In addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intra cellular enzymes. Thus, enzyme regulation represents a second category of mechanisms by which cellular bio chemical functions can be controlled.
Enzyme Inhibition. Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used.
Enzyme inhibition is another example of negative feedback control; it is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances.
Enzyme Activation. Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP; the presence of this cAMP, in turn, immediately activates the glycogen splitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration.
Another interesting instance of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for formation of DNA and RNA. When purines are formed, they inhibit the enzymes that are required for formation of additional purines. However, they activate the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-feed between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times.
Summary. There are two principal mechanisms by which cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation and (2) enzyme regulation. The genes can be either activated or inhibited, and likewise, the enzyme systems can be either activated or inhibited. These regulatory mechanisms most often function as feedback control systems that continually monitor the cell’s biochemical composition and make corrections as needed. However, on occasion, substances from without the cell (especially some of the hormones discussed throughout this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems.
