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, +205 mV at pH 7 and +240 mV at pH 4. Monomer. Possibly located on the extracellular face of the cell membrane. Present in photosynthetically (anaerobically) but not dark (aerobic respiration) grown cells (at protein level). Apparently unprocessed protein is also present, its quantities increase with culture age (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 dark grown cells (PubMed:22249883). The second report is thought to be correct (PubMed:23357331). Belongs to the multicopper oxidase family.