Remember that at equilibrium the rates of forward and reverse reactions are equal. Therefore, the conversion of, for example, 3‐ phosphoglycerate to glyceraldehyde‐3‐phosphate occurs rapidly. In contrast, the reactions far from equilibrium, such as the conversion of phosphoenol pyruvate to pyruvate, have rates that are greater in the forward than in the reverse direction. Imagine a series of pools in a fountain. If two pools are at the same level, there is no point in putting a dam between them to control the flow of water. On the other hand, the rate of water flow can be controlled effectively at any point where one pool spills into a lower one. Think of the compounds in the free energy diagram as pools—where does a pool spill into a lower one, offering the possbility of control? At three enzyme‐catalyzed reactions:
2.Fructose‐6‐phosphate 
fructose‐1,6‐bisphosphate. Glucose‐6‐phosphate has other metabolic fates than simply leading to pyruvate. For example, it can be used to synthesize ribose for DNA and RNA nucleotides. The most important regulatory step of glycolysis is the phosphofructokinase reaction. Phosphofructokinase is regulated by the energy charge of the cell—that is, the fraction of the adenosine nucleotides of the cell that contain high‐energy bonds. Energy charge is given by the formula:
The energy charge of a cell can vary from about 0.95 to 0.7. ATP inhibits the phosphofructokinase reaction by raising the K m for fructose‐6‐phosphate. AMP activates the reaction. Thus, when energy is required, glycolysis is activated. When energy is plentiful, the reaction is slowed down.
Phosphofructokinase is also activated by fructose‐2,6‐ bisphosphate, which is formed from fructose‐1‐phosphate by a second, separate phosphofructokinase enzyme—phosphofructokinase II (as shown in Figure ). The activity of PFK II is itself regulated by hormone action. Fructose‐2,6‐bisphosphate allosterically activates PFK I by decreasing the K m for fructose‐6‐phosphate.
Finally, phosphofructokinase is inhibited by citrate. Citrate is the TCA cycle intermediate where 2‐carbon units enter the cycle. A large number of compounds—for example, fatty acids and amino acids—can be metabolized to TCA cycle intermediates. High concentrations of citrate indicate a plentiful supply of intermediates for energy production; therefore, high activity of the glycolytic pathway is not required.
3.Phosphoenol pyruvate → pyruvate. The third big step in the free‐energy diagram is the pyruvate‐kinase reaction, where ATP is formed from phosphoenol pyruvate. ATP inhibits pyruvate kinase, similar to the inhibition of PFK. Pyruvate kinase is also inhibited by acetyl‐Coenzyme A, the product of pyruvate metabolism that enters the TCA cycle. Fatty acids also allosterically inhibit pyruvate kinase, serving as an indicator that alternative energy sources are available for the cell.
Pyruvate kinase is also activated by fructose‐1,6‐bisphosphate. Why fructose‐1,6‐bisphosphate? It is an example of feed‐forward activation. This glycolytic intermediate is controlled by its own enzyme system. If glycolysis is activated, then the activity of pyruvate kinase must also be increased in order to allow overall carbon flow through the pathway. Feed‐forward activation ensures that the enzymes act in concert to the overall goal of energy production.
Physiologically, glycolysis produces energy at a high rate but for a short duration. Biopsies of animal muscle indicate two types of tissue; the two types have different metabolic activities. The flight muscles in the breasts of chickens and turkeys, for example, are light, while the leg and other muscles are dark. The color of the dark meat comes from the iron present in the cytochromes involved in oxygen‐consuming respiration. In these tissues, metabolism of glucose is largely aerobic. In contrast, flight muscle (a fast‐white muscle) contains few mitochondria; glucose is broken down largely by glycolysis. Because only two ATP molecules are produced per glucose consumed by glycolysis, a limited amount of energy is available for muscle activity. The muscle acts quickly, but for only a short time. In contrast, mitochondrial oxidation of glucose in slow‐red muscle makes more ATP but the process takes longer. Slow‐red muscle doesn't work as quickly as fast‐white muscle but can be active for a longer period of time.
Athletes' muscle composition reflects their relative sports. An untrained adult male's leg muscle is about half of each type. Sprinters contain more fast‐white muscle, while an elite marathon runner can have as much as 90% slow‐red muscle tissue. It is unclear how much of the difference is due to training and how much to heredity.