Proton NMR study of the heme environment in bacterial quinol oxidases

Formamide probes a role for water in the catalytic cycle of cytochrome c oxidase

Formamide is a slow-onset inhibitor of mitochondrial cytochrome c oxidase that is proposed to act by blocking water movement through the protein. In the presence of formamide the redox level of mitochondrial cytochrome c oxidase evolves over the steady state as the apparent electron transfer rate from cytochrome a to cytochrome a(3) slows. At maximal inhibition cytochrome a and cytochrome c are fully reduced, whereas cytochrome a(3) and Cu(B) remain fully oxidized consistent with the idea that formamide interferes with electron transfer between cytochrome a and the oxygen reaction site. However, transient kinetic studies show that intrinsic rates of electron transfer are unchanged in the formamide-inhibited enzyme. Formamide inhibition is demonstrated for another member of the heme-oxidase family, cytochrome c oxidase from Bacillus subtilis, but the onset of inhibition is much quicker than for mitochondrial oxidase. If formamide inhibition arises from a steric blockade of water exchange during catalysis then water exchange in the smaller bacterial oxidase is more open. Subunit III removal from the mitochondrial oxidase hastens the onset of formamide inhibition suggesting a role for subunit III in controlling water exchange during the cytochrome c oxidase reaction.

Magic-angle spinning solid-state NMR of a 144 kDa membrane protein complex: E. coli cytochrome bo3 oxidase

Recent progress in magic-angle spinning (MAS) solid-state NMR (SSNMR) has enabled multidimensional studies of large, macroscopically unoriented membrane proteins with associated lipids, without the requirement of solubility that limits other structural techniques. Here we present initial sample preparation and SSNMR studies of a 144 kDa integral membrane protein, E. coli cytochrome bo(3) oxidase. The optimized protocol for expression and purification yields approximately 5 mg of the enzymatically active, uniformly (13)C,(15)N-enriched membrane protein complex from each liter of growth medium. The preparation retains endogenous lipids and yields spectra of high sensitivity and resolution, consistent with a folded, homogenous protein. Line widths of isolated signals are less than 0.5 ppm, with a large number of individual resonances resolved in the 2D and 3D spectra. The (13)C chemical shifts, assigned by amino acid type, are consistent with the secondary structure previously observed by diffraction methods. Although the structure is predominantly helical, the percentage of non-helical signals varies among residue types; these percentages agree well between the NMR and diffraction data. Samples show minimal evidence of degradation after several weeks of NMR data acquisition. Use of a triple resonance scroll resonator probe further improves sample stability and enables higher power decoupling, higher duty cycles and more advanced 3D experiments to be performed. These initial results in cytochrome bo(3) oxidase demonstrate that multidimensional MAS SSNMR techniques have sufficient sensitivity and resolution to interrogate selected parts of a very large uniformly (13)C,(15)N-labeled membrane protein.

Alteration of the regiospecificity of human heme oxygenase-1 by unseating of the heme but not disruption of the distal hydrogen bonding network

Heme oxygenase regiospecifically oxidizes heme at the alpha-meso position to give biliverdin IXalpha, CO, and iron. The heme orientation within the active site, which is thought to determine the oxidation regiospecificity, is shown here for the human enzyme (hHO1) to be largely determined by interactions between the heme carboxylic acid groups and residues Arg183 and Lys18 but not Tyr134. Mutation of either Arg183 or Lys18 individually does not significantly alter the NADPH-cytochrome P450 reductase-dependent reaction regiochemistry but partially shifts the oxidation to the beta/delta-meso positions in the reaction supported by ascorbic acid. Mutation of Glu29 to a lysine, which places a positive charge where it can interact with a heme carboxyl if the heme rotates by approximately 90 degrees, causes a slight loss of regiospecificity but combined with the R183E and K18E mutations results primarily in beta/delta-meso oxidation of the heme under all conditions. NMR analysis of heme binding to the triple K18E/E29K/R183E mutant confirms rotation of the heme in the active site. Kinetic studies demonstrate that mutations of Arg183 greatly impair the rate of the P450 reductase-dependent reaction, in accord with the earlier finding that Arg183 is involved in binding of the reductase to hHO1, but have little effect on the ascorbate reaction. Mutations of Asp140 and Tyr58 that disrupt the active site hydrogen bonding network impair catalytic rates but do not influence the oxidation regiochemistry. The results indicate both that the oxidation regiochemistry is largely controlled by ionic interactions of the heme propionic acid groups with the protein and that shifts in regiospecificity involve rotation of the heme about an axis perpendicular to the heme plane.