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

Advances in understanding the energetics of muscle contraction

Muscle energetics encompasses the relationships between mechanical performance and the biochemical and thermal changes that occur during muscular activity. The biochemical reactions that underpin contraction are described and the way in which these are manifest in experimental recordings, as initial and recovery heat, is illustrated. Energy use during contraction can be partitioned into that related to cross-bridge force generation and that associated with activation by Ca2+. Activation processes account for 25-45% of ATP turnover in an isometric contraction, varying amongst muscles. Muscle energy use during contraction depends on the nature of the contraction. When shortening muscles produce less force than when contracting isometrically but use energy at a greater rate. These characteristics reflect more rapid cross-bridge cycling when shortening. When lengthening, muscles produce more force than in an isometric contraction but use energy at a lower rate. In that case, cross-bridges cycle but via a pathway in which ATP splitting is not completed. Shortening muscles convert part of the free energy available from ATP hydrolysis into work with the remainder appearing as heat. In the most efficient muscle studied, that of a tortoise, cross-bridges convert a maximum of 47% of the available energy into work. In most other muscles, only 20-30% of the free energy from ATP hydrolysis is converted into work.

Binding of calcium and magnesium to human cardiac troponin C

Cardiac muscle thin filaments are composed of actin, tropomyosin, and troponin that change conformation in response to Ca²⁺ binding, triggering muscle contraction. Human cardiac troponin C (cTnC) is the Ca²⁺-sensing component of the thin filament. It contains structural sites (III/IV) that bind both Ca²⁺ and Mg²⁺ and a regulatory site (II) that has been thought to bind only Ca²⁺. Binding of Ca²⁺ at this site initiates a series of conformational changes that culminate in force production. However, the mechanisms that underpin the regulation of binding at site II remain unclear. Here, we have quantified the interaction between site II and Ca²⁺/Mg²⁺ through isothermal titration calorimetry and thermodynamic integration simulations. Direct and competitive binding titrations with WT N-terminal cTnC and full-length cTnC indicate that physiologically relevant concentrations of both Ca²⁺/Mg²⁺ interacted with the same locus. Moreover, the D67A/D73A N-terminal cTnC construct in which two coordinating residues within site II were removed was found to have significantly reduced affinity for both cations. In addition, 1 mM Mg²⁺ caused a 1.4-fold lower affinity for Ca²⁺. These experiments strongly suggest that cytosolic-free Mg²⁺ occupies a significant population of the available site II. Interaction of Mg²⁺ with site II of cTnC likely has important functional consequences for the heart both at baseline as well as in diseased states that decrease or increase the availability of Mg²⁺, such as secondary hyperparathyroidism or ischemia, respectively.

In depth, thermodynamic analysis of Ca2+ binding to human cardiac troponin C: Extracting buffer-independent binding parameters

BACKGROUND

Characterizing the thermodynamic parameters behind metal-biomolecule interactions is fundamental to understanding the roles metal ions play in biology. Isothermal Titration Calorimetry (ITC) is a "gold-standard" for obtaining these data. However, in addition to metal-protein binding, additional equilibria such as metal-buffer interactions must be taken into consideration prior to making meaningful comparisons between metal-binding systems.

METHODS

In this study, the thermodynamics of Ca²⁺ binding to three buffers (Bis-Tris, MES, and MOPS) were obtained from Ca²⁺-EDTA titrations using ITC. These data were used to extract buffer-independent parameters for Ca²⁺ binding to human cardiac troponin C (hcTnC), an EF-hand containing protein required for heart muscle contraction.

RESULTS

The number of protons released upon Ca²⁺ binding to the C- and N-domain of hcTnC were found to be 1.1 and 1.2, respectively. These values permitted determination of buffer-independent thermodynamic parameters of Ca²⁺-hcTnC binding, and the extracted data agreed well among the buffers tested. Both buffer and pH-adjusted parameters were determined for Ca²⁺ binding to the N-domain of hcTnC and revealed that Ca²⁺ binding under aqueous conditions and physiological ionic strength is both thermodynamically favorable and driven by entropy.

CONCLUSIONS

Taken together, the consistency of these data between buffer systems and the similarity between theoretical and experimental proton release is indicative of the reliability of the method used and the importance of extracting metal-buffer interactions in these studies.

GENERAL SIGNIFICANCE

The experimental approach described herein is clearly applicable to other metal ions and other EF-hand protein systems.

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.

