Feedback Inhibition

∞ generated and posted on 2016.12.21 ∞

Mechanism whereby presence of an entity interferes with generation of more of that entity.

A typical scenario for the functioning of feedback inhibition is seen in terms of the output of enzymes or metabolic pathways. Here a certain end product is produced with too little end product resulting in insufficient supplies within the organism or individual cell. Alternatively, producing more end product than is necessary can be wasteful and even directly harmful. Proper organism functioning therefore usually requires that these metabolic products are maintained at somewhat constant levels. The means by which such levels are maintained usually involves a dual functioning of end products both in terms of their metabolic function and as inhibitors of the enzymes responsible for their production. Inhibition typically occurs allosterically, with end products binding to those enzymes found at the start of metabolic pathways, particularly at the points (enzymes) where pathways become dedicated to producing only that product. The result is a shutting down of synthesis when there is too much of something and a starting up again of synthesis when there is too little.

In actuality, across cells or organisms what one sees is a slowing versus an acceleration of production, rather than necessarily the black and white of being completely enzymatically on versus off. This is because inhibition is a function of how many enzymes are bound by inhibitor at a given time, which rarely will be either 100% or 0% but instead lies somewhere in between such that a steady state is established. The result is that end products often are produced by metabolic pathways just as fast as they are utilized by the producing cell or larger organism.

Figure legend: Illustration of the idea of feedback inhibition, or negative feedback/end-point inhibition, using a home thermostat and furnace. Excess heat in the vicinity of the thermostat – above the setpoint which is indicated as a line in the figure – causes the thermostat to turn the furnace off. Absent heat generation there is a steady loss of heat from the house until the temperature falls below the set point. At this point the furnace is turned back on, again to restore the house's temperature up to or slightly beyond the set point. So too the cruise control on a car detects velocity (the car's speed), cutting back on engine power when velocity exceeds the set point, or increasing engine output when velocity is below the set point. In biological systems these processes usually are based on concentrations of small molecules where excess concentrations results in greater levels of binding to proteins whose function is to be inhibited whereas lower concentrations allow those proteins to function.

Feedback inhibition is key to homeostasis and therefore to the proper functioning of cell and organisms. It is the reason that things can stay more or less the same in the functioning of these entities, even as their surrounding environments vary.