Hemoglobin Rothschild: Structural Rationalization of Decreased O2 Affinity as a Consequence of a β37(C3)Trp->Arg Mutation

Hemoglobin Rothschild is characterized by a {beta}37(C3)Trp[->]Arg mutation that severely impairs wildtype hemoglobin function. This mutation has previously been documented to diminish conformational cooperativity, and thereby uppercut oxygen affinity. While the mutation is known to have direct implications on the hinge region at the 1{beta}2 interface, the immediate and indirect manifestations of this mutation have not been rendered using high-resolution molecular visualization software. Further unexplored is whether low O2 affinity in HbR is an outcome of a stabilized, unliganded, tetrameric T-state, a liganded, dimerized R-state deprived of quaternary enhancement, or a combination of both. Herein, PyMOL is used to rationalize the structural artifacts of the Rothschild variant that govern decreased O2 affinity via a stabilized, tetrameric T-state HbR, and decreased O2 affinity via HbR dimerization and loss of cooperative binding in the R-state. Molecular docking simulations were then performed to determine on what grounds O2 affinity is most attenuated. The result shows that, at the 95% confidence level, reduced O2 affinity in HbR is just as much an outcome of a stabilized tetrameric T-state as it is a dimerized R-state lacking quaternary, subunit cooperativity. The work described here builds a statistical framework to accommodate further, pair-wise comparison of low O2 affinity hemoglobin variants to build intuition on which primary sequence mutations pose the largest clinical consequences.