Final answer:
The sixth coordination site in deoxyhemoglobin and deoxymyoglobin is available for oxygen. While oxygen primarily binds to these proteins to facilitate its transport, other molecules like CO can also bind, albeit with potentially harmful effects.
Step-by-step explanation:
In deoxyhemoglobin and deoxymyoglobin, the sixth coordination site is indeed unoccupied and is available for oxygen to bind. Oxygen is the molecule that has the primary role in binding to these sites to transport oxygen throughout the body. The binding of oxygen to myoglobin and hemoglobin is crucial for efficient oxygen transport and release where it's needed in the body.
However, it is also important to note that other small diatomic molecules like carbon monoxide (CO) and nitric oxide (NO) can bind to these proteins, often with a higher affinity than oxygen. This can lead to issues like carbon monoxide poisoning, where CO competes with oxygen for binding sites on hemoglobin, preventing adequate oxygen delivery to tissues.