Three Spectrally Different States of Cytochromes cc' and c' and Their Interconversion
1969 Volume 65 Issue 2 Pages 225-237
Details
1969 Volume 65 Issue 2 Pages 225-237
1. Oxidized cytochrome cc' from Rhodospirillum rubrum exhibits three different types of absorption spectrum depending on pH; i.e., the neutral type (Type I), intermediate type (Type II), and alkaline type (Type III). The pH-dependent spectral conversion is reversible. In the reduced form, it shows an abnormal spectrum (Type α) in the pH region where the spectrum of the oxidized form is either Type I or II, but is converted to a state having a normal spectrum (Type n) at more alkaline pH's.
2. Alcohols, phenols and ketones induce the spectral conversion from Types I and II to Type III at fixed pH values, and the efficiency of these solvents in causing the conversion parallels their hydrophobicity. Urea and guanidine hydrochloride also effect the conversion at slow rates. On the other hand, high ionic strength not only prevents the conversion of Type II to Type III, but also causes almost complete reversion of Type III to Type II at alkaline pH's. Glycerol shows the same effect to a lesser extent. Deoxycholate shows no effects.
3. The helical content determined from circular dichroism measurements is about 63% in both the states showing Type I and II spectra. The spectral conversion by 2-propanol at pH 7 results in no appreciable change in the helical content, whereas that induced by alkali or urea is accompanied by a considerable increase in the random-coli structure.
4. Cytochrome c' from Rhodopseudomonas palustris responds to pH and the various reagents similarly, but is more resistant to treatments causing the conversion to Type III.
5. It is concluded that the anomalous Type I and II spectra of these cytochromes arise from a unique hydrophobic structure in the vicinity of the heme. Direct and indirect destruction of this structure leads to the conversion to Type III spectrum, which is normal for c-type cytochromes. This conversion does not necessarily involve the alteration in the gross helical conformation of the proteins.
6. These properties of cytochromes cc' and c' are discussed in relation to the atypical spectral properties of P-450 and the ethyl isocyanide compound of protoheme.
