Figure 4.

pH dependence of the catalytic lifespan of CcO III (−) forms with Asp-132, Asp-139, Asp-132 plus Cys 139 or Asp-132 plus Asp-139 as the initial proton acceptors of the D pathway, compared to normal CcO containing subunit III (WT III (+). The green line represents the minimum catalytic lifespan of WT III (+) across this pH range; the long lifespan of WT III (+) is difficult to measure with accuracy. Catalytic lifespan measurements and exponential fits were performed as in Figure 3.