Acid Denaturation and Anion-Induced Folding of Globular Proteins: Multitude of Equilibrium Partially Folded Intermediates

The structure of a globular protein is known to be affected by addition of acids or alkalis. The fact that a protein unfolds or at least denatures in solutions with extreme pH values was known for a long time. Prof. Anthony L. Fink (Tony) brought this field to a new level by showing that acid-unfolded proteins can partially regain their ordered structures in the presence of various anions. This review analyses his contributions to this branch of protein science. His studies provided an explanation for the molecular mechanisms underlying anion-induced refolding of acid-unfolded proteins. He was the first who clearly showed that different anions differed dramatically in their efficiency to bring about this refolding. This difference was shown to be manifested in the amounts of anions needed to complete a structural transition, in the degree of cooperativity of these transitions, and in the amounts of ordered structure induced in the acid-unfolded proteins at the completion of the corresponding transitions. He was also first who undoubtedly demonstrated that, at low pH, proteins can populate discrete partially folded conformations in the presence of different anions. His papers are highly cited, clearly showing that the work of Tony Fink on anion-induced folding of globular proteins made a great impact to the protein folding field.