Controlling ligand binding in myoglobin by mutagenesis.
A quadruple mutant of sperm whale myoglobin was constructed to mimic the structure found in Ascaris suum hemoglobin. The replacements include His(E7)-->Gln, Leu(B10)-->Tyr, Thr(E10)--> Arg, and Ile(G8)-->Phe. Single, double, and triple mutants were characterized to dissect out the effects of the individual substitutions. The crystal structures of the deoxy and oxy forms of the quadruple mutant were determined and compared with that of native Ascaris hemoglobin. Tyr(B10) myoglobin displays low O(2) affinity, high dissociation rate constants, and heterogeneous kinetic behavior, suggesting unfavorable steric interactions between the B10 phenol side chain and His(E7). In contrast, all mutants containing the Tyr(B10)/Gln(E7) pair show high O(2) affinity, low dissociation rate constants, and simple, monophasic kinetic behavior. Replacement of Ile(107) with Phe enhances nanosecond geminate recombination singly and in combination with the Tyr(B10)/Gln(E7)/Arg(E10) mutation by limiting access to the Xe4 site. These kinetic results and comparisons with native Ascaris hemoglobin demonstrate the importance of distal pocket cavities in governing the kinetics of ligand binding. The approximately 150-fold higher O(2) affinity of Ascaris hemoglobin compared with that for Tyr(B10)/Gln(E7)-containing myoglobin mutants appears to be the result of favorable proximal effects in the Ascaris protein, due to a staggered orientation of His(F8), the lack of a hydrogen bonding lattice between the F4, F7, and F8 residues, and the presence of a large polar Trp(G5) residue in the interior portion of the proximal heme pocket.
Draghi
Federica
F
A. Rossi Fanelli Department of Biochemical Sciences, CNR Center of Molecular Biology, University of Rome La Sapienza, P. le. A. Moro 5, 00185 Rome, Italy.
Miele
Adriana Erica
AE
Travaglini-Allocatelli
Carlo
C
Vallone
Beatrice
B
Brunori
Maurizio
M
Gibson
Quentin H
QH
Olson
John S
JS
D020816:Amino Acid Motifs; D000818:Animals; D017165:Ascaris suum / Q000378:metabolism; D018360:Crystallography, X-Ray; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007532:Isoleucine / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008954:Models, Biological; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009211:Myoglobin / Q000737:chemistry* / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011995:Recombination, Genetic; D013997:Time Factors
D006454:Hemoglobins; D008024:Ligands; D009211:Myoglobin; D007532:Isoleucine; D009569:Nitric Oxide; D006418:Heme; D010100:Oxygen