CaATP prolongs strong actomyosin binding and promotes futile myosin stroke

Cardiac Myosin and Thin Filament as Targets for Lead and Cadmium Divalent Cations

Lead and cadmium are heavy metals widely distributed in the environment and contribute significantly to cardiovascular morbidity and mortality. Using Leadmium Green dye, we have shown that lead and cadmium enter cardiomyocytes, distributing throughout the cell. Using an in vitro motility assay, we have shown that sliding velocity of actin and native thin filaments over myosin decreases with increasing concentrations of Pb2+ and Cd2+. Significantly lower concentrations of Pb2+ and Cd2+ (0.6 mM) were required to stop sliding of thin filaments over myosin compared to the stopping actin sliding over the same myosin (1.1-1.6 mM). Lower concentration of Cd2+ (1.1 mM) needed to stop actin sliding over myosin compared to the Pb2++Cd2+ combination (1.3 mM) and lead alone (1.6 mM). There were no differences found in the effects of lead and cadmium cations on relative force developed by myosin heads or number of actin filaments bound to myosin. Sliding velocity of actin over myosin in the left atrium, right and left ventricles changed equally when exposed to the same dose of the same metal. Thus, we have demonstrated for the first time that Pb2+ and Cd2+ can directly affect myosin and thin filament function, with Cd2+ exerting a more toxic influence on myosin function compared to Pb2+.

Direct Effects of Toxic Divalent Cations on Contractile Proteins with Implications for the Heart: Unraveling Mechanisms of Dysfunction

The binding of calcium and magnesium ions to proteins is crucial for regulating heart contraction. However, other divalent cations, including xenobiotics, can accumulate in the myocardium and enter cardiomyocytes, where they can bind to proteins. In this article, we summarized the impact of these cations on myosin ATPase activity and EF-hand proteins, with special attention given to toxic cations. Optimal binding to EF-hand proteins occurs at an ionic radius close to that of Mg2+ and Ca2+. In skeletal Troponin C, Cd2+, Sr2+, Pb2+, Mn2+, Co2+, Ni2+, Ba2+, Mg2+, Zn2+, and trivalent lanthanides can substitute for Ca2+. As myosin ATPase is not a specific MgATPase, Ca2+, Fe2+, Mn2+, Ni2+, and Sr2+ could support myosin ATPase activity. On the other hand, Zn2+ and Cu2 significantly inhibit ATPase activity. The affinity to various divalent cations depends on certain proteins or their isoforms and can alter with amino acid substitution and post-translational modification. Cardiac EF-hand proteins and the myosin ATP-binding pocket are potential molecular targets for toxic cations, which could significantly alter the mechanical characteristics of the heart muscle at the molecular level.

Electrostatic interactions in the SH1-SH2 helix of human cardiac myosin modulate the time of strong actomyosin binding

Two single mutations, R694N and E45Q, were introduced in the beta isoform of human cardiac myosin to remove permanent salt bridges E45:R694 and E98:R694 in the SH1-SH2 helix of the myosin head. Beta isoform-specific bridges E45:R694 and E98:R694 were discovered in the molecular dynamics simulations of the alpha and beta myosin isoforms. Alpha and beta isoforms exhibit different kinetics, ADP dissociates slower from actomyosin containing beta myosin isoform, therefore, beta myosin stays strongly bound to actin longer. We hypothesize that the electrostatic interactions in the SH1-SH2 helix modulate the affinity of ADP to actomyosin, and therefore, the time of the strong actomyosin binding. Wild type and the mutants of the myosin head construct (1-843 amino acid residues) were expressed in differentiated C2C12 cells, and the duration of the strongly bound state of actomyosin was characterized using transient kinetics spectrophotometry. All myosin constructs exhibited a fast rate of ATP binding to actomyosin and a slow rate of ADP dissociation, showing that ADP release limits the time of the strongly bound state of actomyosin. The mutant R694N showed a faster rate of ADP release from actomyosin, compared to the wild type and the E45Q mutant, thus indicating that electrostatic interactions within the SH1-SH2 helix region of human cardiac myosin modulate ADP release and thus, the duration of the strongly bound state of actomyosin.

Switch-1 instability at the active site decouples ATP hydrolysis from force generation in myosin II

Myosin active site elements (i.e., switch‐1) bind both ATP and a divalent metal to coordinate ATP hydrolysis. ATP hydrolysis at the active site is linked via allosteric communication to the actin polymer binding site and lever arm movement, thus coupling the free energy of ATP hydrolysis to force generation. How active site motifs are functionally linked to actin binding and the power stroke is still poorly understood. We hypothesize that destabilizing switch‐1 movement at the active site will negatively affect the tight coupling of the ATPase catalytic cycle to force production. Using a metal‐switch system, we tested the effect of interfering with switch‐1 coordination of the divalent metal cofactor on force generation. We found that while ATPase activity increased, motility was inhibited. Our results demonstrate that a single atom change that affects the switch‐1 interaction with the divalent metal directly affects actin binding and productive force generation. Even slight modification of the switch‐1 divalent metal coordination can decouple ATP hydrolysis from motility. Switch‐1 movement is therefore critical for both structural communication with the actin binding site, as well as coupling the energy of ATP hydrolysis to force generation.