Biochemistry—Multienzyme Complexes Tryptophan synthase of Escherichia coli is a classic bifunctional oligomeric enzyme. What is its subunit composition?
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AA single protein designated A only
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BTwo proteins designated A and B forming a functional complex
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COne protein A plus an unrelated one-subunit “a” fragment
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DA single protein designated B only
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EThree distinct proteins designated A, B, and C
Answer
Correct Answer: Two proteins designated A and B forming a functional complex
Explanation
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:
Step 1: Recognize “bifunctional” implies two catalytic activities housed in distinct subunits.Step 2: Recall the established α2β2 architecture in many organisms; the essential point is the presence of A and B subunits.Step 3: Select the option that explicitly states “two proteins designated A and B.”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