In the urea cycle, N-acetylglutamate (NAG) is an obligate allosteric activator of carbamoyl phosphate synthetase I (CPS-I). NAG synthase (NAGS) is activated by arginine. This regulatory circuit serves as a feed-forward signal because:

• A High arginine signals successful urea cycle completion, activating NAGS to increase NAG, which activates CPS-I to amplify urea cycle throughput ✓
• B Arginine is toxic and must be degraded rapidly; high arginine activates its own elimination
• C NAG activates CPS-I to generate ornithine, which then stimulates arginine synthesis
• D N-acetylglutamate functions as a non-essential amino acid precursor for arginine synthesis

Explanation

This is an elegant feed-forward regulatory mechanism. Arginine is a late product of the urea cycle (formed from argininosuccinate). When protein catabolism is high, both ammonia (increasing CPS-I substrate load) and arginine (from increased amino acid catabolism and urea cycle flux) are elevated. High arginine allosterically activates NAGS (N-acetylglutamate synthase), increasing NAG production. NAG is the obligate allosteric activator of CPS-I (the rate-limiting, committed step of the urea cycle). Thus, arginine—a cycle product—signals that the cycle is running at high capacity and should run faster, amplifying urea formation to handle the ammonia load. This is a positive/feed-forward loop: high cycle activity → more arginine → more NAG → more CPS-I activity → greater cycle flux.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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