A New Non-Canonical Control Mechanism in an Allosteric Protein -An Inert Interface Comes to Life

Abstract

The classical tenet states that allosteric proteins exist in at least two different conformations, each one exhibiting distinct functional characteristics that interconvert each other. Basically, a conformational change leads to a functional change. This is accomplished by proteins that almost in all cases (with very few exceptions) are oligomeric, i.e., are formed by two or more subunits. This implies that, as it is in the minimal case of a dimer, at least one protein interface is present.In the case of the hemoglobin, according to the Monod-Wyman-Changeux, Two-State Concerted Model, the archetypal allosteric protein exists theoretically in either a tense (“T”) or relaxed (“R”) conformation. X-ray crystallographic studies have revealed that these two conformations correspond to two different structures. 1H-NMR studies have shown that either structure is characterized by specific sets of salt bridges formed between residues on both αβ dimers, the so-called α1β2 (or α2β1) interdimeric interactions. The accepted consensus states that the structural change consists of a ∼15°rotation of one dimer over the other when converting from “T” to “R”, and vice versa, whereas the αβ dimers themselves do not experience any conformational change, i.e., the α1β1 (or α2β2) interface remains unaltered. Functionally, “T” and “R” are characterized by low and high affinity for the ligand, respectively.In the present work, we have altered chemically this allegedly inert intradimeric α1β1 interface and found striking functional changes that cannot be explained in terms of the canonical allosteric model, yet “T” and “R” structural traits were unequivocally present. This finding exemplifies the functional versatility that a protein can attained, exceeding the limits of what is called “of physiological significance”. Experimental data that support this finding will be presented.