Molten globule-like state of cytochrome c induced by polyanion poly(vinylsulfate) in slightly acidic pH

The effect of polyanion, poly(vinylsulfate), used as a model of negatively charged surface, on ferric cytochrome c (ferricyt c) structure in acidic pH has been studied by absorbance spectroscopy, circular dichroism (CD), tryp tophan (Trp) fluorescence and microcalorimetry. The polyanion induced only small changes in the native structure of the protein at neutral pH, but it profoundly shifted the acid induced high spin state of the heme in the acti ve center of cyt c to a more neutral pH region. Cooperativity of the acidic transition of ferricyt c in the presence of the polyanion was disturbed, i n comparison with uncomplexed protein, as followed from different apparent pK(a) values observed in a distinct regions of the ferricyt c electronic ab sorbance spectrum (4.55 +/- 0.08 in the 620 nm band region and 5.47 +/- 0.1 5 in the Soret region). The ferricyt c structure in the complex with the po lyanion at acidic pH (below pH 5.0) has properties of a molten globule-like state. Its tertiary structure is strongly disturbed according to CD and mi crocalorimetry measurements; however, its secondary structure, from CD, is still native-like and ferricyt c is in a compact state as evidenced by quen ched Trp fluorescence. These findings are discussed in the context of the m olten globule state of proteins induced on a negatively charged membrane su rface under physiological conditions. (C) 1999 Elsevier Science B.V. All ri ghts reserved.