Structural changes in cytochrome c oxidase induced by cytochrome c binding. A resonance raman study

Electronic wiring of a multi-redox site membrane protein in a biomimetic surface architecture

Bioelectronic coupling of multi-redox-site membrane proteins was accomplished with cytochrome c oxidase (CcO) as an example. A biomimetic membrane system was used for the oriented immobilization of the CcO oxidase on a metal electrode. When the protein is immobilized with the CcO binding side directed toward the electrode and reconstituted in situ into a lipid bilayer, it is addressable by direct electron transfer to the redox centers. Electron transfer to the enzyme via the spacer, referred to as electronic wiring, shows an exceptionally high rate constant. This allows a kinetic analysis of all four consecutive electron transfer steps within the enzyme to be carried out. Electron transfer followed by rapid scan cyclic voltammetry in combination with surface-enhanced resonance Raman spectroscopy provides mechanistic and structural information about the heme centers. Probing the enzyme under turnover conditions showed mechanistic insights into proton translocation coupled to electron transfer. This bioelectronic approach opens a new field of activity to investigate complex processes in a wide variety of membrane proteins.

Redox and redox-coupled processes of heme proteins and enzymes at electrochemical interfaces

Modern bioelectrochemical methods rely upon the immobilisation of redox proteins and enzymes on electrodes coated with biocompatible materials to prevent denaturation. However, even when protein denaturation is effectively avoided, heterogeneous protein electron transfer is often coupled to non-Faradaic processes like reorientation, conformational transitions or acid-base equilibria. Disentangling these processes requires methods capable of probing simultaneously the structure and reaction dynamics of the adsorbed species. Here we provide an overview of the recent developments in Raman and infrared surface-enhanced spectroelectrochemical techniques applied to the study of soluble and membrane bound redox heme proteins and enzymes. Possible biological implications of the findings are critically discussed.

Folding units govern the cytochrome c alkaline transition

The alkaline transition of cytochrome c is a model for protein structural switching in which the normal heme ligand is replaced by another group. Stopped flow data following a jump to high pH detect two slow kinetic phases, suggesting two rate-limiting structure changes. Results described here indicate that these events are controlled by the same structural unfolding reactions that account for the first two steps in the reversible unfolding pathway of cytochrome c. These and other results show that the cooperative folding-unfolding behavior of protein foldons can account for a variety of functional activities in addition to determining folding pathways.

Electron transfer between yeast cytochrome bc(1) complex and cytochrome c: a structural analysis

The structure of the complex between cytochrome c (CYC) and the cytochrome bc(1) complex (QCR) from yeast crystallized with an antibody fragment has been recently determined at 2.97 A resolution [Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 2800]. CYC binds to subunit cytochrome c(1) of the enzyme stabilized by hydrophobic interactions surrounding the heme crevices creating a small, compact contact site. A central cation-pi interaction is an important and conserved feature of CYC binding. Peripheral patches with highly conserved complementary charges further stabilize the enzyme-substrate complex by long-range electrostatic forces and may affect the orientation of the substrate. Size and characteristics of the contact site are optimal for a transient electron transfer complex. Kinetic data show a bell-shaped ionic strength dependence of the cytochrome c reduction with a maximum activity near physiological ionic strength. The dependence is less pronounced in yeast compared to horse heart CYC indicating less impact of electrostatic interactions in the yeast system. Interestingly, a local QCR activity minimum is found for both substrates at 120-140 mM ionic strength. The architecture of the complex results in close distance of both c-type heme groups allowing the rapid reduction of cytochrome c by QCR via direct heme-to-heme electron transfer. Remarkably, CYC binds only to one of the two possible binding sites of the homodimeric complex and binding appears to be coordinated with the presence of ubiquinone at the Q(i) site. Regulatory aspects of CYC reduction are discussed.