subsequently or together as preformed complexes. Metal-ADP inhibition kinetics analysis was performed showing that in all samples Mn(II) enters one catalytic site on a beta subunit. From the HF-EPR spectra, the zero-field splitting (ZFS) parameters of the various samples were obtained, showing that different metal-protein coordination symmetry is induced depending on the metal nucleotide addition order and the protein/metal/nucleotide molar ratios. The electron spin-echo envelope modulation (ESEEM) technique was used to obtain information on the interaction between Mn(II) and the (31)P nuclei of the metal-coordinated nucleotide. In the case of samples containing ADP, the measured (31)P hyperfine couplings clearly indicated coordination changes related to the metal nucleotide addition order and the protein/metal/nucleotide ratios. On the contrary, the samples with AMPPNP showed very similar ESEEM patterns, despite the remarkable differences present among their HF-EPR spectra. This fact has been attributed to changes in the metal-site coordination symmetry because of ligands not involving phosphate groups. The kinetic data showed that the divalent metal always induces in the catalytic site the high-affinity conformation, while EPR experiments in frozen solutions supported the occurrence of different precatalytic states when the metal and ADP are added to the protein sequentially or together as a preformed complex. The different states evolve to the same conformation, the metal(II)-ADP inhibited form, upon induction of the trisite catalytic activity. All our spectroscopic and kinetic data point to the active role of the divalent cation in creating a competent catalytic site upon binding to MF1, in accordance with previous evidence obtained for Escherichia coli and chloroplast F(1).

Characterization of the functional coupling of bovine brain vacuolar-type H(+)-translocating ATPase. Effect of divalent cations, phospholipids, and subunit H (SFD)

Vacuolar-type H+-translocating ATPases (V-ATPases or V-pumps) are complex proteins containing multiple subunits and are organized into two functional domains: a peripheral catalytic sector V1 and a membranous proton channel V0. The functional coupling of ATP hydrolysis activity to proton transport in V-pumps requires a regulatory component known as subunit H (SFD) as has been shown both in vivo and in vitro (Ho, M. N., Hirata, R., Umemoto, N., Ohya, Y., Takatsuki, A., Stevens, T. H., and Anraku, Y. (1993) J. Biol. Chem. 268, 18286-18292; Xie, X. S., Crider, B. P., Ma, Y. M., and Stone, D. K. (1994) J. Biol. Chem. 269, 25809-25815). Ca2+ is thought to uncouple V-pumps because it is found to support ATP hydrolysis but not proton transport, while Mg2+ supports both activities. The direct effect of phospholipids on the coupling of V-ATPases has not been reported, likely due to the fact that phospholipids are constituents of biological membranes. We now report that Ca2+-induced uncoupling of the bovine brain V-ATPase can be reversed by imposition of a favorable membrane potential. Furthermore we report a simple "membrane-free" assay system using the V0 proton channel-specific inhibitor bafilomycin as a probe to detect the coupling of V-ATPase under certain conditions. With this system, we have characterized the functional effect of subunit H, divalent cations, and phospholipids on bovine brain V-ATPase and have found that each of these three factors plays a critical role in the functional coupling of the V-pump.

Characterization of the binding of Fe(III) to F1ATPase from bovine heart mitochondria

The binding Fe(III) to F1ATPase purified from beef heart mitochondria has been characterized by chemical analyses and EPR spectroscopy. F1ATPase binds 2 mol of Fe(III)/mol of protein selectively in the presence of saturating concentrations of ATP. In the absence of nucleotides or in the presence of either saturating ADP or limiting ATP concentrations, the enzyme binds 1 equivalent of Fe(III). F1ATPase pretreated with 5'-p- fluorosulfonylbenzoyladenosine, that selectively modifies the non-catalytic sites, binds only 1 mol of Fe(III)/mol of protein in the presence of either saturating ATP or ADP, Fe(III)-loaded F1ATPase containing either 1 or 2 equivalents of Fe(III) show identical EPR signals at g=4.3. The signals are not perturbed by the binding of nucleotides to the enzyme while they are altered by phosphate addition. These results indicate that F1ATPase contains two distinct Fe(III)-binding sites, which differ from nucleotide-binding sites, and that one of these sites is opened up for Fe(III) uptake by conformational changes induced by binding of ATP to the loose non-catalytic site.

