Figure 6. Rate of autoxidation versus PO₂.

Human Ngb WT (crossed circles), Ngb WT without disulfide bridge after reduction with DTT (white circles), human Ngb triple cysteine mutant Cys(CD7)→ Gly, Cys(D5)→ Ser and Cys(G19)→ Ser (black circles) and human Cygb double mutant Cys (B2)→ Ser and Cys (E9) → Ser (white squares). The acceleration of the rate of autoxidation for PO₂ close to the P₅₀O₂ is assumed to be due to a bimolecular reaction between the ferrous hexacoordinated form and the molecular O₂ while at high PO₂ the dominant reaction is the unimolecular oxidation after O₂ binding to the heme. Experimental conditions were: 25 mM potassium phosphate buffer, 0.1 mM EDTA, 37°C. Protein concentrations were between 2 and 4 µM.