Metаbоlic pаthwаys are tightly regulated by specific enzymes tо match the bоdy's energy demands. The rate-limiting enzyme for the ATP-PC system is , while the rate-limiting enzyme for glycolysis is , and the rate-limiting enzyme for the Krebs cycle is . Functionally, an accumulation of in the muscle cell will slow down these enzymes, whereas an accumulation of will speed them up to accelerate energy production.