Spectral identification of intermediates generated during the reaction of dioxygen with the wild-type and EQ(I-286) mutant of Rhodobacter sphaeroides cytochrome c oxidase

Cytochrome c oxidase oxygen reduction reaction induced by cytochrome c on nickel-coordination surfaces based on graphene oxide in suspension

BACKGROUND

The electrochemical and spectroscopic investigation of bacterial electron-transfer proteins stabilized on solid state electrodes has provided an effective approach for functional respiratory enzyme studies.

METHODS

We assess the biocompatibility of carboxylated graphene oxide (CGO) functionalized with Nickel nitrilotriacetic groups (CGO-NiNTA) ccordinating His-tagged cytochrome c oxidase (CcO) from Rhodobacter sphaeroides.

RESULTS

Kinetic studies employing UV-visible absorption spectroscopy confirmed that the immobilized CcO oxidized horse-heart cytochrome c (Cyt c) albeit at a slower rate than isolated CcO. The oxygen reduction reaction as catalyzed by immobilized CcO could be clearly distinguished from that arising from CGO-NiNTA in the presence of Cyt c and dithiothreitol (DTT) as a sacrificial reducing agent. Our findings indicate that while the protein content is about 3.7‰ by mass with respect to the support, the contribution to the oxygen consumption activity averaged at 56.3%.

CONCLUSIONS

The CGO-based support stabilizes the free enzyme which, while capable of Cyt c oxidation, is unable to carry out oxygen consumption in solution on its own under our conditions. The turnover rate for the immobilized CcO was as high as 240 O₂ molecules per second per CcO unit.

GENERAL SIGNIFICANCE

In vitro investigations of electron flow on isolated components of bacterial electron-transfer enzymes immobilized on the surface of CGO in suspension are expected to shed new light on microbial bioenergetic functions, that could ultimately contribute toward the improvement of performance in living organisms.

The Reactions of O2 and NO with Mixed-Valence ba3 Cytochrome c Oxidase from Thermus thermophilus

Earlier CO flow-flash experiments on the fully reduced Thermus thermophilus ba₃ (Tt ba₃) cytochrome oxidase revealed that O₂ binding was slowed down by a factor of 10 in the presence of CO (Szundi et al., 2010, PNAS 107, 21010-21015). The goal of the current study is to explore whether the long apparent lifetime (∼50 ms) of the Cu_B⁺-CO complex generated upon photolysis of the CO-bound mixed-valence Tt ba₃ (Koutsoupakis et al., 2019, Acc. Chem. Res. 52, 1380-1390) affects O₂ and NO binding and the ability of Cu_B to act as an electron donor during O-O bond splitting. The CO recombination, NO binding, and the reaction of mixed-valence Tt ba₃ with O₂ were investigated by time-resolved optical absorption spectroscopy using the CO flow-flash approach and photolabile O₂ and NO carriers. No electron backflow was detected after photolysis of the mixed-valence CO-bound Tt ba₃. The rate of O₂ and NO binding was two times slower than in the fully reduced enzyme in the presence of CO and 20 times slower than in the absence of CO. The purported long-lived Cu_B⁺-CO complex did not prevent O-O bond splitting and the resulting P_M formation, which was significantly faster (5-10 times) than in the bovine heart enzyme. We propose that O₂ binding to heme a₃ in Tt ba₃ causes CO to dissociate from Cu_B⁺ in a concerted manner through steric and/or electronic effects, thus allowing Cu_B⁺ to act as an electron donor in the mixed-valence enzyme. The significantly faster O₂ binding and O-O bond cleavage in Tt ba₃ compared to analogous steps in the aa₃ oxidases could reflect evolutionary adaptation of the enzyme to the microaerobic conditions of the T. thermophilus HB8 species.

The CO Photodissociation and Recombination Dynamics of the W172Y/F282T Ligand Channel Mutant of Rhodobacter sphaeroides aa3 Cytochrome c Oxidase

In the ligand channel of the cytochrome c oxidase from Rhodobacter sphaeroides (Rs aa3 ) W172 and F282 have been proposed to generate a constriction that may slow ligand access to and from the active site. To explore this issue, the tryptophan and phenylalanine residues in Rs aa3 were mutated to the less bulky tyrosine and threonine residues, respectively, which occupy these sites in Thermus thermophilus (Tt) ba3 cytochrome oxidase. The CO photolysis and recombination dynamics of the reduced wild-type Rs aa3 and the W172Y/F282T mutant were investigated using time-resolved optical absorption spectroscopy. The spectral changes associated with the multiple processes are attributed to different conformers. The major CO recombination process (44 μs) in the W172Y/F282T mutant is ~500 times faster than the predominant CO recombination process in the wild-type enzyme (~23 ms). Classical dynamic simulations of the wild-type enzyme and double mutant showed significant structural changes at the active site in the mutant, including movement of the heme a3 ring-D propionate toward CuB and reduced binuclear center cavity volume. These structural changes effectively close the ligand exit pathway from the binuclear center, providing a basis for the faster CO recombination in the double mutant.

The pathway of O₂to the active site in heme-copper oxidases

The route of O₂to and from the high-spin heme in heme-copper oxidases has generally been believed to emulate that of carbon monoxide (CO). Time-resolved and stationary infrared experiments in our laboratories of the fully reduced CO-bound enzymes, as well as transient optical absorption saturation kinetics studies as a function of CO pressure, have provided strong support for CO binding to CuB⁺ on the pathway to and from the high-spin heme. The presence of CO on CuB⁺ suggests that O₂binding may be compromised in CO flow-flash experiments. Time-resolved optical absorption studies show that the rate of O₂and NO binding in the bovine enzyme (1 × 10⁸M⁻¹s⁻¹) is unaffected by the presence of CO, which is consistent with the rapid dissociation (t½ = 1.5μs) of CO from CuB⁺. In contrast, in Thermus thermophilus (Tt) cytochrome ba3 the O₂and NO binding to heme a3 slows by an order of magnitude in the presence of CO (from 1 × 10⁹ to 1 × 10⁸M⁻¹s⁻¹), but is still considerably faster (~10μs at 1atm O₂) than the CO off-rate from CuB in the absence of O₂(milliseconds). These results show that traditional CO flow-flash experiments do not give accurate results for the physiological binding of O₂and NO in Tt ba3, namely, in the absence of CO. They also raise the question whether in CO flow-flash experiments on Tt ba3 the presence of CO on CuB⁺ impedes the binding of O₂to CuB⁺ or, if O₂does not bind to CuB⁺ prior to heme a3, whether the CuB⁺-CO complex sterically restricts access of O₂to the heme. Both possibilities are discussed, and we argue that O₂binds directly to heme a3 in Tt ba3, causing CO to dissociate from CuB⁺ in a concerted manner through steric and/or electronic effects. This would allow CuB⁺ to function as an electron donor during the fast (5μs) breaking of the OO bond. These results suggest that the binding of CO to CuB⁺ on the path to and from heme a3 may not be applicable to O₂and NO in all heme-copper oxidases. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.