INTERACTIONS OF THE NUCLEOSOMAL CORE HISTONES - A CALORIMETRIC STUDY OF OCTAMER ASSEMBLY

Enthalpies of assembly for [calf thymus chromatin] histone subunits into the nucleosomal core octamer were studied by isothermal heatburst microcalorimetry. The assembly follows a mixed association reaction scheme D + T .dblarw. DT (hexamer) (1a), DT + D .dblarw. D2T (octamer) (1b), where D is the H2A-H2B dimer and T is the (H3-H4)2 tetramer. The standard enthalpies for reactions 1a and 1b are large and unequal to pH 7.5, 2 M NaCl, 25.degree. C (.DELTA.H.degree.1 = -30.8 kcal; .DELTA.H.degree.2 = -22.5 kcal). The determined equilibrium constants are in agreement with previous results that indicate cooperativity in the assembly process, the intrinsic equilibrium constants for reaction 1b being about 4 times as great as that of reaction 1a. The calorimetric heats determined in different buffers indicate a small net proton release for overall octamer assembly. Direct titration shows the proton release to be .apprx. 0.2 proton/octamer formed at pH 7.5. This number increases as the pH is lowered. The bearing of results on the types of dominant interactions stabilizing the octamer is discussed.