Conformations ferricytochrome

Our assignments of the near infra-red spectra have important implications for the interpretation of the visible spectra. The spectra of low-spin ferric haemoproteins in the region 13—25 kK are fairly typical of metalloporphyrins, and can be explained in terms of the theory discussed in Section IIB. However, a weak band is sometimes seen at 14—16 kK which is z-polarized in ferricytochrome-c (90), ferrihaemoglobin azide (110) and ferrimyoglobin azide (92). The band at 14.4 kK in ferricytochrome-c has been the subject of much discussion. At neutral pH it has an intensity of e=800 (59) but this is very sensitive to the pH and the conformation of the protein (55). It has been suggested that this band arises from electron transfer from a distal cysteine or tyrosine to the metal (4), and that the ease of electron transfer in this way could play... 

A comparison of CD spectra (200—600 nm) between bovine heart fenicytochrome c and ferricytochrome from Rhodospirillum rubrum showed marked differences, while the spectra were more similar in the reduced forms. These differences were attributed to subtle variations around the hemes rather than to differences in the polypeptide chain conformations (216). Optical activity studies of cytochromes C3 showed similarities among three species of Desulfovibrio but revealed differences from mammalian cytochrome c. Rotatory strengths of the resolved CD... 

CD measurements were used as a sensitive indicator for monitoring conformational changes in horse heart ferricytochrome c effected by urea, pH, temperature and extrinsic ligands (221), by alcohols (222), or by chemical modifications (223). 

Ferricytochrome c SWCNTs CD UV-Vis The presence of CNTs decreased the mobility of the protein and induced a more tight conformation. [139]... 

Cytochrome c exhibits several pH-dependent conformational states. In particular, an alkaline transition with a pXa —9.1 has been observed for ferricy-tochrome c. This transition is believed to be associated with the dissociation of Met-80 the reduetion potential decreases dramatically, and the 695-nm absorption band, associated with a sulfur iron charge-transfer transition, disappears. The NMR resonance due to ( H3C-) Met-80 in deuterium-enriched ferricytochrome c disappears from its hyperfine-shifted upheld position without line broadening, and reappears coincident with the ( H3C-)Met-65 resonance. In contrast, ferrocytochrome c maintains an ordered structure over the pH range 4 to 11. The heme iron in ferricytochrome c remains low-spin throughout this... 

To this point we have been considering the cytochrome c molecule as a static object, which is far from correct. As Theorell first reported ferricytochrome c has five distinct pH-dependent conformational states, separated by the dissociation of single protons ... 

Cytochrome c exists in several pH-dependent conformational states. Native ferricytochrome c, known as State III, undergoes a conformational change to State IV as solution pH is raised from neutral values. This conformational change has a pK of 9.35. Serre et used differential pulse... 

Assfalg et have determined the solution structure of enriched yeast iso-l-ferricytochrome c which contained three mutations designed to produce a single conformer. The global structure of the protein was determined using NOE s, backbone angle constraints, and pseudocontact shifts as constraints. Yao et have determined the solution structure of cyanoferricytochrome c. 

Wilson (1967) has reported on the use of low temperatures (— 196 C) to sharpen the absorption bands of some nonheme iron proteins, e.g., ferredoxins of spinach and of Clostridium acidi-urici, and an adrenal protein. All three of these nonheme proteins have absorption maxima near 700 nm which disappear on reduction by dithionite. The oxidized spinach ferredoxin has two additional absorptions, a shoulder near 830 nm and a weak maximum at 930 nm. Wilson (1967) investigated the possibility that the absorbance of these bands, particularly the 714 nm band of spinach ferre-doxin, is sensitive to protein conformational changes in a manner analogous to the 695 nm band of ferricytochrome c (Schejter and George, 1964). 

The reduction of ferricytochrome c by at neutral pH appears to be a three-step process. In the first step (A =4.5x 10 lmol- s ) a transient complex is formed between the cytochrome and the hydrated electron, in the second (k= 5 X 10 s ) the haem iron is reduced, and in the third (/ = 1.3 x 10 s ) the protein conformation changes from that appropriate for Fe to that appropriate for Fe. The authors favour a specific pathway for the movement of the electron from the surface of the molecule to the haem iron (step 1). No intermediate complexes were observed in the reduction of ferricytochrome c by the superoxide radical ion. At 20 °C the rate constant for the reaction at pH 4.7—6.7 is 1.4 x 10 1 mol s and as the pH increases above 6.7 the rate constant steadily decreases (eventually reaching zero, indicating that the neutral and high-pH forms if ferricytochrome c are un-reactive). The activation enthalpy is 18 kJ mol and it seems that little protein rearrangement is required for the formation of the activated complex. The kinetics have been reported for the reduction by Cr + of 2-hydroxy-5-nitrobenzyltryptophyl cytochrome c and of iV-formyltryptophyl cytochrome c. ... 

Sutin s kinetic studies on the oxidation of horse-heart ferrocytochrome c by tris-(phen)cobalt(iii) have recently been extended to acid pH. The reaction is first-order with respect to each reactant but the dependence of the rate on [H+] is not simple. Measurements in chloride medium (7=0.13 mol 1 ) over the pH range 1—7 revealed a rate maximum at pH 2.9 (A =6.7x 10 1 mol" s at 25 °C). By contrast, the rate constants at pH 1.0 and 5.8 are 3.2 x 10 and 2.1 x 10 1 mol" S", respectively. Below pH 1.7, biphasic kinetics are observed, the slower reaction having a rate constant of ca. 2 s" (independent of oxidant concentration). The slow process is ascribed to a conformational change in the ferricytochrome c which is produced in... 

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