Labeling of a thiol residue in sarcoplasmic reticulum ATPase by pyrene maleimide. Solvent accessibility studied by fluorescence quenching

Synthesis and spectral characterization of a thiol-reactive long-lifetime Ru(II) complex

We report the synthesis and spectral properties of a long-lifetime luminescent Ru complex containing a sulfhydryl-reactive maleimide group, [Ru (2,2'-bipyridine)2(1, 10-phenanthroline-5-maleimide)](PF6)2. [Ru(bpy)2(phen-mi)]2+ was covalently linked to human serum albumin, immunoglobulin G, and beta-galactosidase. The lifetimes for probe bound to proteins were near 1.1 micros. In the absence of rotational motions, the probe displayed an anisotropy near 0.17 for excitation near 475 nm. Anisotropy decay data were used to determine rotational correlation times of the proteins, which showed local probe motions in addition to overall rotational diffusion. This long-lifetime sulfhydryl-reactive probe can be used to recover microsecond rotational motions and/or domain motions of proteins and/or macromolecular complexes.

Intrinsic fluorescence as a probe of structure-function relationships in Ca(2+)-transport ATPases

Applications of intrinsic fluorescence measurements in the study of Ca(2+)-transport ATPases are reviewed. Since the initial reports showing that the fluorescence emission was sensitive to Ca2+ binding, a substantial amount of work has focused on the use of both steady-state and time-resolved fluorescence spectroscopy to investigate structure-function relationships in sarcoplasmic reticulum and plasma membrane Ca(2+)-ATPases. These studies have revealed ligand-induced conformational changes, as well as provided information on protein-protein, protein-solvent and/or protein-lipid interactions in different functional states of these proteins. The main results of these studies, as well as possible future prospects are discussed.

Reaction of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole with the (Ca2+ + Mg2+)- ATPase protein of sarcoplasmic reticulum at low temperature

Modification of the (Ca2+ + Mg2+)-ATPase protein of rabbit skeletal sarcoplasmic reticulum (SR) with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, NBD-Cl, at 4 degrees C for 5 min caused a 63% loss of the Ca(2+)-dependent ATPase activity when 1 mol of the adenine analog was incorporated per 10(5) g of protein. At 25 degrees C, above the lipid phase transition, the extent of labeling was 3-fold higher although the Ca(2+)-ATPase activity was inhibited to the same extent. MgATP protected the ATPase activity at 4 degrees C and 25 degrees C but there was little change in the extent of labeling at 4 degrees C suggesting that changes in the fluidity of the lipid moiety made different sites on the ATPase protein accessible to the reagent. At 4 degrees C, addition of sodium deoxycholate enhanced the inactivation (6% ATPase activity remained) but the labeling of the SR-ATPase protein did not increase significantly. Incubation with MgATP prior to solubilization with deoxycholate resulted in the protection of the Ca(2+)-ATPase activity and only a small decrease in the labeling occurred. At 25 degrees C, a similar pattern was found with deoxycholate but the loss of ATPase activity was less dramatic and the extent of labeling by NBD-Cl was greater than that at 4 degrees C. MgATP induced changes in the conformation of the ATPase protein protecting essential cysteine residues while shifting the reaction of NBD-Cl with the ATPase protein to non-essential sites in the absence or presence of deoxycholate. An analysis of tryptic digests of the NBD-ATPase protein showed that MgATP shifted the labeling from the A2 subfragment to the A1 subfragment in the absence of deoxycholate and from the A1 subfragment to the A2 subfragment in the presence of deoxycholate. The reagent, NBD-Cl, can distinguish between different temperature dependent conformational states of the ATPase protein.

