Final answer:
Endurance training typically leads to increased activity of citrate synthase due to higher aerobic metabolism demands, while phosphofructokinase activity might decrease or remain unchanged due to negative feedback by ATP and citrate.
Step-by-step explanation:
If after endurance training, there is an increase in mitochondria size and count, we would expect to see an increase in citrate synthase activity and a potential decrease or no change in phosphofructokinase activity. This is because endurance training enhances aerobic metabolism, leading to an increased demand for ATP production through aerobic pathways, which includes the citric acid cycle where citrate synthase is a key enzyme. Consequently, citrate synthase activity increases to meet the higher demand for aerobic energy production. In contrast, phosphofructokinase, which is a rate-limiting enzyme in glycolysis, is negatively regulated by high levels of ATP and citrate, both of which are indicative of sufficient energy supplies and thus may reduce the need for this pathway to be highly active. Therefore, with more mitochondria functioning efficiently, ATP levels would be high, and thus, phosphofructokinase activity might decrease or remain unaffected.