Thermodynamics and molecular dynamics simulations of calcium binding to the regulatory site of human cardiac troponin C: evidence for communication with the structural calcium binding sites

Human cardiac troponin C (HcTnC), a member of the EF hand family of proteins, is a calcium sensor responsible for initiating contraction of the myocardium. Ca(2+) binding to the regulatory domain induces a slight change in HcTnC conformation which modifies subsequent interactions in the troponin-tropomyosin-actin complex. Herein, we report a calorimetric study of Ca(2+) binding to HcTnC. Isotherms obtained at 25 °C (10 mM 2-morpholinoethanesulfonic acid, 50 mM KCl, pH 7.0) provided thermodynamic parameters for Ca(2+) binding to both the high-affinity and the low-affinity domain of HcTnC. Ca(2+) binding to the N-domain was shown to be endothermic in 2-morpholinoethanesulfonic acid buffer and allowed us to extract the thermodynamics of Ca(2+) binding to the regulatory domain. This pattern stems from changes that occur at the Ca(2+) site rather than structural changes of the protein. Molecular dynamics simulations performed on apo and calcium-bound HcTnC(1-89) support this claim. The values of the Gibbs free energy for Ca(2+) binding to the N-domain in the full-length protein and to the isolated domain (HcTnC(1-89)) are similar; however, differences in the entropic and enthalpic contributions to the free energy provide supporting evidence for the cooperativity of the C-domain and the N-domain. Thermograms obtained at two additional temperatures (10 and 37 °C) revealed interesting trends in the enthalpies and entropies of binding for both thermodynamic events. This allowed the determination of the change in heat capacity (∆C(p)) from a plot of ∆H verses temperature and may provide evidence for positive cooperativity of Ca(2+) binding to the C-domain.

Calcium binding to troponin C as a primary step of the regulation of contraction. A microcalorimetric approach

Microcalorimetric titration studies of EF-hand Ca-binding proteins (troponin C, calmodulin and parvalbumins) resulted in the notion that Ca binding to the "active" Ca site, which is involved in the regulation of contraction, induces a characteristic anomalous enthalpy and heat-capacity changes indicating an exposure of hydrophobic residues to the solvent, which enables the proteins to interact with their targets. There is a good agreement between the results of the calorimetric and the structural studies in frog and chicken skeletal troponin C. In both species one of the N-terminal low-affinity Ca-sites is the "active" Ca site regulating muscle contraction. The results from calorimetry have shown, however, that the situation in rabbit skeletal troponin C may be more complex. Moreover, in both calorimetric and structural studies, the situation in cardiac troponin C is quite different. These results suggest the need for further studies to elucidate the mechanism of regulation by Ca. These characteristic changes do not occur in Ca-buffering proteins.

The importance of the carboxyl-terminal domain of cardiac troponin C in Ca2+-sensitive muscle regulation

The interactions between troponin I and troponin C are central to the Ca(2+)-regulated control of striated muscle. Using isothermal titration microcalorimetry we have studied the binding of human cardiac troponin C (cTnC) and its isolated domains to human cardiac troponin I (cTnI). We provide the first binding data for these proteins while they are free in solution and unmodified by reporter groups. Our data reveal that the C-terminal domain of cTnC is responsible for most of the free energy change upon cTnC.cTnI binding. Importantly, the interaction between cTnI and the C-terminal domain of cTnC is 8-fold stronger in the presence of Ca(2+) than in the presence of Mg(2+), suggesting that the C-terminal domain of cTnC may play a modulatory role in cardiac muscle regulation. Changes in the affinity of cTnI for cTnC and its isolated C-terminal domain in response to ionic strength support this finding, with both following similar trends. At physiological ionic strength the affinity of cTnC for cTnI changed very little in response to Ca(2+), although the thermodynamic data show a clear distinction between binding in the presence of Ca(2+) and in the presence of Mg(2+).

Heat capacity and entropy changes of the major isotype of the toad (Bufo) parvalbumin induced by calcium binding

