Fluorescence Indicates Substrate Binding

Steps:

• Enzyme fluoresces due to phosphate group; structural changes alter intensity.
• Serine addition increases fluorescence, indicating serine binds to enzyme, forming enzyme-serine complex (supports 2).
• Indole addition with serine decreases fluorescence, indicating indole binds to enzyme-serine complex, forming enzyme-serine-indole complex (supports 3).
• Reaction (serine + indole → tryptophan) and fluorescence changes confirm both substrates attach at active site (supports 1).

Why A is correct:

• All observations align with enzyme kinetics: binary complex enhances fluorescence, ternary complex quenches it, confirming active site binding.

Why the others are wrong:

• B omits 3; decrease with indole shows ternary complex formation.
• C omits 2; increase with serine shows enzyme-serine complex.
• D omits 1; substrates' role in reaction ties binding to active site.

Final answer: A