Oxymyohemerythrin: Discriminating between O₂ release and autoxidation

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, ΔV(Double Dagger) (21.5°C)=+28±3 cm³mol^-1, ΔH(Double Dagger) (1 atm)=+22±1 kcalmol^-1, and ΔS(Double Dagger) (1 atm)=+ 28±4 eu. The autoxidation reaction (pH 8.0, 21.5°C, 1 atm) displays different kinetic parameters: ΔVDouble Dagger)= -8±2 cm³mol^-1, ΔH(Double Dagger)=+24.1±0.7 kcalmol^-1, and ΔS(Double Dagger)=+1±1 eu. Autoxidation in the presence of sodium azide is orders of magnitude faster than solvolytic autoxidation. The ΔV(Double Dagger) parameters for azide anation and azide-assisted autoxidation reaction are +15±2 and +59±2 cm³mol^-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 ~10³-10⁵ times faster than the wild-type protein, emphasizing the importance of leucine-103, which may function as a protein 'gate' in stabilizing bound dioxygen. Copyright (C) 2000 Elsevier Science B.V.