The irreversible phosphorylation reaction catalyzed by phospho - fructokinase-1 ( PFK-1 ) is the most important control point and the rate-limiting and committed step of glyco l ysis (Figure 8.16). PFK-1 is controlled by the available concentrations of the substrates ATP and fructose 6- phosphate, and by regulatory substances described below. 1. Regulation by energy levels within the cell: PFK-1 is inhibited allosterically by elevated levels of ATP, which act as an “energy- rich” signal indicating an abundance of high-energy compounds. Elevated levels of citrate, an intermediate in the TCA cycle (see p. 109), also inhibit PFK-1 . Conversely, PFK-1 is activated alloster- ically by high concentrations of AMP, which signal that the cell’s energy stores are depleted. [Note: Citrate inhibition favors the use of glucose for glycogen synthesis, see p.125.] 2. Regulation by fructose 2,6-bisphosphate: Fructose 2,6-bisphos- phate is the most potent activator of PFK-1 (see Figure 8.16), and is able to activate the enzyme even when ATP levels are high. Fructose 2,6-bisphosphate is formed by phosphofructokinase-2 ( PFK-2 ), an enzyme different than PFK-1 . PFK-2 is a bifunctional protein that has both the kinase activity that produces fructose 2,6-bisphosphate and a phosphatase activity that dephosphory- lates fructose 2,6-bisphosphate back to fructose 6-phosphate. In liver, the kinase domain is active if dephosphorylated and is inac- tive if phosphorylated (Figure 8.17). [Note: Fructose 2,6-bisphos