Two types of proton-modulated calcium binding in the sarcoplasmic reticulum Ca(2+)-ATPase. II. Characteristics of their calcium bindings

Ca2+-ATPase Molecules as a Calcium-Sensitive Membrane-Endoskeleton of Sarcoplasmic Reticulum

The Ca²⁺-transport ATPase of sarcoplasmic reticulum (SR) is an integral, transmembrane protein. It sequesters cytoplasmic calcium ions released from SR during muscle contraction, and causes muscle relaxation. Based on negative staining and transmission electron microscopy of SR vesicles isolated from rabbit skeletal muscle, we propose that the ATPase molecules might also be a calcium-sensitive membrane-endoskeleton. Under conditions when the ATPase molecules scarcely transport Ca²⁺, i.e., in the presence of ATP and ≤ 0.9 nM Ca²⁺, some of the ATPase particles on the SR vesicle surface gathered to form tetramers. The tetramers crystallized into a cylindrical helical array in some vesicles and probably resulted in the elongated protrusion that extended from some round SRs. As the Ca²⁺ concentration increased to 0.2 µM, i.e., under conditions when the transporter molecules fully carry out their activities, the ATPase crystal arrays disappeared, but the SR protrusions remained. In the absence of ATP, almost all of the SR vesicles were round and no crystal arrays were evident, independent of the calcium concentration. This suggests that ATP induced crystallization at low Ca²⁺ concentrations. From the observed morphological changes, the role of the proposed ATPase membrane-endoskeleton is discussed in the context of calcium regulation during muscle contraction.

Susceptibility test of two Ca(2+)-ATPase conformers to denaturants and polyols to outline their structural difference

To determine the effect of denaturants [guanidine hydrochloride (GdnHCl) and urea] and polyols [with various molecular masses (62.1-600)] on calcium binding at the two hypothesized conformers (A and B forms) of the chemically equivalent sarcoplasmic reticulum Ca(2+)-ATPase, which bind two calcium ions in different manners, we examined the effect of these reagents on the calcium dependence of ATP-supported phosphorylation of the ATPase molecules and of their calcium-activated, acetyl phosphatate hydrolytic activity. (1) GdnHCl (~0.05 M) and urea (~0.5 M) increased the apparent calcium affinity (K (0.5)) of 2-6 μM of noncooperative binding [Hill coefficient (n (H)) ~ 1] of the A form to 10-40 μM. (2) The employed polyols transformed the binding of the A form into cooperative binding (n (H) ~ 2), accompanying the approach of its K (0.5) value to that (K (0.5) = 0.04-0.2 μM) of the cooperative binding (n (H) ~ 2) of the B form; the transition concentration (0.025-2 M) of the polyols, above which such transformation occurs, was in inverse relation to their molecular mass. (3) The binding of the B form was resistant to these denaturants and polyols. Based on these data, a structural model of the two forms, calcium-binding domains of which are loosely and compactly folded, is presented.

Side-chain protonation and mobility in the sarcoplasmic reticulum Ca2+-ATPase: implications for proton countertransport and Ca2+ release

Protonation of acidic residues in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 1a) was studied by multiconformation continuum electrostatic calculations in the Ca(2+)-bound state Ca(2)E1, in the Ca(2+)-free state E2(TG) with bound thapsigargin, and in the E2P (ADP-insensitive phosphoenzyme) analog state with MgF(4)(2-) E2(TG+MgF(4)(2-)). Around physiological pH, all acidic Ca(2+) ligands (Glu(309), Glu(771), Asp(800), and Glu(908)) were unprotonated in Ca(2)E1; in E2(TG) and E2(TG+MgF(4)(2-)) Glu(771), Asp(800), and Glu(908) were protonated. Glu(771) and Glu(908) had calculated pK(a) values larger than 14 in E2(TG) and E2(TG+MgF(4)(2-)), whereas Asp(800) titrated with calculated pK(a) values near 7.5. Glu(309) had very different pK(a) values in the Ca(2+)-free states: 8.4 in E2(TG+MgF(4)(2-)) and 4.7 in E2(TG) because of a different local backbone conformation. This indicates that Glu(309) can switch between a high and a low pK(a) mode, depending on the local backbone conformation. Protonated Glu(309) occupied predominantly two main, very differently orientated side-chain conformations in E2(TG+MgF(4)(2-)): one oriented inward toward the other Ca(2+) ligands and one oriented outward toward a protein channel that seems to be in contact with the cytoplasm. Upon deprotonation, Glu(309) adopted completely the outwardly orientated side-chain conformation. The contact of Glu(309) with the cytoplasm in E2(TG+MgF(4)(2-)) makes this residue unlikely to bind lumenal protons. Instead it might serve as a proton shuttle between Ca(2+)-binding site I and the cytoplasm. Glu(771), Asp(800), and Glu(908) are proposed to take part in proton countertransport.

