Physical heterogeneity of muscle glycogen phosphorylase revealed by hydrostatic pressure dissociation

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Abstract
Four independent methods that employ fluorescence spectroscopy show that the tetramer of glycogen phosphorylase A (GPA) from rabbit muscle is reversibly dissociated into monomers by hydrostatic pressures under 2.5 kbar, if aggregation of the monomers is prevented by the addition of 8% glycerol. The free energy of association at 20 degrees C (-32 kcal mol-1) depends upon a large entropy increase (T delta S = +65 kcal mol-1) that counteracts an unfavorable enthalpy of association of +33 kcal mol-1. The association volumes calculated from the pressure dependence of the dissociation are nearly 4-fold smaller than those calculated from the shift in dissociation pressure with concentration. The dimer obtained by dilution of GPA at atmospheric pressure differs from the hypothesized dimer intermediate in the pressure dissociation by the much larger monomer affinity of the former. Like other tetramers, GPA shows hysteresis of the pressure profile upon decompression and conformational drift of the dissociated monomers. By use of the energy transfer method it is demonstrated that the relaxation time for half-dissociation (5 min) is over an order of magnitude shorter than that for subunit exchange (118 min). In all three tetramers studied, lactate dehydrogenase, glyceraldehyde phosphate dehydrogenase, and glycogen phosphorylase, the deterministic character of the dissociation equilibrium under pressure and the anomalous concentration dependence of the pressure dissociation demonstrate that these tetramers are heterogeneous populations with regard to their free energy and/or volumes of association.

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