1H Nuclear-Magnetic-Resonance Studies of Porcine Lutropin and Its α and β Subunits.

The titration curves of the histidine residues of porcine lutropin and its isolated α and β subunits have been determined by following the pH-dependence of the imidazole C-2 proton resonances. The isolated α subunit contains a buried histidine, whose C-2 proton does not exchange with solvent, and which has the unusually low pK of 3.3. In the native hormone all the histidine residues have relatively normal pK values (between 5.7 and 6.2). The four histidine C-2 proton resonances have been assigned to specific residues in the amino-acid sequence, by means of deuterium and tritium exchange experiments on the α subunit and its des(92-96) derivative. The histidine with a pK of 3.3 is identified as His-α87. The effects of pH on tyrosine and methyl proton resonances show that the titration of His-87 in the isolated α subunit is accompanied by a significant conformational change which involves loosening of the protein structure but which is not a normal unfolding transition. The role of conformational changes in the generation of biological activity by subunit association in the glycoprotein hormones is discussed. [ABSTRACT FROM AUTHOR]