Comparison of electron transfer kinetics between redox proteins free in solution and electrostatically complexed to a lipid bilayer membrane

Kinetic, Thermodynamic, and Mechanistic Patterns for Free (Unbound) Cytochromec at Au/SAM Junctions: Impact of Electronic Coupling, Hydrostatic Pressure, and Stabilizing/Denaturing Additives

Combined kinetic (electrochemical) and thermodynamic (calorimetric) investigations were performed for an unbound (intact native-like) cytochrome c (CytC) freely diffusing to and from gold electrodes modified by hydroxyl-terminated self-assembled monolayer films (SAMs), under a unique broad range of experimental conditions. Our approach included: 1) fine-tuning of the charge-transfer (CT) distance by using the extended set of Au-deposited hydroxyl-terminated alkanethiol SAMs [-S-(CH(2))(n)-OH] of variable thickness (n=2, 3, 4, 6, 11); 2) application of a high-pressure (up to 150 MPa) kinetic strategy toward the representative Au/SAM/CytC assemblies (n=3, 4, 6); 3) complementary electrochemical and microcalorimetric studies on the impact of some stabilizing and denaturing additives. We report for the first time a mechanistic changeover detected for "free" CytC by three independent kinetic methods, manifested through 1) the abrupt change in the dependence of the shape of the electron exchange standard rate constant (k(o)) versus the SAM thickness (resulting in a variation of estimated actual CT range within ca. 15 to 25 A including ca. 11 A of an "effective" heme-to-omega-hydroxyl distance). The corresponding values of the electronic coupling matrix element vary within the range from ca. 3 to 0.02 cm(-1); 2) the change in activation volume from +6.7 (n=3), to approximately 0 (n=4), and -5.5 (n=6) cm(3) mol(-1) (disclosing at n=3 a direct pressure effect on the protein's internal viscosity); 3) a "full" Kramers-type viscosity dependence for k(o) at n=2 and 3 (demonstrating control of an intraglobular friction through the external dynamic properties), and its gradual transformation to the viscosity independent (nonadiabatic) regime at n=6 and 11. Multilateral cross-testing of "free" CytC in a native-like, glucose-stabilized and urea-destabilized (molten-globule-like) states revealed novel intrinsic links between local/global structural and functional characteristics. Importantly, our results on the high-pressure and solution-viscosity effects, together with matching literature data, strongly support the concept of "dynamic slaving", which implies that fluctuations involving "small" solution components control the proteins' intrinsic dynamics and function in a highly cooperative manner as far as CT processes under adiabatic conditions are concerned.

Pro-oxidant effects of cross-linked haemoglobins explored using liposome and cytochrome c oxidase vesicle model membranes

The therapeutic use of cell-free haemoglobin as a blood substitute has been hampered by toxicological effects. A model asolectin (phosphatidylcholine/phosphatidylethanolamine) liposome system was utilized to study the pro-oxidant efficiency of several chemically modified haemoglobins on biological membranes. Lipid peroxidation, resulting from the interactions between haemoglobin and liposomes, was measured by conjugated diene formation and the maximal rates of oxygen uptake. Spectral changes gave insight into the occurrence of the ferryl iron species. The residual reactivity of oxidatively damaged haemoglobins with ligands during incubation with liposomes was assessed from rapid kinetic carbon monoxide-binding experiments. Liposomes in which cytochrome c oxidase was embedded show both haemoglobin and the enzyme to be oxidatively damaged during incubation. The functional state of cytochrome c oxidase was monitored in the presence and absence of a free radical scavenger. Once in contact, both unmodified and modified haemoglobins triggered and maintained severe radical-mediated membrane damage. Differences in the pro-oxidant activities among haemoglobins may be explained by either the differential population of their ferryl intermediates or disparate dimerization and transfer of haem into the membrane with subsequent haem degradation. This study may contribute to a better understanding of the molecular determinants of haemoglobin interactions with a variety of biological membranes.