Interaction of ADP with skeletal and cardiac myosin and their active fragments observed by proton release

A metabolite-sensitive, thermodynamically constrained model of cardiac cross-bridge cycling: implications for force development during ischemia

We present a metabolically regulated model of cardiac active force generation with which we investigate the effects of ischemia on maximum force production. Our model, based on a model of cross-bridge kinetics that was developed by others, reproduces many of the observed effects of MgATP, MgADP, Pi, and H(+) on force development while retaining the force/length/Ca(2+) properties of the original model. We introduce three new parameters to account for the competitive binding of H(+) to the Ca(2+) binding site on troponin C and the binding of MgADP within the cross-bridge cycle. These parameters, along with the Pi and H(+) regulatory steps within the cross-bridge cycle, were constrained using data from the literature and validated using a range of metabolic and sinusoidal length perturbation protocols. The placement of the MgADP binding step between two strongly-bound and force-generating states leads to the emergence of an unexpected effect on the force-MgADP curve, where the trend of the relationship (positive or negative) depends on the concentrations of the other metabolites and [H(+)]. The model is used to investigate the sensitivity of maximum force production to changes in metabolite concentrations during the development of ischemia.

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.

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.

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.