Yeast cytochrome constants

The reaction Fe ccp/Fe cytc + Fe ccp/Fe cytc proceeds with AE s 0.4V. The reaction h j been moj ored both by pulse radiolysis, and by simple mixing of Fe ccp + Fe1 cytc, with equivalent results k 0.25 0.07 s (figure 10) It is interesting that a dependence of rate on the primary structure of the protein is observed (at constant AG) for horse cy1j.c/ccp(yeast) k = 0.25 s but for yeast. cytc/(yeast) ccp k = 4 s 1 and for tuna cytc/yeast ccp k s 0.1 s, even though the general three dimensional structures are essentially identical for horse, tuna and yeast cytochromes c. These determinations disprove an earlier suggestion based on modulated excitation spectroscopy, that k - 10 s. Clearly the rate is slow,... 

Table I. Rate Constants for Intramolecular Electron Transfer in H39C C102T Variant of Yeast Cytochrome c Covalently Bound to Ruthenium Complexes at Cys39.

Figure 3. Plot of ln(l yhj vs. the free energy for the intramolecular electron transfer reactions of the H39C C102T variant of yeast cytochrome c. The solid curve is a plot of equation 1 with Hab = 0.06 cm and A = 1 eV. The data points can be identified by using Table I. The rate constant kj, are indicated by and...

The complex between yeast cytochrome c peroxidase and cytochrome c, however, seams to present a different type of behavior with respect to the association process. As for most known complexes formed between electron transfer proteins, this is a week complex with a binding constant of the order of 10. In contrast to the other cases, these two proteins do not show a great extent of surface matching at the docking interface, nor do they show very specific amino acid interactions across the intermolecular surface. As a result, the predicted model of the structure of this complex is less well defined. 

Another system (77) in which the direct electrochemistry of cytochrome c has been usefully exploited is in driving the turnover of yeast flavocytochrome 62 (L-lactate/cytochrome c oxidoreductase E.C. 1.1.2.3). A second-order rate constant of 5 x 10 M sec" was obtained for turnover of the cytochrome c by the enzyme, which agreed well with the value from conventional spectrophotometric methods. 

When dealing with pleiotropic respiratory-deficient mutants of yeast, three practical difficulties constantly arise. The major inconvenience is the general genetic instability of the chromosomal pleiotropic respiratory-deficient mutants in S. cerevisiae where an additional p mutation spontaneously arises with very high frequency Since both the nuclear and the mitochondrial mutations are often expressed by deficient cytochromes aas and b, as well as by the loss of oligomycin sensitivity of the ATPase, the biochemical study of these double mutants is very difficult. An easy way out of this difficulty is to use chromosomal respiratory-deficient mutants of a petite-negative yeast species like 5. pombe, where no viable p mutation can be induced. 

---