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

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Abstract
The titration curves of the histidine residues of porcine lutropin and its isolated .alpha. and .beta. subunits were determined by following the pH-dependence of the imidazole C-2 proton resonances. The isolated .alpha. subunit contained a buried histidine, whose C-2 proton did not exchange with solvent and which had the unusually low pK of 3.3. In the native hormone all the histidine residues had relatively normal pK values (5.7-6.2). The 4 histidine C-2 proton resonances were assigned to specific residues in the amino acid sequence, by means of deuterium and tritium exchange experiments on the .alpha. subunit and its des(92-96) derivative. The histidine with a pK of 3.3 was identified as His-.alpha.87. The effects of pH on tyrosine and methyl proton resonances show that the titration of His-87 in the isolated .alpha. 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 to subunit association in the glycoprotein hormones is discussed.