Biochemistry—Multienzyme Complexes Tryptophan synthase of Escherichia coli is a classic bifunctional oligomeric enzyme. What is its subunit composition?

Introduction / Context:
Many biosynthetic pathways use multienzyme complexes that channel intermediates between active sites. Tryptophan synthase is a well-studied example that couples two sequential reactions to produce tryptophan from indole-3-glycerol phosphate and serine, minimizing diffusion losses of the reactive intermediate indole.

Given Data / Assumptions:

• Tryptophan synthase is described as bifunctional and oligomeric.
• The canonical E. coli enzyme is known to have distinct subunits often labeled A (α) and B (β).
• Subunits assemble into a higher-order complex for efficient catalysis.

Concept / Approach:
Identify the minimal accurate description: two different subunits (A and B) associate to form the complete enzyme. This composition underlies substrate channeling: the α-subunit generates indole, which is directly delivered to the β-subunit active site for condensation with serine to form tryptophan.

Step-by-Step Solution:

Verification / Alternative check:
Structural and kinetic studies show inter-subunit tunnels guiding indole from A to B, consistent with the AB composition.

Why Other Options Are Wrong:

• Only A or only B: would delete half the catalytic function.
• “A plus one-subunit a”: nonstandard and incorrect nomenclature.
• Three proteins A, B, C: not supported for the classic bacterial enzyme.

Common Pitfalls:
Confusing the two-subunit composition with other multienzyme assemblies that include additional regulatory subunits; conflating stoichiometry details with the essential identity of A and B.

Final Answer:
Two proteins designated A and B forming a functional complex