Intramolecular distances within the Ca(2+)-ATPase from sarcoplasmic reticulum as estimated through fluorescence energy transfer between probes

Fluorescence energy transfer measurements have been carried out to estimate intramolecular distances between probes bound to Ca(2+)-transporting ATPase (Ca(2+)-ATPase) as well as distances between these probes and the phospholipid headgroup. The nucleotide binding site was monitored by using 1,N6-ethenoadenosine 5'-triphosphate, a fluorescent analogue of ATP, and also by labelling Lys515 with fluorescein 5'-isothiocyanate. Three different cysteine residues were individually labelled using the following probes: 5-[(2-iodoacetyl)aminoethyl]amino-naphthalene-1-sulfonic acid (I-AEDANS), 7-chloro-4-nitro-2,1,3-benzoxadiazole (NBD-Cl) and fluorescent maleimides. The surface of the membrane was labelled by reconstitution with fluorescent phospholipids (fluorescein and rhodamine derivatives). We found a distance of 4.1 nm from the nucleotide binding site to NBD (at Cys344), and the same distance to fluorescent maleimides (at Cys364). The AEDANS label (at Cys670,672) was found separated 3.5 nm from NBD, 4.4 nm from fluorescent maleimides, and 3.9 nm from the lipid matrix. The NBD label was 3.2 nm apart from fluorescent maleimides and 2.2 nm from the lipid matrix. Finally, fluorescent maleimides were found to be located 4.2 nm above the membrane surface. All these distances agree with a molecular model in which NBD is located in the stalk portion of the Ca(2+)-ATPase, near the surface of the membrane, and the rest of the probes are above it, in the globular domain of the protein.

Are there different water requirements in different steps of a catalytic cycle? Hydration effects at the E1 and E2 conformers of sarcoplasmic reticulum Ca(2+)-ATPase studied in organic solvents with low amounts of water

The Ca(2+)-ATPase from sarcoplasmic reticulum was transferred in an active form to a low-water system composed of toluene, phospholipids, and Triton X-100 (TPT). The Ca(2+)-ATPase activity in the TPT system with 4.0% water (by vol. was about 50% of the activity observed in all-aqueous mixtures. Phosphate formation was linear with time up to 20% of ATP hydrolysis and, as expected from an enzyme-catalysed reaction, activity was linear with protein concentration. No ATPase activity was detected in the presence of 3 mM EGTA, indicating that the enzyme retained its Ca2+ dependence in the TPT system. A hyperbolic response to ATP concentration was observed with a Km of 0.15 mM. There was no detectable ATPase activity at water concentrations below 1.5% (by vol.). With 2.0% water, activity became detectable and increased as the water content was progressively raised to 7.0% (by vol.). Higher amounts of water produced unstable emulsions. Enzyme phosphorylation by ATP and dephosphorylation took place in the TPT system. The velocities of both enzyme phosphorylation and dephosphorylation increased with increments in the water content. The enzyme could also be phosphorylated in the TPT system by inorganic phosphate. However, in comparison to ATP, phosphorylation by phosphate took place with significantly lower amounts of water. It is suggested that at low amounts of water, the enzyme is in a relatively rigid conformation and, as the water content is increased, the ATPase acquires more flexibility and, hence, the capacity to carry out catalysis at higher rates. Nevertheless, the release of conformational constraints of the catalytic site of the E2 conformer takes place at water concentrations much lower than those needed for the expression of catalytic activity by the E1 conformer.

Fluorescence properties of the Ca2+,Mg2(+)-ATPase protein of sarcoplasmic reticulum labeled with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

The fluorescence intensity of the Ca2+,Mg2(+)-ATPase protein of rabbit skeletal sarcoplasmic reticulum that incorporated about 2 mol of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) was enhanced at high MgATP concentrations with or without 50 microM calcium. The observed enhancement indicates that the fluorophore, NBD-Cl, can detect conformational changes in the ATPase protein.

Modification of the (Ca2+ + Mg2+)-ATPase protein of sarcoplasmic reticulum with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

