Calorimetric studies of the interaction of myosin with ADP

Energetics of muscle contraction: further trials

Knowledge accumulated in the field of energetics of muscle contraction has been reviewed in this article. Active muscle converts chemical energy into heat and work. Therefore, measurements of heat production and mechanical work provide the framework for understanding the process of energy conversion in contraction. In the 1970s, precise comparison between energy output and the associated chemical reactions was performed. It has been found that the two do not match in several situations, resulting in an energy balance discrepancy. More recently, efforts in resolving these discrepancies in the energy balance have been made involving chemical analysis, phosphorus nuclear magnetic resonance spectroscopy, and microcalorimetry. Through reviewing the evidence from these studies, the energy balance discrepancy developed early during isometric contraction has become well understood on a quantitative basis. In this situation energy balance is established when we take into account the binding of Ca to sarcoplasmic proteins such as troponin and parvalbumin, and also the shift of cross-bridge states. On the other hand, the energy balance discrepancy observed during rapid shortening still remains to be clarified. The problem may be related to the essential mechanism of cross-bridge action.

Heat, phosphorus NMR and microcalorimetry in relation to the mechanism of filament sliding

For muscle heat measurements the methods available are sensitive and rapid, and the heat is related to the chemical changes in a manner that provides a firm outline for understanding the mechanism of contraction. For example linear dependence of the shortening heat on the sarcomere length has shown that the rate of turnover of cross-bridges increases during shortening. However, heat is bound to lack specificity. In order to cope with this problem, various methods such as rigorous chemical analyses, phosphorus NMR and microcalorimetry have been introduced. As a result of ultra-rapid freezing and chemical analysis by D. R. Wilkie (Gilbert, Kretzchmar, Wilkie and Woledge, 1971), the energy balance discrepancy between (heat + work) and the amount of phosphocreatine (PCr) split emerged, i.e. the unexplained enthalpy. Calcium ions move from the sarcoplasmic reticulum to the calcium-receptive proteins in the sarcoplasm during contraction. In an attempt to find the cause of the unexplained enthalpy, microcalorimetry of calcium binding to calcium-receptive proteins has been performed. The results have shown that calcium ions dislocated between sites within the sarcoplasm on activation may produce about 1/3 of the unexplained heat. In addition calcium pump should operate by consuming PCr to relocate the calcium after the contraction. Time-resolved phosphorus NMR has also shown that a certain amount of PCr splitting continues during early minute of recovery period after the contraction without Pi released. This delayed splitting of PCr is most likely caused by the kinetic properties of the contractile proteins and can explain another 1/3 of the unexplained enthalpy. The mechanism of how muscle is regulated is another important question. Studies of calcium binding to calcium-receptive proteins in the sarcoplasm by using titration microcalorimetry has shown that troponin C has a characteristic single calcium-binding site that is most likely to be involved in the regulation of contraction.

Anomalous binding of MgADP to myosin of skeletal muscle

Binding of adenosine diphosphate to skeletal muscle myosin was studied using a range of concentrations from 0 to 2 mM. Up to 0.2 mM adenosine diphosphate two equivalent and independent nucleotide binding sites were detected, characterized by the single association constant of 5 x 10(4)M(-1). At greater adenosine diphosphate concentrations a decreasing binding capacity was noticed, bound nucleotide being essentially approximately 0.1 mol/mol at a 1-2mM adenosine diphosphate concentration. We tentatively propose that nucleotides act indirectly on myosin by promoting the perturbation of the solvent, which is supported by the fact that polyphosphates are known powerful kosmotropes.

Isotachophoresis of nucleic acid constituents

Determination methods for purine and pyrimidine bases, nucleosides, nucleotides and related compounds using analytical capillary isotachophoresis are reviewed. First, the isotachophoretic characterization of these compounds, as well as methods for sample preparation prior to analysis, and the different ways of detecting unknown substances in complex biological systems are described. Then applications of isotachophoretic analysis in medical diagnosis and biomedical research are reviewed. In particular, the analysis of purines, pyrimidines, nucleosides and related compounds in blood and serum for the diagnosis and treatment of inherited diseases and cancer is described. Selected applications of nucleotide analysis in biomedical research using different tissue extracts are also reviewed, and some examples of nucleotide-dependent enzymic reactions, which were performed by means of analytical isotachophoresis, are presented.

