Site-directed mutagenesis was employed to examine the role played by specific surface residues in the activity of cytochrome c peroxidase. The double charge, aspartic acid to lysine, point mutations were constructed at positions 37, 79, and 217 on the surface of cytochrome c peroxidase, sites purported to be within or proximal to the recognition site for cytochrome c in an electron-transfer productive complex formed by the two proteins. The resulting mutant peroxidases were examined for catalytic activity by steady-state measurements and binding affinity by two methods, fluorescence binding titration and cytochrome c affinity chromatography. The cloned peroxidases exhibit similar UV-visible spectra to the wild-type yeast protein, indicating that there are no major structural differences between the cloned peroxidases and the wild-type enzyme. The aspartic acid to lysine mutations at positions 79 and 217 exhibited similar turnover numbers and binding affinities to that seen for the "wild type-like" cloned peroxidase. The same change at position 37 caused more than a 10-fold decrease in both turnover of and binding affinity for cytochrome c. This empirical finding localizes a primary recognition region critical to the dynamic complex. Models from the literature proposing structures for the complex between peroxidase and cytochrome c are discussed in light of these findings.

Mitochondrial holocytochrome c contains heme that is covalently attached to the protein in a reaction catalyzed by the enzyme cytochrome c heme lyase. In the absence of heme, apocytochrome c, the precursor to holocytochrome c, is unfolded. We find that purified apocytochrome c binds noncovalently to heme. Binding is accompanied by changes in the optical absorption spectrum of heme and by quenching of the tryptophan fluorescence of the protein. The affinity of apocytochrome c for heme, as well as the stoichiometry of binding, appears to depend on whether or not cyanide is present and on the oxidation state of heme. Under reducing conditions, in the presence of cyanide, the association appears to be 1:1, with a binding constant of about 10(7) M-1. Under oxidized conditions, there may be multiple hemes bound per molecule of apocytochrome c. Upon binding to heme, apocytochrome c exhibits a mobility similar to that of holocytochrome c in gel filtration chromatography and velocity gradient ultracentrifugation, indicating that the heme-protein complex adopts a structure that is almost as compact as that of holocytochrome c. Changes in the circular dichroism spectrum of apocytochrome c are consistent with an increase in the alpha-helical content of the protein on binding heme. The compact structure of the noncovalent heme-apocytochrome c complex may represent an intermediate in the de novo folding of cytochrome c.

Flash photolysis and pulse radiolysis measurements demonstrate a conformational dependence of electron transfer rates across a 16-mer helical bundle (three-helix metalloprotein) modified with a capping Co'II(bipyridine)3 electron acceptor at the N terminus and a 1-ethyl-1'-ethyl-4,4'-bipyridinium donor at the C terminus. For the CoIII(peptide)3-1-ethyl-1'-ethyl-4,4'-bipyridinium maquettes, the observed transfer is a first order, intramolecular process, independent of peptide concentration or laser pulse energy. In the presence of 6 M urea, the random coil bundle (0% helicity) has an observed electron transfer rate constant of kobs = 900 ± 100 s-1. In the presence of 25% trifluoroethanol (TFE), the helicity of the peptide is 80% and the k.bs increases to 2000 + 200 s-1. Moreover, the increase in the rate constant in TFE is consistent with the observed decrease in donoracceptor distance in this solvent. Such bifunctional systems provide a class of molecules for testing the effects of conformation on electron transfer in proteins and peptides.De novo design of redox proteins represents a significant challenge for biological and biomimetic chemistry (1, 2). Several maquettes have been designed toward systems in which electrons can be translocated across proteins (3,4). A wealth of data now exist for modified natural proteins like cytochrome c (5, 6). Significant data are also available for modified single peptide systems (7), but conformational equilibria often complicate the interpretation of simple systems (8). Two particularly attractive structural maquettes for the design and study of de novo redox proteins were reported by Ghadhiri et al. (9) and Lieberman and Sasaki (10). Both systems consist of a three helix bundle, whose stoichiometry and topology are defined by the capping metal bipyridyl complex. These bundles have been well characterized in the literature (2, 9-11). Because there are numerous tris-bipyridyl complexes, using this motif to create three helix bundles allows ready access to the many varied spectroscopic, photophysical, and redox properties offered by these metal compounds. Moreover, the ability to control the conformation of these three helix bundles under different solvent conditions provides a facile system in which to study the effects of secondary structure on rates of electron transfer (ET), while maintaining a constant bond connectivity. By probing the ET rates of a designed metalloprotein in both folded and unfolded states, the role of helical secondary structure in mediating ET can be investigated. MATERIALS AND METHODSPeptide Modifications. A minor elaboration on these metalloproteins provides a model bifunctional redox system in which redox active 1-ethyl-1'-ethyl-4,4'-bipyridinium is covalently linked to the C terminus of a bipyridine-modified 16-mer peptide, called "16-mer," and a redox active metal (cobalt) is incorporated into the N terminus as shown in Fig. 1. The sequence of the 16-mer and the structure of the C-terminal modifier, 1-ethyl-i '-ethyl-4,4'-bipyridinium, ...

