2spm
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A NOVEL SITE-DIRECTED MUTANT OF MYOGLOBIN WITH AN UNUSUALLY HIGH O2 AFFINITY AND LOW AUTOOXIDATION RATE
OverviewOverview
Mutants of sperm whale myoglobin were constructed at position 29 (B10 in, helix notation) to examine the effects of distal pocket size on the rates, of ligand binding and autooxidation. Leu29 was replaced with Ala, Val, and, Phe using the synthetic gene and Escherichia coli expression system of, Springer and Sligar (Springer, B. A., and Sligar, S. G. (1987) Proc. Natl., Acad. Sci. U. S. A. 84, 8961-8965). Structures of the ferric forms of, Val29 and Phe29, and the oxy form of Phe29 myoglobin were determined to, 1.7 A by x-ray crystallography. The ferric mutant proteins are remarkably, isomorphous with the wild type protein except in the immediate vicinity of, residue 29. Thus, the protein structure in the distal pocket of myoglobin, can accommodate either a large "hole" (i.e. Ala or Val) or a large side, chain (i.e. Phe) at position 29 without perturbation of tertiary, structure. Phe29 oxymyoglobin is also identical to the native oxy protein, in terms of overall structure and interactions between the bound O2 and, His64, Val68, Phe43, and Ile107. The distance between the nearest side, chain atom of residue 29 and the second atom of the bound oxygen molecule, is 3.2 A in the Phe29 protein and 4.9 A in native myoglobin. The, equilibrium constants for O2 binding to Ala29, Val29, and Leu29 (native), myoglobin are the same, approximately 1.0 x 10(6) M-1 at 20 degrees C, whereas that for the Phe29 protein is markedly greater, 15 x 10(6) M-1., This increase in affinity is due primarily to a 10-fold decrease in the O2, dissociation rate constant for the Phe29 mutant and appears to be the, result of stabilizing interactions between the negative portion of the, bound O2 dipole and the partially positive edge of the phenyl ring., Increasing the size of residue 29 causes large decreases in the rate of, autooxidation of myoglobin: k(ox) = 0.24, 0.23, 0.055, and 0.005 h-1 for, Ala29, Val29, Leu29 (native), and Phe29 myoglobin, respectively, in air at, 37 degrees C. Thus, the Leu29----Phe mutation produces a reduced protein, that is remarkably stable and is expressed in E. coli as 100% MbO2. The, selective pressure to conserve Leu29 at the B10 position probably, represents a compromise between reducing the rate of autooxidation and, maintaining a large enough O2 dissociation rate constant to allow rapid, oxygen release during respiration.
About this StructureAbout this Structure
2SPM is a Single protein structure of sequence from Physeter catodon with SO4 and HEM as ligands. Full crystallographic information is available from OCA.
ReferenceReference
A novel site-directed mutant of myoglobin with an unusually high O2 affinity and low autooxidation rate., Carver TE, Brantley RE Jr, Singleton EW, Arduini RM, Quillin ML, Phillips GN Jr, Olson JS, J Biol Chem. 1992 Jul 15;267(20):14443-50. PMID:1629229
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