Cells are very efficient. They do not waste. energy and do not make substance which they do not need. Following process keeps the metabolism of different compounds under control:
Control of anabolism
Sometimes, an amino acid is present in larger amount. The anabolic pathway synthesizes amino acid from an intermediate in the Krebs cycle. This pathway is turned off.
It is the most common mechanism for this control: It uses end-product (feedback) inhibition. The end product of the anabolic pathway inhibits the enzyme. This enzyme catalyzes a key step in the pathway.
Control of catabolism
Cells can also control their catabolism. For example, a cell like a muscle cell is working very hard. Therefore, its ATP concentration decreases. So aerobic respiration increases. When ATP is sufficient to meet demand, aerobic respiration slows down. It saves organic molecules for other necessary functions The control is based on regulating enzyme activity. This activity is present at special points in the catabolic pathway. Therefore, the metabolism can be controlled at these points.
Control of metabolism by phosphofructokinase:
One of the main controlling points in aerobic respiration is the enzyme phosphofructokinase. The speed of functioning of this enzyme is controlled. So a cell can speed or slow the entire metabolic process. Phosphofructokinase has specific receptor sites for specific inhibitors and activators.
ATP inhibits it
ADP or AMP stimulates it.
Phosphofructokinase is sensitive to the energy needs of the cell and the ratio of *TP to ADP or AMP.
(i) If ATP begins to accumulate, this enzyme shuts down glycolysis.
(ii) If ADP or AMP begins to accumulate, phosphofructokinase becomes active and turns on the glycolytic pathway.
(iii) Citric acid in the cytoplasm also inhibits phosphofructokinase.
Thi control pathway synchronizes the rates of glycolysis and the Krebs cycle. For example, when citric acid begins to accumulate, glycolysis slows down. It red ces the supply of acetyl-coeniyme A to the Krebs cycle. On the other hand, if •itric acid consumption increases, glycolysis accelerates and meets the de and for more acetyl-coenzyme.