Effects of nucleotides on the protein ligands to metals at the M2 and M3 metal-binding sites of the spinach chloroplast F1-ATPase

We have identified the most probable protein ligands at the catalytic M3 and noncatalytic M2 metal-binding sites in the spinach chloroplast F1-ATPase (CF1) and here propose possible residues in the protein sequence for these ligands in latent CF1 in the absence of nucleotide. The changes in the metal ligands at these sites upon binding of nucleotide to the N2 and N3 sites and upon activation of latent CF1 provide a possible molecular basis for inhibition of ATPase activity by free metal, for the lack of activity in the latent state, and for the gating mechanism of the ATPase H+ pump. To these ends, the Mg2+ analogue, vanadyl (VIV = O)2+, was used as a paramagnetic probe at the M2 and M3 metal-binding sites. EPR and ESEEM spectra of VO2+ were obtained, and simulations of the full EPR spectra imply the ligand sets at the different metal-binding sites. When VO2+ is added to CF1 in the absence of ATP, the most likely set of ligands at the M2 site are 1 ROH (alpha T176), 2 H2O, and 1 RCOO- (alpha D269 or alpha D270), where the suggested amino acid designations of the residues are given in parentheses according to the mitochondrial sequence. Evidence suggests a possible axial nitrogen ligand at this site (alpha K175). When the M2 site is filled by addition of VO2+ and ATP, the metal binds as a second species in which N2-bound ATP and M2-bound VO2+ form a monodentate complex with concomitant exchange of the equatorial protein ligands by 3 H2O.(ABSTRACT TRUNCATED AT 250 WORDS)

A radiotracer probe to study metal interaction with human lactate dehydrogenase isoenzymes

The electrophilic Ag+ ion was found to destroy completely the enzymatic activity of lactate dehydrogenase isoenzyme LDH-1 while other transition metal ions reduced its activity in varying degrees. A radiotracer probe involving 110mAg-labeled silver ion was used to understand the mechanism of denaturation of LDH and also to determine the number of active sites, if any, for substrate binding with the enzyme. Purified LDH-l was reacted with 110mAg-labeled silver ion and the mixture was passed through the sephadex G-75-120 gel to separate the 110mAg-LDH complex that might be formed during the reaction. The resulting elution curve revealed that a stable complex was formed. From the total radioactivity of 110mAg bound LDH, the specific activity of labeled Ag+ and the amount of LDH used the ratio of the number of moles of Ag+ reacted with 1 mol of LDH was computed. This was found to be approximately 4.0, indicating that there are four binding sites in LDD, probably one on each subunit. Kinetic studies of LDH catalysis of L-P reaction in the presence and absence of Ag+ ion suggest that silver ion is involved in competitive inhibition and that the interaction conforms to the "lock-and-key" model. The inhibition of catalysis by other metals is presumably of a noncompetitive type.

The effect of inorganic pyrophosphate on the activity and Pi-binding properties of mitochondrial F1-ATPase

Interaction of F1-ATPase from beef heart mitochondria with PPi has been investigated. The presence of PPi in the ATPase assay medium does not affect the initial rate of ATP hydrolysis by F1-ATPase, but slows down the decrease of enzyme activity in the course of ATP hydrolysis and increases the steady-state rate of ATP hydrolysis. Being present in the ATPase assay medium, PPi accelerates the ATP-dependent reactivation of an inactive complex formed by F1-ATPase and ADP. This inactive complex is also reactivated after preincubation with PPi. F1-ATPase, preincubated with PPi, is inactivated by azide much more slowly than is the non-preincubated enzyme. PPi stimulates the binding of Pi to F1-ATPase by decreasing mainly the Kd for Pi and only slightly raising the stoichiometry of high-affinity Pi binding. It follows from the results obtained that PPi interacts with the non-catalytic site(s) of F1-ATPase.

Regulation of the chloroplast H+-ATPase by light. The involvement of Mg2+ ions

The involvement of Mg2+ ions in the light-dependent regulation of the chloroplast H+-ATPase was studied in both type C and osmotically shocked type A chloroplasts. The following results were obtained. ATPase activity measured under dark, partially uncoupling conditions, following light activation with dithiothreitol and pyocyanine, was markedly enhanced by the presence of Mg2+ in the activation stage. This Mg2+ effect required concentrations in the millimolar range, was rather slow (time range of minutes), reversible, rather unspecific and did not involve changes in the affinity to dithiothreitol. Dark deactivation of the ATPase in the absence of substrate was accelerated by Mg2+. The dark effect of Mg2+ also required millimolar concentrations, but was fast (time range of seconds), highly specific for Mg2+, and did not involve thiol oxidation. The major effect of the absence of Mg2+ from the light-activation stage or of its presence in the dark interval between activation and assay was the induction of an 'abnormal' sensitivity to uncouplers: after these treatments ATP hydrolysis was not stimulated but rather inhibited by NH4Cl or other uncouplers. The pretreatments in the light without Mg2+ or dark with Mg2+ did not affect the membrane proton permeability, nor the proton pumping coupled to ATPase activity. The results are discussed in terms of Mg2+-dependent regulation of the enzyme complex at the level of subunit interaction and its effect on the affinity to protons.