Structural transformations of cytochrome c upon interaction with cardiolipin

Abstract

Interactions of cytochrome c (cyt c) with cardiolipin (CL) play a critical role in early stages of apoptosis. Upon binding to CL, cyt c undergoes changes in secondary and tertiary structure that lead to a dramatic increase in its peroxidase activity. Insertion of the protein into membranes, insertion of CL acyl chains into the protein interior, and extensive unfolding of cyt c after adsorption to the membrane have been proposed as possible modes for interaction of cyt c with CL. Dissociation of Met80 is accompanied by opening of the heme crevice and binding of another heme ligand. Fluorescence studies have revealed conformational heterogeneity of the lipid-bound protein ensemble with distinct polypeptide conformations that vary in the degree of protein unfolding. We correlate these recent findings to other biophysical observations and rationalize the role of experimental conditions in defining conformational properties and peroxidase activity of the cyt c ensemble. Latest time-resolved studies propose the trigger and the sequence of cardiolipin-induced structural transitions of cyt c.

The key role played by charge in the interaction of cytochrome c with cardiolipin

“…In particular, the rupture of the heme-Fe-M80 coordination and the tertiary rearrangement of cyt c, involving the 40's Ω loop and the M80-containing Ω loop, have been reported to occur in CL-bound cyt c [7,20,21]. The difficulties encountered in the study of the cyt c/CL binding process are correlated with several factors, such as the ionic strength of the solution, the membrane curvature, the CL/ cyt c molar ratio at which the binding reaction is followed, which significantly affect the results obtained [7,17,19,45,46]. Moreover, the finding that CL-bound cyt c is present in solution as a heterogeneous ensemble of forms characterized by different heme coordination represents a further complication [15,16,28].…”

“…In the present study, CD Soret spectra show that the Lys72Arg and Lys73Arg mutants have structure similar to that of the wt protein, while the Lys72Asn and Lys73Asn mutants are the variants that most differ from it. The decreased rotational strength of the 414-nm band observed for the Asn mutants, is likely attributable to the presence of a minor heterogeneous subpopulation with non-native X-Fe-His18 axial coordination (where X is a misligated endogenous ligand) in equilibrium with the major Met80-Fe-His18 form [14,15,17]. Upon mutation, also the near-UV (270-300 nm) dichroic band decreases on passing from Arg to Ala to Asn (Fig.…”

“…The models proposed to describe the CL/cyt c interaction identified some regions involved in the protein-liposome interaction [5,6,17,18,26,47]; among others, the Met80-containing 66-92 region, which comprises the cleft formed by the 67-71 and 82-85 residues and a network of positively charged residues (i.e., Lys72, Lys73, Lys86) that may facilitate the insertion of the acyl chain, is particularly noteworthy [5,6,26]. This prompted us to focus our studies on this region, which is considered crucial for the cyt c/CL binding process.…”

“…CL-bound cyt c shows altered tertiary structure, a perturbed heme crevice and the displacement of Met80 from the sixth coordination position of the heme-Fe atom [5][6][7][8][16][17][18][19]. Depending on the experimental conditions, the sixth coordination position of the heme iron remains unbound or is occupied by another side chain (likely a Lys or a His residue) [7,20,21].…”

Myxococcus CsgA, Drosophila Sniffer, and human HSD10 are cardiolipin phospholipases

“…The CytC structure in this role is well-studied and involves a heme moiety covalently attached by two thioether linkages. In a separate function, CytC can also bind CL and induces a disordered structure that has presented a challenge to study but has also led to an appreciation that these changes provoke the transition from electron carrier to peroxidase (Muenzner and Pletneva 2014). Once bound to CytC, the polyunsaturated fatty acids of CL are oxidized in the presence of hydrogen peroxide (Kagan et al 2005(Kagan et al , 2009).…”