Erik Sedla ´k 1 Maria ´ n Antalı B k 2 Coulombic and Noncoulombic 1 Department of Biochemistry, Faculty of Science, Effect of Polyanions on P. J. S ˘ afa ´ rik University, Moyzesova 11, Cytochrome c Structure 04167 Kos ˘ ice, Slovakia 2 Department of Biophysics, Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 043 53 Kos ˘ ice, Slovakia Received 3 October 1997; accepted 18 February 1998 Abstract: The properties of the complexes of ferricytochrome c with two different polyan- ions—poly(vinylsulfate) and poly(4-styrene-sulfonate) —with a comparable charge density but with the different size of the uncharged part of their molecules have been studied by means of optical spectroscopy, differential scanning calorimetry, and gel chromatography. Ferriccytochrome c formed a complex with the former one through coulombic interactions and remained in a native-like state. The addition of the second polyanion to a solution of ferric cytochrome c at a low ionic strength, pH 7.0, resulted in profound conformational change in the hydrophobic core of protein (opening of the heme crevice with a perturbation of the methionine 80–heme iron bond and the hydrophobic core of the protein). These may be understood as an involvement of noncoulombic ( hydrophobic, H-bonding ) interactions of the uncharged part of the polyanion molecule. Conformational changes and the observed shift in acidic transition from low spin to high spin state of ferric cytochrome c detected in the presence of the polyanions may have biological implication in understanding the origin of conformational changes in proteins induced in the course of their interaction with membrane lipids and membrane proteins.  1998 John Wiley & Sons, Inc. Biopoly 46: 145–154, 1998 Keywords: cytochrome c; conformational change; polyanion binding; coulombic interaction; hydrophobic interaction; microcalorimetry INTRODUCTION Mitochondrial cyt c (a basic protein of pI Å 10.1) forms tight complexes with its biological redox part- ners mainly through electrostatic interactions. Since Cytochrome c (cyt c) is an essential component of the respiratory chain in bacteria and mitochondria. complex formation induces conformational changes Correspondence to: Maria ´n Antalik Contract grant sponsor: Slovak Grant Agency Contract grant number: 4173 and 2053 Biopolymers, Vol. 46, 145–154 (1998)  1998 John Wiley & Sons, Inc. CCC 0006-3525/98 / 030145-10 145 5601 / 8k4a$$5601 06-18-98 15:36:08 bpa W: Biopolymers