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Oxymyohemerythrin: discriminating between O2 release and autoxidation
C R Lloyd1, G M Raner, A Moser
1Department of Chemistry, University of Utah, Salt Lake City 84112, USA.
Abstract:
Myohemerythrin (Mhr) is a non-heme iron O2 carrier (with two irons in the active site) that is typically found in the retractor muscle of marine 'peanut' worms. OxyMhr may either release O2, or undergo an autoxidation reaction in which hydrogen peroxide is released and diferric metMhr is produced. The autoxidation reaction can also be promoted by the addition of certain anions to Mhr solutions. This work, using recombinant Themiste zostericola Mhrs, contrasts the results of environmental effects on these reactions. For the O2 release reaction, deltaVdouble dagger(21.5 degrees C) = +28+/-3 cm3 mol(-1), deltaHdouble dagger(1 atm) = +22+/-1 kcal mol(-1), and deltaSdouble dagger(1 atm) = +28+/-4 eu. The autoxidation reaction (pH 8.0, 21.5 degrees C, 1 atm) displays different kinetic parameters: deltaVdouble dagger = -8+/-2 cm3 mol(-1), deltaHdouble dagger = +24.1+/-0.7 kcal mol(-1), and deltaSdouble dagger = +1+/-1 eu. Autoxidation in the presence of sodium azide is orders of magnitude faster than solvolytic autoxidation. The deltaVdouble dagger parameters for azide anation and azide-assisted autoxidation reaction are +15+/-2 and +59+/-2 cm3 mol(-1), respectively, indicating that the rate-limiting steps for the Mhr autoxidation and anation reactions (including O2 uptake) are not associated with ligand binding to the Fe2 center. The L103V and L103N oxyMhr mutants autoxidize approximately 10(3)-10(5) times faster than the wild-type protein, emphasizing the importance of leucine-103, which may function as a protein 'gate' in stabilizing bound dioxygen.
Insights
Myohemerythrin (Mhr), an oxygen carrier, undergoes distinct reactions including O2 release and autoxidation. Environmental factors significantly influence these processes, with specific mutations like L103V/N dramatically accelerating autoxidation.
Area of Science:
- Biochemistry
- Protein Chemistry
- Biophysical Chemistry
Background:
- Myohemerythrin (Mhr) is a non-heme iron O2 carrier found in marine worms.
- OxyMhr can release O2 or undergo autoxidation, producing hydrogen peroxide and metMhr.
- Anions can promote Mhr autoxidation.
Purpose of the Study:
- To investigate environmental effects on Mhr's O2 release and autoxidation reactions.
- To compare kinetic parameters of O2 release versus autoxidation.
- To explore the role of specific mutations and anions in Mhr reactivity.
Main Methods:
- Utilized recombinant Themiste zostericola Mhrs.
- Measured kinetic parameters (deltaV‡, deltaH‡, deltaS‡) for O2 release and autoxidation.
- Investigated reactions with and without sodium azide, and studied L103V/N mutants.
Main Results:
- O2 release and autoxidation exhibit distinct thermodynamic and kinetic parameters.
- Autoxidation is significantly accelerated by sodium azide.
- L103V and L103N oxyMhr mutants autoxidize 10^3-10^5 times faster than wild-type Mhr.
Conclusions:
- Leucine-103 is crucial for stabilizing bound dioxygen, potentially acting as a protein gate.
- The rate-limiting steps for Mhr autoxidation and anation are not solely ligand binding.
- Environmental factors and protein structure play key roles in Mhr's oxygen-binding and reaction dynamics.