Final answer:
The amino acid residue that provides the fourth electron to cytochrome c oxidase is tyrosine, due to its ability to stabilize a radical through its phenolic hydroxyl group and resonance.
Step-by-step explanation:
The student has asked which amino acid residue is involved in providing a fourth electron to cytochrome c oxidase, leading to the formation of a transient radical.
Among the choices provided, the amino acid tyrosine has the capability to form a radical due to its ability to stabilize the unpaired electron through resonance within its aromatic ring structure. This property is not present in phenylalanine, threonine, tryptophan, or serine.
Furthermore, tyrosine is well known for its role in electron transfer processes in enzymes due to the phenolic hydroxyl group it possesses.
This is related to the protein phosphorylation process where amino acids like serine, threonine, and tyrosine can be phosphorylated due to the presence of a hydroxyl group in their side chains, which is necessary for the attachment of a phosphate group. The phosphate group addition alters the enzyme activity or signaling pathways in cells.