Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 2 / Section A / 4 / Solution

The enzyme adenylate kinase rapidly interconverts the adenine nucleotides:For approximately fixed conditions and near-equilibrium activity ratios,The AMP:ATP ratio responds quadratically to ADP:ATP. Doubling ADP:ATP therefore quadruples AMP:ATP. In working skeletal muscle, ATP is also buffered by creatine kinase and continuing ATP production, so its fractional change is relatively small. The initially smaller AMP pool can undergo a much larger fractional increase, providing a sensitive signal of energetic stress to regulators such as AMP-activated protein kinase. This concerns sensitivity of relative changes; it does not mean that the absolute AMP concentration exceeds ATP.