Transient absorption techniques were used to measure the intracomplex electron transfer rates between four recombinant yeast cytochrome c peroxidases and iso-1 cytochrome c (cytc). The binding affinities and catalytic activities with cytc were previously examined [Corin et al. (1991) Biochemistry 30, 11585]. The four include a wild-type peroxidase (ECcP) and three others, each of which has one surface aspartic acid converted to lysine at position 37, 79, or 217. These sites have been suggested to be within or proximal to the recognition site for cytc. These mutants conduct electron transfer with cytc but differ with respect to the ionic strength profiles of their limiting rate constants. At pH and mu = 114 mM, ECcP and D217K show similar limiting rate constants for electron transfer with cytc, k(lim), of ca. 2000 s-1. In the same peroxidase concentration range, the D37K mutant exhibits a k(obs) of ca. 100 s-1. Instability of the compound I form of D79K prevented a complete study of the intracomplex kinetics of this mutant by this technique. At pH 6 and low ionic strength (8 mM), D37K exhibits a dramatic increase in k(obs) to ca. 800 s-1 while the other two recombinants show a marked decrease to values < 150 s-1. D37K displays much lower affinity for cytc than do the other peroxidases at higher ionic strengths [Hake et al. (1992) J. Am. Chem. Soc. 114, 5442], thus preventing adequate complexation necessary for efficient electron transfer. Variations in binding affinity do not explain the more subtle ionic strength kinetic profile observed for D217K.(ABSTRACT TRUNCATED AT 250 WORDS)

A fluorescence quenching experiment confirms that in the redox reaction between cytochrome c-551 and azurin, protein complexing is negligible. Azurin-pH indicator T-jump experiments show that Pseudomonas aeruginosa (Ps.) azurin exhibits a slow time constant, tau, in its return to pH equilibrium but Alcaligenes faecalis (Alc.) azurin does not. The decrease of l/tau with increasing pH shows that the rate-determining process is a slow transformation of the imidazolium form of histidine-35 from a conformation where it cannot ionize to one in which it can. The fast relaxation time constant of the redox reaction varies little with pH, but the slow time constant increased by a factor of approximately 2.5 increasing pH between pH 5 and pH 8. The corresponding amplitudes, especially the slow one, vary with pH. On the basis of all the present evidence it is concluded that, while some differences of redox reactivity do occur on protonation, these differences are not major. In general, the two proteins cyt c-551 and azurin react with each other with rates only weakly dependent upon pH. A classical pH titration was carried out on the reduced and oxidized form of Ps. and Alc. azurin with the result that two protons were released between pH 6 and pH 8, in the former from His-35 and -83 and in the latter from His-83 and Ala-1.