Substrate regulation of calcium binding in Ca2+-ATPase molecules of the sarcoplasmic reticulum. II. Effect of CTP, GTP, ITP, and UTP

To examine the effect of CTP, GTP, ITP, and UTP on calcium binding of Ca2+-ATPase molecules of the sarcoplasmic reticulum, the calcium dependence of the Ca2+-activated hydrolysis activities of these NTPs of the enzyme molecules was examined by comparison with that of calcium binding of the molecules in the absence of the NTPs at pH 7.40. In the sarcoplasmic reticulum membrane, CTP, GTP, and ITP did not affect the noncooperative (Hill value (n(H)) of approximately 1, apparent calcium affinity (K(0.5)) of 2-6 microm)) and cooperative (n(H) approximately 2, K(0.5) approximately 0.2 microm) calcium binding of the molecules, whereas UTP caused the molecules to highly cooperatively (n(H) approximately 4) bind calcium ions with a lowered K(0.5) of approximately 0.04 microm. When the enzyme molecules were solubilized with detergent, all of these NTPs reversibly degraded the calcium affinity of the molecule (from K(0.5) = 3-5 to >40 microm), although the effect of the NTPs on the negatively cooperative manner (n(H) approximately 0.5) of calcium binding was not experimentally obtained. Taking into account the first part of this study (Nakamura, J., Tajima, G., Sato, C., Furukohri, T., and Konishi, K. (2002) J. Biol. Chem. 277, 24180-24190) showing the improving effect of ATP on calcium binding of the membranous and solubilized molecules, the results show that ATP is the only intrinsic substrate for the enzyme molecule. This NTP regulation is discussed in terms of the oligomeric structure of the molecules.

Substrate regulation of calcium binding in Ca2+-ATPase molecules of the sarcoplasmic reticulum. I. Effect of ATP

The effect of ATP on calcium binding of the Ca2+-ATPase of the sarcoplasmic reticulum has not been clarified. By comparing the calcium dependence of the ATPase activity and of phosphorylation of the ATPase molecules with that of calcium binding in the absence of ATP, we show the existence of two types of regulatory site of the enzyme molecules at which ATP binding variously improves the calcium binding performance of the molecules depending on the aggregation state of the molecules and pH; the two regulatory sites bind ATP at submillimolar (0.25 mm) and millimolar (5 mm) ATP, respectively. The results are discussed based on a model of two conformational variants (A and B forms) of the chemically equivalent ATPase molecules (Nakamura, J., and Furukohri, T. (1994) J. Biol. Chem. 269, 30818-30821). For example, in the sarcoplasmic reticulum membrane at pH 7.40, submillimolar ATP converted the calcium binding manner of the A form from noncooperative (Hill number (n(H)) of approximately 1) to cooperative (n(H) approximately 2), concurrent with a decrease in the apparent calcium affinity (K(0.5)) from 2-6 to 0.1-0.3 microm. The binding of the A form became almost the same as that of the B form (n(H) approximately 2, K(0.5) approximately 0.2 microm), which was not affected by ATP. Millimolar ATP further decreased the K(0.5) of the cooperative binding of the two forms to approximately 0.05 microm. Regulation of the calcium binding performance by ATP is discussed in terms of monomeric and oligomeric pathway models.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Independence of two conformations of sarcoplasmic reticulum Ca2+-ATPase molecules in hydrolyzing acetyl phosphate. A two-pair model of the ATPase structural unit

