In step 3 of the citric acid cycle, the enzyme isocitrate dehydrogenase is regulated by NADH. Compare and contrast the regulation of this enzyme with the regulation of phosphofructokinase in glycolysis.

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1. Identify the enzymes and pathways involved: Isocitrate dehydrogenase is an enzyme in the citric acid cycle, which is regulated by NADH. Phosphofructokinase is an enzyme in glycolysis, primarily regulated by ATP and AMP.
2. Understand the type of regulation: Isocitrate dehydrogenase is inhibited by high levels of NADH, indicating a feedback inhibition mechanism. This is because NADH is a product of the citric acid cycle, and its accumulation suggests that energy production needs to slow down.
3. Contrast with phosphofructokinase regulation: Phosphofructokinase is inhibited by ATP (high-energy molecule) and activated by AMP (low-energy molecule). This regulation is also a form of feedback inhibition but focuses on the energy status of the cell rather than the concentration of a specific product of the pathway.
4. Discuss the role of allosteric sites: Both enzymes are allosterically regulated, but the molecules that bind to these sites differ. Isocitrate dehydrogenase has allosteric sites for NADH, while phosphofructokinase has allosteric sites for ATP and AMP.
5. Examine the physiological implications: The regulation of these enzymes ensures that energy production is balanced according to the cell's needs. Isocitrate dehydrogenase slows the citric acid cycle during high-energy conditions (high NADH), while phosphofructokinase slows glycolysis during energy-rich conditions (high ATP) and speeds it up during energy-poor conditions (high AMP).