Oxidation
of cardiolipin (CL) by cytochrome c (cytc) has been proposed to initiate the intrinsic pathway of
apoptosis. Domain-swapped dimer (DSD) conformations of cytc have been reported both by our laboratory and by others.
The DSD is an alternate conformer of cytc that could
oxygenate CL early in apoptosis. We demonstrate here that the cytc DSD has a set of properties that would provide tighter
regulation of the intrinsic pathway. We show that the human DSD is
kinetically more stable than horse and yeast DSDs. Circular dichroism
data indicate that the DSD has a less asymmetric heme environment,
similar to that seen when the monomeric protein binds to CL vesicles
at high lipid:protein ratios. The dimer undergoes the alkaline conformational
transition near pH 7.0, 2.5 pH units lower than that of the monomer.
Data from fluorescence correlation spectroscopy and fluorescence anisotropy
suggest that the alkaline transition of the DSD may act as a switch
from a high affinity for CL nanodiscs at pH 7.4 to a much lower affinity
at pH 8.0. Additionally, the peroxidase activity of the human DSD
increases 7-fold compared to that of the monomer at pH 7 and 8, but
by 14-fold at pH 6 when mixed Met80/H2O ligation replaces
the lysine ligation of the alkaline state. We also present data that
indicate that cytc binding shows a cooperative effect
as the concentration of cytc is increased. The DSD
appears to have evolved into a pH-inducible switch that provides a
means to control activation of apoptosis near pH 7.0.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.