Abstract
The solution 1H NMR structure of the active site and ligand dissociation rate for the cyanomet complex have been determined for a sperm whale myoglobin triple mutant Leu29(B10)-->Tyr, His64(E7)-->Gln, Thr67(E10)-->Arg that mimics the distal residue configuration of the oxygen-avid hemoglobin from Ascaris suum. A double mutant that retains Leu29(B10) was similarly investigated. Two-dimensional NMR analysis of the iron-induced dipolar shifts, together with the conserved proximal side structure for the two mutants, allowed the determination of the orientations of the paramagnetic susceptibility tensor for each complex. The resulting magnetic axes, together with paramagnetic relaxation and steady-state NOEs, led to a quantitative description of the distal residue orientations. The distal Tyr29(B10) in the triple mutant provides a strong hydrogen bond to the bound cyanide comparable to that provided by His64(E7) in wild-type myoglobin. The distal Gln64(E7) in the triple mutant is sufficiently close to the bound cyanide to severe as a hydrogen bond donor, but the angle is not consistent with a strong hydrogen bond. Dipolar contacts between the Arg67(E10) guanidinium group and the Gln64(E7) side chain in both mutants support a hydrogen-bond to the Gln64(E7) carbonyl group. The much lower oxygen affinity of this triple mutant relative to that of Ascaris hemoglobin is concluded to arise from side-chain orientations that do not allow hydrogen bonds between the Gln64(E7) side-chain NHs and both the ligand and Tyr29(B10) hydroxyl oxygen. Cyanide dissociation rates for the reduced cyanide complexes are virtually unaffected by the mutations and are consistent with a model of the rate-determining step as the intrinsically slow Fe-C bond breaking that is largely independent of any hydrogen bonds to the cyanide nitrogen.