Which enzyme detects high levels of ADP to trigger ATP resynthesis in the ATP-PC system?

• A. Hexokinase
• B. Creatine Kinase
• C. Phosphofructokinase
• D. Lactate Dehydrogenase

Answer: (B)

The main idea is how ATP is rapidly regenerated in the ATP-PC system when energy demand is high. This quick resynthesis is driven by the enzyme creatine kinase, which transfers the phosphate from phosphocreatine to ADP to form ATP and creatine. When ADP levels rise, the reaction is pushed forward, so creatine kinase acts as a detector and responder to the energy deficit, instantly restoring ATP for muscle contraction. The other enzymes listed aren’t involved in this immediate phosphagen system: hexokinase starts glycolysis by using ATP to phosphorylate glucose; phosphofructokinase is a rate-limiting glycolysis enzyme that governs carbohydrate breakdown rather than directly resynthesizing ATP in the phosphagen system; lactate dehydrogenase handles the conversion of pyruvate to lactate in anaerobic glycolysis.