Hydrogen Bonding of Tyrosine B10 to Heme-bound Oxygen in Ascaris Hemoglobin

Abstract

The hemoglobin from Ascaris suum, a parasitic nem- atode, has a spontaneous dissociation rate for the dioxy- gen ligand that is 3 orders of magnitude less than for mammalian myoglobins or hemoglobins. In this hemo- globin, the distal histidine is replaced with a glutamine which is capable of forming a stabilizing hydrogen bond to the bound dioxygen. A single hydrogen bond from a glutamine is, under typical circumstances, not sufficient to account for the low off rate for oxygen. Several stud- ies point to a second hydrogen bond to the heme-bound dioxygen originating from tyrosine B10 as the source of this unusual reactivity. In this study ultraviolet (UV) resonance Raman spectroscopy is used to directly ob- serve the formation of this hydrogen bond upon oxygen binding. The study reveals that both oxygen and carbon monoxide induce similar conformational changes in the globin upon binding to the heme; however, in the case of oxygen, a strong hydrogen bond involving a tyrosine is also observed. Similar studies on the QE7L mutant of this Hb suggest that the glutamine plays a role in stabi- lizing a rigid tertiary structure associated with the dis- tal heme pocket. This conformation maintains the tyro- sine in an orientation conducive to hydrogen bond formation with a heme-bound dioxygen ligand.