The (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase (Ca2+-transporting), EC 3.6.1.38) protein of rabbit skeletal sarcoplasmic reticulum (SR) rapidly incorporated 2 mol of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) per 10(5) g of protein with little change in the Ca2+-dependent ATPase activity. When 2 additional mol of the reagent were bound the Ca2+-ATPase, activity was inhibited. The same pattern was found for modified intact SR and the Ca2+ uptake ability was inhibited. MgATP, CaATP and MgADP protected the Ca2+-ATPase activity concurrent with a decrease of about 1 mol of the NBD group per 10(5) g protein, but the Ca2+ uptake ability was not protected. Calcium alone had no effect on the modification. The modified ATPase protein or SR formed non-serial oligomers or aggregates, but the ATPase protein remained the predominant species present. In the presence of MgATP, oligomer formation was reduced partially but the major changes in the Ca2+-ATPase activity were due to the modification of the ATPase monomer. Thiolysis of the NBD-ATPase protein with dithiothreitol did not restore the Ca2+-ATPase activity, although more than 1 mol of the NBD group was removed from cysteine residues. Cysteine residues were modified in the NBD-ATPase protein or SR when the enzyme activity was inhibited. Trypsin digestion of NBD-SR or its ATPase protein released the A, B, A1, and A2 fragments. The A fragment and its subfragment A2 contained most of the label. Substrate MgATP protection studies showed that the A1 and A2 fragments were involved in maintaining the Ca2+-ATPase activity. Reagent-induced conformational changes of these fragments rather than direct active site group labeling accounted for the loss of ATPase activity.

Role of water activity on the rates of acetyl phosphate and ATP hydrolysis

The rates of hydrolysis of acetyl phosphate in the presence of 0.1 M NaOH and of ATP in the presence of either 1 M HCl or 1 M NaOH were measured at different temperatures and in the presence of different concentrations of the organic solvents dimethyl sulfoxide or ethylene glycol. Under all conditions tested, there was a progressive increase in the rate constant of hydrolysis of both phosphate compounds as the water activity of the medium was decreased by the addition of organic solvents. At 25 degrees C, substitution of 70% of the water of the medium by dimethyl sulfoxide promoted an increase of two orders of magnitude in the rate constant of acetyl phosphate hydrolysis. In the presence of 80% and 90% dimethyl sulfoxide the rate of acetyl phosphate hydrolysis increased by more than two orders of magnitude and was so fast that it could not be measured with the method used. The effect of organic solvents on the rate of ATP hydrolysis was less pronounced than that observed for acetyl phosphate hydrolysis. At 30 degrees C, substitution of 90% of water by an organic solvent promoted a 4-6-fold increase of the rate of ATP hydrolysis. Acceleration of either acetyl phosphate or ATP hydrolysis rates was promoted by a decrease in both activation energies (Ea) and in entropies of activation delta S. The data obtained are discussed with reference to the mechanism of catalysis of enzymes involved in energy transduction such as the Ca2+-ATPase of sarcoplasmic reticulum and the F1-ATPase of mitochondria.

Inhibition of mitochondrial F1 ATPase and sarcoplasmic reticulum ATPase by hydrophobic molecules

The hydrophobic nature of the active site of two energy-transducing ATPases was explored by comparing interactions between Pi and each of three hydrophobic drugs in the absence and presence of organic solvents. The drugs tested were the Fe . bathophenanthroline complex and the anticalmodulin drugs, calmidazolium and trifluoperazine. All inhibit the Pi in equilibrium with ATP exchange reaction catalyzed by submitochondrial particles and the ATPase activity of both submitochondrial particles and soluble F1 ATPase. The inhibition by the three drugs is reversed by either raising the Pi concentration or by adding organic solvent (dimethylsulfoxide, ethyleneglycol or methanol) to the medium. The inhibition of the Pi in equilibrium with ATP exchange by trifluoperazine becomes more pronounced when the electrochemical proton gradient formed across the membrane of the submitochondrial particles is decreased by the addition to the medium of the proton ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone. The ATPase activity and the Ca2+ uptake by sarcoplasmic reticulum vesicles are inhibited by the Fe . bathophenanthroline complex, calmidazolium and trifluoperazine. Phosphorylation of the ATPases by Pi, synthesis of ATP from ADP and Pi and the fast efflux of Ca2+ observed during reversal of the Ca2+ pump are inhibited by the three drugs. The inhibition is reversed by raising the concentration of Pi or dimethylsulfoxide. The three drugs tested appear to compete with Pi for a common binding site on the Ca2+-ATPase. The data presented are interpreted according to the proposal that the catalytic site of an enzyme involved in energy transduction undergoes a hydrophobic-hydrophilic transition during the catalytic cycle.