The sarcoplasmic reticulum Ca2+-ATPase molecules have been shown to exist in two conformations (A and B) that result from intermolecular interaction of ATPase molecules (Nakamura, J., and Tajima, G. (1995) J. Biol. Chem. 270, 17350-17354). The A form binds two calcium ions noncooperatively, whereas the B form binds the calcium ions cooperatively. Here, we examined the independence of these two forms in the calcium-activated hydrolysis of acetyl phosphate (AcP) under asynchronous and synchronous conditions of their E1-E2 transitions at 0-5 and 25 degrees C. Irrespective of their synchronism and temperature, the two forms hydrolyzed AcP due to calcium that was bound to each of the forms, indicating the independence of the two forms in hydrolyzing AcP. Taking into account the monomer-dimer transition of the ATPase molecules on the sarcoplasmic reticulum membrane accompanying E1-E2 transition of the molecules (Dux, L., Taylor, K. A., Ting-Beall, H. P., and Martonosi, A. (1985) J. Biol. Chem. 260, 11730-11743), the two types of molecules seem to independently carry out such monomer-dimer transition of the same type of molecules. Two pairs, each consisting of the same type of molecules, are suggested to be the structural unit of the ATPase molecules.

The modulation of Ca2+ binding to sarcoplasmic reticulum ATPase by ATP analogues is pH-dependent

Excess ATP is known to enhance Ca(2+)-ATPase activity and, among other effects, to accelerate the Ca2+ binding reaction. In previous work, we studied the pH dependence of this reaction and proposed a 3H+/2Ca2+ exchange at the transport sites, in agreement with the H+/Ca2+ counter transport. Here we studied the effect of ADP and nonhydrolyzable ATP analogues on the Ca2+ binding reaction at various pH values. At pH 6, where Ca2+ binding is monophasic and slow, ADP, adenosine 5'-(beta,gamma-methylene)triphosphate (AMP-PCP), or adenyl-5'-yl imidodiphosphate (AMPPNP) increased the Ca2+ binding rate constant 20-fold. At pH 7 and 8, where Ca2+ binding is biphasic, the nucleotides induce fast and monophasic Ca2+ binding. At pH 7, AMP-PCP accelerated Ca2+ binding with an apparent dissociation constant of 10 microM. At acidic pH, ADP, AMPPCP, or AMPPNP increased the equilibrium affinity of Ca2+ for ATPase, whereas at alkaline pH, these nucleotides had no effect. At pH 5.5, AMPPCP increased equilibrium Ca2+ binding with an apparent dissociation constant of 1 microM.

Negative or positive cooperation in calcium binding to detergent-solubilized ATPase of the sarcoplasmic reticulum. Its modulation by a high concentration of ATP

Two different conformations of chemically equivalent Ca(2+)-ATPase molecules in the sarcoplasmic reticulum have been shown to non- and positive cooperatively bind two calcium ions, respectively (Nakamura, J. (1994) J. Biol. Chem. 269, 30822-30827). At pH 7.40, these ATPase molecules split into E1 (high affinity state for calcium), and E2 (low affinity state for calcium), respectively, before calcium binding. At this pH, calcium binding to the monomeric ATPase, solubilized with dodecyloctaethylenglycol monoether, was studied by examining 45Ca2+ binding to the ATPase and calcium dependence of its phosphorylation, fluorescence intensity, ATP-hydrolysis at a low (5 microM) concentration of ATP, and acetyl phosphate hydrolysis. The results suggest that the solubilized ATPase molecules predominantly preexist in E2 and negative cooperatively (the Hill value (nH) = 0.5-0.6) bind 2 mol of calcium/mol of the ATPase with an apparent calcium affinity (K0.5) of 3-5 microM. The nonequivalences of calcium bindings at the membranous ATPase molecules seem to result from the intermolecular interaction of the molecules. A high concentration (5 mM) of ATP modulated the binding manner so that it became positively cooperative (nH approximately 2) and increased the K0.5 to 0.1 microM.