The Reversible Denaturation Of Oxidized Cytochrome c At pH 2.9
The physico-chemical study of biological molecules is a rapidly expanding area of investigation which allows an examination of the relation of biological process to molecular structure. The structural relationship of DNA to cell reproduction and genetics is a prime example of this quest to explain life processes in terms of structure and function at the molecular level. The heme proteins are particularly interesting in this regard. These proteins are composed of a polypeptide chain and a non-peptide prosthetic group called a heme which consists of a porphyrin bonded to an iron atom. The way that the peptide is attached to the heme and the manner in which the peptide is folded about the heme to a large extent governs the physiological activity of the molecule. For example, hemoglobin's ability to bind molecular oxygen for transport in the blood stream is affected by the oxidation state of the heme iron; the ferrous state will bind oxygen while the ferric state will not. The mode of peptide attachment has been found to be critical for the stabilization of the ferrous state. Catalase and peroxidase are heme enzymes which catalyze reactions involving the decomposition of hydrogen peroxide. Unlike hemoglobin, these molecules require that the heme iron be in the ferric state. The folding of the protein plays an important part in the determination of the electronic properties of the heme iron and the retention of the catalytic activity of these molecules. The heme protein studied in this work is the oxidized form of cytochrome c which has a molecular weight of 13,000 and is composed of a peptide chain of 10lj. amino acids and a heme group. Cytochrome c is important biologically because it is cart of the electron transfer chain found in cellular mitochondria. The oxidation-reduction of the heme iron provides the vehicle of electron transfer for this process. Before discussing cytochrome c, it is useful to consider some physico-chemical and structural features of proteins in general. Protein structure is divided in three categories, primary, secondary, and tertiary, according to the method of K. Linderst/rm-Lang. (1 ) Primary structure concerns itself with the peptide skeleton of the molecule; features resulting from the joining together of amino acids in a fixed, way by means of peptide bonds. The primary structure of a protein can be determined by hydrolyzing the molecule and determining the amino acid sequence by methods such as ion exchange chromatography or, electronhoresis. Coiling of the peptide chain into ordered patterns, usually beta-pleated, sheets, spirals, or helices, accounts for the secondary level of protein structure. The most probable secondary structural form is the alpha-helix proposed by Pauling and his co-workers. (2) This coil is 4 stablized by formation of a series of hydrogen bonds between a amine hydrogen and a carbonyl oxygen three amino acid units along the chain. The fact that the atoms comprising the peptide bond lie in the same plane without strain also stabilizes secondary structure.