<P> 1 - Phosphofructokinase is a tetramer of 4 identical subunits that each have a catalytic site . In addition to the catalytic binding sites, there are 4 additional binding sites for allosteric regulation . </P> <P> 1 - phosphofructokinase catalyzes the committed step of glycolysis . This step can be a rate limiting step in glycolysis, and may be regulated to establish the rate of glucose oxidation in the cell . This reaction is a phosphoryl group transfer from ATP to fructose 6 - phosphate, yielding fructose 1, 6 - biphosphate . The reaction is highly exergonic and is irreversible within the cell, though can be bypassed in gluconeogenesis via the enzyme fructose 1, 6 - bisphosphatase . </P> <P> The regulation of 1 - phosphofructokinase occurs primarily by allosteric effectors, and is based on the cell's energy needs . There are two ATP binding sites; a substrate site where the phosphate transfer occurs, and an allosteric site where allosteric regulation occurs . ATP acts as an allosteric inhibitor and when cellular concentrations of ATP are high, and the cell's energy needs are low, the reaction catalyzed by 1 - phosphofructokinase is inhibited . When the cell is actively consuming ATP and stores deplete, the concentration of ATP lowers while the concentrations of AMP and ADP increase . AMP and ADP are both positive effectors of 1 - phosphofructokinase and bind allosterically to activate the reaction . This activation encourages glycolysis and ATP production . Inhibition can also occur via citrate, a product of glycolysis and intermediate in the citric acid cycle . An increased concentration of citrate indicates the cell is meeting current energy needs, and therefore encourages allosteric inhibition of 1 - phosphofructokinase allosterically via ATP . </P> <P> Allosteric regulation of 1 - phosphofructokinase is facilitated hormonally to help the liver to maintain blood glucose levels . This is achieved in part by increasing or decreasing rates of glycolysis . When blood glucose levels are low, the secretion of glucagon leads to the phosphorylation of phosphofructokinase 2 which inhibits formation of fructose 2, 6 - bisphosphate . Therefore, when glucagon inhibits phosphofructokinase 2, cellular levels of fructose 2, 6 - bisphosphate decrease, and reduce activation of 1 - phosphofructokinase, ultimately reducing the rate of glycolysis within the cell . When 1 - phosphofructokinase is inhibited, rates of gluconeogenesis increase, further aiding an increase in blood sugar . When blood sugar is high, however, the secretion of insulin produces the opposite effect by removing the phosphate group from phosphofructokinase 2, which leads to activation, and formation of fructose 2, 6 - bisphosphate . As concentrations of fructose 2, 6 - bisphosphate increase, 1 - phosphofructokinase is allosterically activated, and rates of glycolysis increase, consuming glucose . At the same time the rate of gluconeogenesis decrease, also contributing to lowering blood sugar levels . </P>

How many binding sites for atp are there in pfk