Enthalpy, entropy and heat capacity changes induced by binding of calcium ions to cardiac troponin C

Microcalorimetric titrations have been used to study the binding of Ca2+ to cardiac troponin C. Measurements were made both in the presence and in the absence of Mg2+, and at temperatures of 5 degrees, 15 degrees and 25 degrees C. Changes in enthalpy, entropy and heat capacity of troponin C associated with Ca binding have been determined. Cardiac troponin C exhibited a decrease in enthalpy and an increase in entropy associated with Ca binding. Enthalpy changes increased linearly with temperature, indicating that the Ca binding causes negative changes in the heat capacity of troponin C. These results show that the Ca binding causes a strong hydrophobic effect and a tightening of the molecular structure of cardiac troponin C.

Interaction of ADP and magnesium with the active site of myosin subfragment-1 observed by reactivity changes of the critical thiols and by direct binding methods at low and high ionic strength

Comprehensive binding studies using direct and indirect methods yield stoichiometry and affinities for the binding of Mg X ADP and uncomplexed ADP to the active site of myosin subfragment-1. Additionally, the binding parameters for Mg2+ in the ternary complex protein X Mg X ADP are presented for the first time. The indirect method makes use of reactivity changes of the critical thiol-1 and thiol-2 groups, which occur upon the binding of the ligand at the active site. The affinity constants derived by this method are corroborated by two independent direct methods, equilibrium dialysis and centrifugation transport. For Mg2+, ADP and Mg X ADP just one mole of ligand binds/mole subfragment-1. The affinity of Mg X ADP at low ionic strength is 2.1 X 10(6) M-1 and only five-times lower in the absence of Mg2+. In the ternary complex Mg2+ has a low affinity of 4.1 X 10(4) M-1. At high ionic strength the uncomplexed ADP binds with a 43-times-lower affinity than Mg X ADP, whose affinity is 6.9 X 10(5) M-1. In this case Mg2+ interacts in the ternary complex with the higher affinity of 3.2 X 10(5) M-1, implying that at high salt concentration it plays a more prominent role in anchoring ADP at the active site.

Kinetic and thermodynamic properties of the ternary complex between F-actin, myosin subfragment 1 and adenosine 5'-[beta, gamma-imido]triphosphate

Equilibrium constants for the formation of a ternary complex between actin, myosin subfragment 1 (S1) and the non-hydrolyzable ATP analog adenosine 5'-[beta, gamma-imido]triphosphate (Ado PP[NH]P) were determined from light-scattering titrations under a variety of conditions. The affinities of S1 (binding constant K1) and acto . S1 (K4) for AdoPP[NH]P have relatively low dependencies on temperature (delta H degrees approximately equal to - 15 - 30 kJ mol-1) and ionic strength, in contrast to the affinities of S1 (K2) and S1 . AdoPP[NH]P (K3) for actin which are influenced quite strongly by temperature (delta H degrees approximately equal to 50 - 65 kJ mol-1) and ionic strength, K2 decreasing by a factor of 10 - 15 between I = 0.05 M and I = 0.2 M and K3 decreasing by a factor of 5.K1, and by detailed balance K2 as well, were found to be about 10-times higher than hitherto reported values (K1 = 3.4 X 10(7) M-1, K2 = 6 X 10(8) M-1, at 24 degrees C,I = 0.09 M, pH 8.0). The binding of ADP to S1 is about 10-fold weaker than that of AdoPP[NH]P, being however much more exothermic (delta H degrees = - 70 kJ mol-1 at I = 0.1 M) and having a negative standard entropy change (delta S = - 125 J mol-1 K-1), in contrast to AdoPP[NH]P binding for which the calculated delta S had positive values. The observed rate constant of dissociation of acto . S1 by AdoPP[NH]P showed an almost hyperbolic dependence on the nucleotide concentration, reaching a maximum of 15 s-1 at I = 0.055 M and 5 s-1 at I = 0.275 M, pH 8.0, 23 degrees C; at 5 degrees C this value was somewhat higher. The rate constant of dissociation of AdoPP[NH]P from its complex with acto . S1 was estimated to exceed 400 s-1 at 23 degrees C, and to be of the order of 150 s-1 at 4 degrees C. The observed rate constant for the association of the S1 . nucleotide complex and actin was proportional to actin concentrations up to 60 microM, thus defining an apparent second-order rate constant of 2 X 10(4) M-1 s-1 at I = 0.125 M and 23 degrees C. A reaction scheme is proposed in which isomerizations of the acto . S1 and acto . S1 . nucleotide complexes can occur.

