Quarternary structure amplification of protein folding differences observed in 'native' platelet factor-4

Platelet factor 4 (PF4), a protein of 70 residues, exists in solution as a distribution of monomer, dimer and tetramer (M-D-T) states in slow exchange on a 600 MHz 1H-NMR chemical shift time scale. Well-resolved Y60 ring proton resonances in each aggregate state allow derivation of M-D-T populations. Under the same set of solution conditions, M-D-T aggregate state distributions vary from prep to prep or with repeated freeze-drying or raising/lowering the solution pH. These variations are not the result of chemical modifications and reflect differences in the strength of subunit associations and therefore folding. Variations are greatest at pH values at or below the pKas of carboxylate groups, supporting the idea that electrostatic interactions modulate PF4 subunit interactions. Treating these distributions as true equilibria results in free energy differences of about 0.5 kcal/mol subunit. Quarternary structure amplifies free energy differences among various folded states of monomer PF4.