HYDROGEN-BONDING OF TYROSINE B10 TO HEME-BOUND OXYGEN IN ASCARIS HEMOGLOBIN - DIRECT EVIDENCE FROM UV RESONANCE RAMAN-SPECTROSCOPY

The hemoglobin from Ascaris suum, a parasitic nematode, has a spontane ous dissociation rate for the dioxygen ligand that is 3 orders of magn itude less than for mammalian myoglobins or hemoglobins. In this hemog lobin, the distal histidine is replaced with a glutamine which is capa ble of forming a stabilizing hydrogen bond to the bound dioxygen. A si ngle hydrogen bond from a glutamine is, under typical circumstances, n ot sufficient to account for the low off rate for oxygen. Several stud ies point to a second hydrogen bond to the heme-bound dioxygen origina ting from tyrosine B10 as the source of this unusual reactivity, In th is study ultraviolet (UV) resonance Raman spectroscopy is used to dire ctly observe 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 i s also observed. Similar studies on the QE7L mutant of this Hb suggest that the glutamine plays a role in stabilizing a rigid tertiary struc ture associated with the distal heme pocket. This conformation maintai ns the tyro sine in an orientation conducive to hydrogen bond formatio n with a heme-bound dioxygen ligand.