Phosphorus-31 nuclear magnetic resonance pH titration studies of the phosphoproteins tropomyosin and glycogen phosphorylase a.

31P nuclear magnetic resonance (NMR), pH titration studies of the phosphoproteins tropomyosin and glycogen phosphorylase a (in the presence of the inhibitor glucose) show that the resonances for the phosphoserine regulatory sites shift with pH. Analysis of line widths indicates that both residues have considerable mobility. These results are in agreement with studies on similar phosphorylated sites on other proteins, leading us to propose that mobility is a general feature of such regulatory sites. pH titration studies on a series of model compounds have indicated that an empirical correlation exists between the Hill coefficient n (a measure of the cooperativity of the titration curve) and the presence of charged groups in the vicinity of the phosphoryl moiety. Moreover, these studies showed that within one class of similarly substituted phosphorus compounds the chemical shifts, the titration behaviour, and the pKa2 were comparable and allow for easy identification of these compounds. The pKa2 values for phosphoserine residues of proteins are in general slightly higher than those of phosphomonoester-containing small compounds. The titration data prompt our estimation that the maximal amount of energy associated with a salt linkage between a phosphoseryl side chain and a positively charged group is about 5 kcal (1 cal = 4.1868 J).