Which statement correctly describes the committed, rate-limiting step of de novo purine synthesis?

• A It condenses glycine with phosphoribosylamine and is inhibited by PRPP
• B It combines carbamoyl phosphate with aspartate and is inhibited by UTP
• C It transfers an amino group from aspartate onto SAICAR and is stimulated by GMP alone
• D It combines glutamine with PRPP and is inhibited by AMP and GMP ✓

Explanation

Glutamine-PRPP amidotransferase catalyzes transfer of the amide nitrogen of glutamine to PRPP, forming phosphoribosylamine, the first committed step. The enzyme is allosterically inhibited by the end products AMP and GMP synergistically and is driven forward when PRPP accumulates. Option B describes aspartate transcarbamoylase logic from pyrimidine synthesis, and option A reverses the regulatory direction, since rising PRPP stimulates rather than inhibits this step.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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