Studies on the oligomeric state of isolated cytochrome oxidase using cross-linking reagents

The chloroplast cytochrome b 6 f complex can exist in monomeric and dimeric states

The cytochrome b 6 f complex isolated from spinach chloroplast membranes can be resolved into two forms, a monomeric and a dimeric form, by centrifugation on sucrose gradients. The conversion of the dimeric form of the complex into the monomeric form could be prevented by cross-linking with the homobifunctional reagent, dithiobis(succinimidylpropionate) but not by cross-linking with disuccinimidyltartrate or glutaraldehyde. SDS-PAGE analyses of the monomeric and dimeric forms of the cytochrome complex showed the presence of specific cross-linked products in each respective form of the complex. For example, the monomeric form contained a cross-linked product of cytochrome f, cytochrome b 6 f and subunit IV while the dimeric form contained a cross-linked dimer of cytochrome b 6 f. The presence of the former in the isolated cytochrome b 6 f complex prepared by the method of Hurt and Hauska (Eur J Biochem 117: 591-599, 1981) indicates the presence of the monomer in his preparation.

Purification and characterization of two-subunit cytochrome aa3 from Bacillus cereus

Cytochrome c-oxidase type aa3 (EC 1.9.3.1) was purified to homogeneity from vegetative Bacillus cereus by ion-exchange and hydroxylapatite chromatography in the presence of Triton X-100. Gel filtration analysis suggested a dimeric structure apparently 172 kDa in size; however, only a monomer of 81 kDa was detected when analysed by non-denaturing gel electrophoresis. Denaturing gel electrophoresis analysis of the protein showed the presence of two subunits (51 and 30 kDa). Atomic absorption and visible spectroscopy showed typical aa3 redox centres with haem a iron and copper in a ratio of 22 nmol and 35 ng-atom per mg protein, respectively. No haem c was found associated with the purified enzyme in the conditions reported here. Oxidase activity was fully reconstituted by phospholipids in the presence of N,N,N',N'-tetramethyl-p-phenylenediamine or reduced yeast cytochrome c (but not horse cytochrome c) as electron donors. This activity was abolished by cyanide and carbon monoxide.

Role of supernumerary subunits in mitochondrial cytochrome c oxidase

Role of supernumerary subunits of bovine heart cytochrome c oxidase has been investigated by examining the influence on the enzymatic activity of their removal by chromatographic procedures or controlled digestion by trypsin. Is has been shown that partial proteolytic cleavage of subunit IV results in depression of respiratory activity and of redox-linked proton translocation. Selective removal by gel-filtration of subunit Vlb has no significant influence on the redox and protonmotive activity of the oxidase.

Monomerization of cytochrome oxidase may be essential for the removal of subunit III

1. Incubation of cytochrome oxidase, under conditions used as initial steps in treatment to remove subunit III, causes at least partial monomerization of the enzyme. 2. The extent of removal of subunit III by anion-exchange fast protein liquid chromatography (FPLC) is much increased if the enzyme is fully monomerized before it is applied to the column. 3. Subunit III is incompletely removed by chymotrypsin treatment. A digestion product of subunit III migrating in SDS-PAGE like subunit IV, is detected with specific antibodies. The amount of this product is reduced when monomerization is increased by raising the detergent/protein ratio. 4. The results suggest that monomerization facilitates removal of subunit III and exposes it to further chymotrypsin digestion. We propose that subunit III is at least in part located in the junction between the monomers in the cytochrome oxidase dimer.