Symmetry and asymmetry in the contractile protein myosin

The subunit composition of the myosin molecule which is built up from 3 pairs of identical polypeptide chains (2 heavy chains and 2 pairs of light chains), gives it the appearance of having symmetric structure. This homodimeric arrangement in the molecule is in fact asymmetric in its construction as a result of the natural folding of the chains. There are also heterodimers which result from combinations of pairs of heavy chains and/or light chains which are not identical in their amino acid sequence. Enzyme kinetics and ligand binding are characterised by homogeneous processes in studies on isolated myosin heads. With the double-headed molecular species, myosin and its water-soluble fragment heavy meromyosin, the enzyme kinetics, nucleotide and metal ion binding exhibit negative cooperativity. Binding of Mg-ADP to active centres induces site-site and therefore head-head interaction, thus intact myosin is designed to be able to function asymmetrically. It is suggested that the ligand-induced asymmetry between the heads plays a central role in crossbridge function. The two heads, even in rest, adopt non-equivalent conformations and it is argued that this built-in constraint complements the asymmetric mode of interaction they subsequently undergo with their reaction partners on the actin filament. It is concluded that the enzyme is so constructed that during contraction the heads can perform their function in an alternating cooperative way.

Thermodynamic studies on the binding of adenosine diphosphate and calcium to beef cardiac myosin

Thermodynamic quantities for the binding of MgADP, CaADP and Ca2+ to purified beef cardiac myosin have been determined by flow calorimetry at 25 degrees C and by equilibrium dialysis at 4 degrees C in 0.5 M KCl, 20 mM tris-HCl (pH 7.5). About 1.65 +/- 0.15 mol MgADP and 1.9 +/- 0.1 mol CaADP were bound per mol myosin. Free energies of binding of MgADP and CaADP were -6.7 and -5.7 kcal/mol, respectively. Enthalpies for binding of MgADP and CaADP were about -12.5 and -19.0 kcal/mol, respectively. Furthermore, there were 1.8 +/- 0.2 mol high affinity Ca2+ binding sites per mol myosin with an affinity constant of about 10(5) M-1. The enthalpy of Ca2+ binding was about zero. It is concluded that CaADP binds to cardiac myosin with a much greater negative enthalpy than MgADP. Also, the free energy of MgADP binding to cardiac myosin is similar to values reported for skeletal myosin. However, the enthalpy of binding is much less negative than the value obtained for skeletal myosin by Kodama and Woledge (J. Biol. Chem. (1976) 251, 7499--7503). The latter results suggest a subtle difference in the nucleotide binding sites of these myosins.

Optical spectroscopic study of the ADP-myosin interaction

The binding of ADP to heavy meromyosin, and the separated subfragment 1 components S-1(A1) and S-1 (A2), has been observed by ultraviolet spectrophotometry. The results are compatible with the presence of spectroscopically equivalent and independent sites, one per head, at both 10 degrees C and 25 degrees C. We do not observe the heterogeneity of binding and of the spectroscopic response that has been reported. The binding has also been followed by other methods sensitive to the effect of ligand on the aromatic residues of the protein, viz. intrinsic fluorescence of heavy meromyosin and changes in the near-ultraviolet Cotton effects of myosin, and its active fragments. Within the limits of our experimental precision, the binding profiles, based on concentration of myosin heads, are the same for myosin as for subfragment 1. A perturbation in the circular dichroism is also generated by pyrophosphate, which competes with ADP. The spectra suggest that subsites for the purine ring and the diphosphate can be recognized. The sensitivity of binding profiles obtained by methods of the kind used here to cooperative or antagonistic interactions between the binding sites has been analysed. It is clear that sizeable effects of this nature could be concealed by the binding curves, even for high experimental precision.