Which amino acid enzyme reacts with tyrosinase to form natural melanin in hair?

The process of melanin formation in hair is primarily dependent on the amino acid tyrosine. Tyrosinase is the key enzyme that catalyzes the conversion of tyrosine into various forms of melanin, which is responsible for the color of hair, skin, and eyes.

Tyrosine itself is derived from phenylalanine through the action of the enzyme phenylalanine hydroxylase, but it is tyrosine that directly interacts with tyrosinase to initiate the series of reactions that ultimately produce melanin. This makes tyrosine a critical precursor in the melanin synthesis pathway, confirming that its reaction with tyrosinase is fundamental in the formation of natural pigmentation in hair.

The other amino acids listed, such as phenylalanine, tryptophan, and cysteine, do not have a direct role in the enzymatic reaction with tyrosinase to form melanin. Thus, recognizing the specific function of tyrosine in this biochemical pathway helps clarify why it is the correct answer.