Biological life is generally driven by enzymatic reactions. During a reaction, inputs, or substrates, are converted into outputs, or products. In a biological system like a human cell, these reactions could theoretically occur in a spontaneous manner, but they would occur so slowly that the cell would not function properly. Special proteins called enzymes are capable of speeding up reactions, but if there was no way to control their activity, the results would be just as catastrophic for the cell. Feedback inhibition is one of the methods a cell can use to hinder unnecessary enzymatic activity.
In a laboratory setting, many reactions can be performed without enzymes by heating a solution full of substrates, which adds energy to the system and increases the chances of the substrates randomly bumping into each other in the exact positions necessary to produce the desired product. Living cells do not have this option, so they produce enzymes to bring substrates together and facilitate the reaction between the different compounds. Enzymatic reactions still require energy, but not nearly as much as would be needed in the absence of the catalytic enzyme.
There are generally three ways a cell can control the activity of its enzymes. It could control how much enzyme is produced or destroyed, but these methods are not as useful to a cell. The third method, feedback inhibition, can be used to immediately react to cellular conditions. Feedback inhibition occurs when one of the products of a chain of reactions hinders the activity of an enzyme at the beginning or in the middle of the chain. This is a reversible process. When concentration of the inhibiting compound falls, it will dissociate from the enzyme, allowing it to catalyze reactions again.
One of the most critical reaction chains in eukaryotic cells is glycolysis and the citric acid cycle. When glucose enters a cell, a chain of 20 different enzymes plus several protein complexes in the mitochondria work together to transform the glucose into ATP, an energy carrier necessary to drive cellular life. If animal cells could not control this enzyme chain, glucose would be continuously drawn from the bloodstream, lowering blood sugar levels to a dangerous level. Without inhibition, enzyme chains would constantly produce compounds the cell does not currently need. This would be a massive waste of cellular resources.
ATP, the end product of glucose metabolism, is the key feedback inhibitor for the enzyme chain. When the cell has an abundance of free ATP molecules -- meaning it has plenty of energy reserves and does not need to produce more -- the compound binds with several enzymes along the chain, most notably phosphofructokinase and pyruvate kinase. ATP inhibition occurs at critical, irreversible points in the process. Glucose metabolism is therefore put on hold until the cell runs low on ATP, at which point the energy molecules break away from the enzymes, allowing them to continue metabolizing sugar into energy.