The possible structural changes in the major isotype of parvalbumin from the toad (Bufo bufo japonicus) skeletal muscle caused by Ca2+ and Mg2+ binding have been analyzed by microcalorimetric titrations. Parvalbumin was titrated with Ca2+ in both the absence and presence of Mg2+ and with Mg2+ in the absence of Ca2+, at pH 7.0, and at 5 degrees, 15 degrees, and 25 degrees C. The two sites in a molecule were equivalent on Mg2(+)-Ca2+ exchange, but distinguishable on Ca2+ and Mg2+ binding. The reactions of parvalbumin with Ca2+ are exothermic at every temperature in both the absence and presence of Mg2+, but those with Mg2+ are always endothermic except for the binding to site 1 at 25 degrees C. The magnitudes of the hydrophobic and internal vibrational contributions to the heat capacity and entropy changes of parvalbumin on Ca2+ and Mg2+ binding and Mg2(+)-Ca2+ exchange have been estimated by the empirical method of Sturtevant [Sturtevant, J. M. (1977) Proc. Natl Acad. Sci. USA 74, 2236-2240]. Although no major conformational changes were noted between Ca2(+)- and Mg2(+)-bound forms of toad parvalbumin, the conformational difference was larger in Ca2+ (or Mg2+) binding to site 1 than site 2. This may indicate that the metal-free form is much less stable than any form with Ca2+ (or Mg2+) bound at one site at least. On Mg2(+)-Ca2+ exchange, the vibrational as well as hydrophobic entropy is only slightly increased in a parallel manner. In contrast, on Ca2+ (or Mg2+) binding, the hydrophobic entropy increases but the vibrational entropy decreases; the former indicates the sequestering of nonpolar groups from the surface to the interior of a molecule, and the latter suggests that the overall structures are tightened on Ca2+ (or Mg2+) binding but loosened on Mg2(+)-Ca2+ exchange. Despite the clear distinctions in the thermodynamic features, the conformational changes of toad parvalbumin are essentially the same as those of the two isotypes of bullfrog parvalbumins on Ca2+ binding and Mg2(+)-Ca2+ exchange.

Modification of temperature dependence of myofilament Ca sensitivity by troponin C replacement

The Ca sensitivity of chemically skinned right ventricular trabeculae from the rat heart was determined at 22 and 8 degrees C. Endogenous troponin C (TnC) was then extracted with EDTA and replaced with either bovine cardiac TnC or rabbit fast-twitch skeletal TnC. The temperature dependence of myofilament Ca sensitivity was then reevaluated. Cooling native cardiac tissue from 22 to 8 degrees C reduced the pCa (-log10 [Ca2+]), generating half-maximal tension (K1/2) from 5.20 +/- 0.07 to 4.89 +/- 0.08 (SD, n = 14), and also reduced maximum Ca-activated force to 33 +/- 6% of its value at 22 degrees C. After extraction of endogenous TnC and reconstitution with cardiac TnC, cooling from 22 to 8 degrees C caused a similar shift in mean K1/2 from 4.93 +/- 0.08 to 4.69 +/- 0.06 (n = 7). When skeletal TnC was reconstituted into TnC-extracted ventricular fibers, cooling from 22 to 8 degrees C led to a much smaller mean shift in K1/2 from 4.88 +/- 0.07 to 4.78 +/- 0.04 (n = 7). The results show that the magnitude of the cooling-induced shift in myofilament Ca sensitivity observed in the native state (or after reconstitution with cardiac TnC) is significantly reduced if the fiber is reconstituted with skeletal TnC (P less than 0.001). This indicates that the temperature dependence of myofilament Ca sensitivity of cardiac muscle can be modified by incorporation of skeletal TnC. Thus Ca binding to TnC plays an important role in determining the temperature dependence of myofilament Ca sensitivity.

The thermodynamics of calcium binding to thermolysin

Calcium binding isotherms were determined for thermolysin in the range pH 5.6-10.5, and from 5 to 45 degrees C. An extensive statistical analysis of the binding data suggests that at least two of the four binding sites bind Ca2+ with complete positive cooperativity and independently of the other two. Nonlinear regression analysis of the binding data was used to calculate cooperative (K1) and independent (K2) binding constants for the four calcium sites. Thermodynamic parameters obtained from a van't Hoff analysis indicate that calcium binding to both cooperative and independent sites is an entropy-driven process. At pH 7.0, delta H1 = 90.4 kJ/mol; delta H2 = 97.5 kJ/mol; delta S1 = 456 J K-1 mol-1; delta S2 = 262 J K-1 mol-1. These results are compared to those obtained for other calcium-binding proteins. An analysis of the pH dependence of the calcium binding constants indicates that the binding of four protons at the cooperative site and one to two protons at the independent sites, modulates the calcium affinity. This confirms an earlier structural assignment of the double-site as the locus of the two cooperatively binding Ca2+. Calcium binding to thermolysin is enhanced in the presence of an active site directed inhibitor, suggesting that there may be positive cooperativity between substrate and calcium binding.