Surath K. Banerjee

Atrial and ventricular cardiac myosins contain different heavy chain species

Myosin isolated from the atria of the heart has been reported to have greater ATPase activity and a different light chain composition than myosin prepared from the ventricles [ 13. However, no differences between the heavy chains of these myosins were found by immunodiffusion techniques. We wish to report here that atria1 and ventricular myosins isolated from calf heart possess heavy chains with different primary structures. We have compared cyanogen bromide digests of atria1 and ventricular myosins in a two-dimensional gel electrophoresis system which resolves the myosin heavy chain peptides quite well. The results show firstly that there are differences in the patterns of heavy chain peptides from these myosins. Secondly, they demonstrate a difference in the distribution of [r4C]carboxymethylated cysteine containing heavy chain peptides. To our knowledge, this is the first demonstration of a structural difference between cardiac atria1 and ventricular myosin heavy chains.

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.

Binding of N-acetylglucosamine tetrasaccharide to lysozyme

The β(1→4)-linked tetrasaccharide derived from N -acetylglucosamine binds to lysozyme differently than the trimer. The apparent enthalpy of binding of the tetramer is 2.8 kcal/mole less negative than that of the trimer. Because of enthalpy-entropy compensation, the free energy of binding of tetramer and trimer differ only slightly (by 0.3 kcal at 25°C). Consideration of these results in conjunction with other properties of lysozyme suggests that there are two tetramer-lysozyme complexes, one of which has the characteristics of the trimer-lysozyme complex. The pentamer and hexamer of N -acetylglucosamine bind like tetramer.

Nucleotide-induced changes in the heat capacity of beef cardiac myosin

The heat of reaction of ATP with beef cardiac myosin has been determined in a flow microcalorimeter at two different temperatures. The reaction heat obtained by extrapolation to infinite flow rate is interpreted to be the heat of formation of the steady-state set of myosin-nucleotide complexes. The large temperature dependence of this heat, an apparent change in heat capacity, could be caused by an isomerization between two myosin conformations. The enthalpies and heat capacities of binding of ADP, AMP, and pyrophosphate have also been measured and are discussed in terms of this model.