Myoglobin with chlorophyllous chromophores: Influence on protein stability

The stabilities of myoglobin, apo-myoglobin, and of two myoglobins with chlorophyllous chromophores (Zn-pheophorbide a and Zn-bacteriopheophorbide a), have been studied by thermal and chemical denaturation. With guanidinium chloride, the stability order is myoglobin > Zn-pheophorbide-myoglobin > Zn-bacteriopheophorbide-myoglobin ∼ apo-myoglobin. The thermal behavior is more complex. The transition temperature of thermal unfolding of the apoprotein (62.4 °C) is increased by Zn-pheophorbide a (83.9 °C) and Zn-bacteriopheophorbide a (82.6 °C) to a similar degree as by the native chromophore, heme (83.5 °C). The recovery with Zn-pheophorbide (92-98%) is even higher than with heme (74-76%), while with Zn-bacteriopheophorbide (40%) it is as low as with the apoprotein (42%). Recovery also depends on the rates of heating, and in particular the time spent at high temperatures. It is concluded that irreversibility of unfolding is related to loss of the chromophores, which are required for proper re-folding.