Final answer:
The compound that binds to heme a3 in complex 4 is oxygen, which is held between iron and copper ions until fully reduced and combined with hydrogen ions to form water.
Step-by-step explanation:
The compounds that bind to heme a3 in complex 4 are oxygen molecules. Complex 4, also known as cytochrome c oxidase, is composed of cytochrome proteins including a and a3. This complex is crucial in the electron transport chain, where it tightly holds an oxygen molecule between the iron (Fe) and copper (Cu) ions until the oxygen is fully reduced. The fully reduced oxygen then combines with hydrogen ions (protons) to form water (H₂O).