Cytochrome oxidase in Anacystis nidulans: stoichiometries and possible functions in the cytoplasmic and thylakoid membranes

Abstract

Three biochemically distinct membrane fractions were obtained from the cyanobacterium Anacystsis nidulans grown under salt-stressed conditions (0.4 M NaCl). These were classed as ICM (thylakoid), CM-1 (light cell membrane) and CM-II (heavy cell membrane). All three fractions were tested for cytochrome c oxidase activity. Two of the three, ICM and CM-II, exhibit such activity towards horse heart cytochrome c in both spectrophotometric and polarographic assay systems. In these two fractions, the oxidase, immunologically identical with other cytochrome aa 3 oxidases, was present at a much lower concentration than that of other cytochromes. ICM contain cytochrome b 6 f approximately equimolar with the photosystems (0.25μmol (g protein) −1 ); [cyt. aa 3 ] is between 2 and 5% [ b 6 f ]. CM-I has 1.5 μmol c -type cytochrome (g protein) −1 and a small amount of b -type cytochrome; [cyt. aa 3 ] is c ]. CM-II has 2.0 μmol c -type cytochrome (g protein) −1 and some b -type including a small b 6 f component; its [cyt. aa 3 ] is between 1 and 2% [cyt c ]. ICM oxidize cyanobacterial cytochrome c -554 ( c 6 ) in a reaction with the kinetic characteristics of an aa 3 -type oxidase. The donor for CM-II aa 3 also appears to be a (periplasmic) cytochrome c 6 . Small amounts of cytochrome aa 3 in ICM may be associated with a need to control the level of available reductant in a cyclic pathway. Slow reductant-level control could also be required in the heavy plasma membrane fraction (CM-II); the functions of extra c -type cytochromes in this membrane and in the oxidase-free CM-I fraction are at present unknown.