If any buried hydrogen bond donors do not have an acceptor, they are listed.
Oxygen is only involved at the very last stage of respiration as the final electron acceptor, but without the whole respiratory chain stops.
Methylene blue is an example of an artificial hydrogen acceptor, or redox indicator.
Some microorganisms however obtain their ATP by anaerobic respiration using a chemiosmotic mechanism where oxygen is not the ultimate electron acceptor.
Yz, the primary electron acceptor of the OEC, sits to the left, approximately 3.4 Angstroms away from the chloride.