Comparison of the Size and Properties of the Cytochrome c/Cardiolipin Nanospheres in a Sediment and Non-polar Medium.

Abstract

Apoptosis, as the major type of programmed cell death, plays an important role in the organism renewal and removal of defective and transformed cells, including cancer cells. One of the earliest apoptotic events is lipid peroxidation in the inner mitochondrial membrane catalyzed by a complex of cytochrome c (CytC) with the mitochondrial phospholipid cardiolipin (CL). It was shown that mixing CytC and CL solutions results in the formation of CytC/CL complexes (Cyt-CL nanospheres) with a diameter of 11-12 nm composed of the molten globule protein molecule and a CL monolayer. Using the methods of dynamic light scattering for the Cyt-CL chloroform solution and small-angle X-ray scattering for the Cyt-CL sediment, it was found that in both cases, Cyt-CL formed nanospheres with a diameter of 8 and 11 nm, which corresponded to the earlier determined lipid/protein ratios of 13-14 and 35-50, respectively. These results showed that the Cyt-CL nanospheres can form not only during crystallization but also in a hydrophobic medium. CytC in the complex exists as a molten globule, as evidenced by the emergence of tryptophan and tyrosine fluorescence (absent in the native protein) due to the Förster resonance transfer of the electron excitation energy onto the heme. At the same time, the coordinate bond between the heme iron and the sulfur atom of methionine 80 in Cyt-CL is disrupted (the absorption band at ~700 nm disappears). Similar disruption of the iron-sulfur bond in Cyt-CL was observed in 50% methanol. These changes were reversible, which corroborates the conclusion on the CytC transition to the molten globule conformation in methanol-containing solutions.