Question

the formation of the amino acid tryptophan is catalyzed by the enzyme tryptophan synthase. this enzyme is made up of alpha and beta subunits. each type of subunit catalyzes a different step in the tryptophan biosynthesis pathway. the alpha subunit forms an indole molecule from indole-3-glycerol phosphate (igp), and the beta subunit forms tryptophan from this indole. this process is summarized in the following diagram. diagram showing igp to indole via alpha subunits (with g3p as a byproduct) and indole to tryptophan via beta subunits (with ser as a reactant) within the tryptophan synthase protein, the active sites of the alpha and beta subunits are connected by a hydrophobic channel. this channel allows the small, hydrophobic indole molecule to diffuse directly from the active site of an alpha subunit to the active site of a beta subunit. which of the following statements describes a likely effect of the channel on the process carried out by tryptophan synthase? choose 1 answer: a the indole molecule does not diffuse across the cell membrane before it can be used by the beta subunit. b the product of the reaction catalyzed by the alpha subunit can enter an alternative metabolic pathway occurring near tryptophan synthase in the cell. c the energy required to maintain the channel reduces the amount of tryptophan the enzyme can produce. d the beta subunit’s reaction product is transferred efficiently to the alpha subunit, where it serves as a reactant.

Explanation:

The hydrophobic channel directly transports indole from the alpha subunit's active site to the beta subunit's active site. This keeps indole from escaping into the cell's cytoplasm, so it cannot diffuse across the cell membrane before being used by the beta subunit. Option B is incorrect because the channel prevents indole from entering alternative pathways. Option C is incorrect as the channel does not consume energy that would limit tryptophan production. Option D is incorrect because the beta subunit's product (tryptophan) is not a reactant for the alpha subunit.

Answer:

A. The indole molecule does not diffuse across the cell membrane before it can be used by the beta subunit.