Corrections - Time-Resolved Optical Absorption Studies of Intramolecular Electron Transfer in Cytochrome c Oxidase

Cytochrome c oxidase oxidizes several hydrogen donors, including TMPD (N,N,N′,N′-tetramethyl-p-phenylenediamine) and DMPT (2-amino-6,7-dimethyl-5,6,7,8-tetrahydropterine), in the absence of the physiological substrate cytochrome c. Maximal enzyme turnovers with TMPD and DMPT alone are rather less than with cytochrome c, but much greater than previously reported if extrapolated to high reductant levels and (or) to 100% reduction of cytochrome a in the steady state. The presence of cytochrome c is, therefore, not necessary for substantial intramolecular electron transfer to occur in the oxidase. A direct bimolecular reduction of cytochrome a by TMPD is sufficient to account for the turnover of the enzyme. CuA may not be an essential component of the TMPD oxidase pathway. DMPT oxidation seems to occur more rapidly than the DMPT – cytochrome a reduction rate and may therefore imply mediation of CuA. Both "resting" and "pulsed" oxidases contain rapid-turnover and slow-turnover species, as determined by aerobic steady-state reduction of cytochrome a by TMPD. Only the "rapid" fraction (≈70% of the total with resting and ≈85% of the total with pulsed) is involved in turnover. We conclude that electron transfer to the a3CuB binuclear centre can occur either from cytochrome a or CuA, depending upon the redox state of the binuclear centre. Under steady-state conditions, cytochrome a and CuA may not always be in rapid equilibrium. Rapid enzyme turnover by either natural or artificial substrates may require reduction of both and two pathways of electron transfer to the a3CuB centre.Key words: cytochrome c oxidase, cytochrome a, respiration, cyanide, stopped flow.

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‘CO2-ligated’ cytochrome c oxidase: characterization and comparison with the Cl- -ligated enzyme

Biochemical Journal ◽

10.1042/bj3020821 ◽

1994 ◽

Vol 302 (3) ◽

pp. 821-826 ◽

Cited By ~ 6

Author(s):

A J Moody ◽

M Richardson ◽

J P Spencer ◽

U Brandt ◽

P R Rich

Keyword(s):

Electron Transfer ◽

Cytochrome C ◽

Ligand Binding ◽

Cytochrome C Oxidase ◽

Absorption Spectra ◽

Intramolecular Electron Transfer ◽

Endogenous Ligand ◽

Binding Properties ◽

Visible Absorption ◽

Bovine Heart

A form of fully oxidized bovine heart cytochrome c oxidase that is induced by CO2/HCO3- is described. The ligand-binding properties of this form are similar to those of Cl(-)-ligated oxidase [Moody, Cooper and Rich (1991) Biochim. Biophys. Acta 1059, 189-207]. Both bind cyanide at a rate (0.2 M-1.s-1 at pH 6.5) intermediate between the rate of binding to the fast and slow forms of the enzyme, and binding of formate to both is almost undetectable. They are also similar in showing poor reactivity with H2O2, or with CO in the presence of O2, which, with fast oxidase, induce the formation of the ‘ferryl’ and ‘peroxy’ states respectively. However, there is a clear difference in the near-u.v./visible absorption spectra of the two forms; that induced by CO2/HCO3- has a Soret maximum at 427 nm whereas Cl(-)-ligated oxidase has a Soret maximum similar to that of fast oxidase at about 424 nm. It appears that both CO2/HCO3- and Cl- are members of a class of ligands that lowers the reactivity of the binuclear centre but does not impede intramolecular electron transfer from haem a to the binuclear centre, unlike the putative endogenous ligand responsible for slow oxidase.

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