Control of the redox potential in c-type cytochromes: importance of the entropic contribution

Expression and characterization of Pseudomonas aeruginosa cytochrome c-551 and two site-directed mutants: role of tryptophan 56 in the modulation of redox properties

The gene coding for Pseudomonas aeruginosa cytochrome c-551 was expressed in Pseudomonas putida under aerobic conditions, using two different expression vectors; the more efficient proved to be pNM185, induced by m-toluate. Mature holo-(cytochrome c-551) was produced in high yield by this expression system, and was purified to homogeneity. Comparison of the recombinant wild-type protein with that purified from Ps. aeruginosa showed no differences in structural and functional properties. Trp56, an internal residue in cytochrome c-551, is located at hydrogen-bonding distance from haem propionate-17, together with Arg47. Ionization of propionate-17 was related to the observed pH-dependence of redox potential. The role of Trp56 in determining the redox properties of Ps. aeruginosa cytochrome c-551 was assessed by site-directed mutagenesis, by substitution with Tyr (W56Y) and Phe (W56F). The W56Y mutant is similar to the wild-type cytochrome. On the other hand, the W56F mutant, although similar to the wild-type protein in spectral properties and electron donation to azurin, is characterized by a weakening of the Fe-Met61 bond, as shown in the oxidized protein by the loss of the 695 nm band approx. 2 pH units below the wild-type. Moreover, in W56F, the midpoint potential and its pH-dependence are both different from the wild-type. These results are consistent with the hypothesis that hydrogen-bonding to haem propionate-17 is important in modulation of the redox properties of Ps. aeruginosa cytochrome c-551.

Protein folding triggered by electron transfer

Rapid photochemical electron injection into unfolded ferricytochrome c titrated with 2.3 to 4.6 M guanidine hydrochloride (GuHCL) at pH 7 and 40 degrees C produced unfolded ferrocytochrome, which then converted to the folded protein. Two folding phases were observed: a fast process with a time constant of 40 microseconds (4.6 M GuHCL), and a slower phase with a rate constant of 90 +/- 20 per second (2.3 M GuHCL). The activation free energy for the slow step varied linearly with GuHCL concentration; the rate constant, extrapolated to aqueous solution, was 7600 per second. Electron-transfer methods can bridge the nanosecond to millisecond measurement time gap for protein folding.