Probably a soluble electron acceptor for the integral membrane protein electron transfer alternative complex III (ACIII). Binds 1 copper ion per subunit. E(0) is +190 mV at pH 9, +215 mV at pH 7 and +240 mV at pH 4. Possibly located on the extracellular face of the cell membrane. Present in both photosynthetically (anaerobically) and dark (aerobic respiration) grown cells (at protein level) (PubMed:19190939). A later paper showed this protein to be present in both chemoheterotrophically (dark) and photoheterotrophically (light) grown cells, but with more protein present in light grown cells (PubMed:22249883). The second report is thought to be correct (PubMed:23357331). Glycosylated. Belongs to the multicopper oxidase family.