Biosynthesis and Degradation

Metabolic pathways for biosynthesis and degradation (anabolism and catabolism) are not simple reversals of each other. Though there may be individual reactions that are reversals of each other in the paired catabolic/anabolic pathways (such as there are in glycolysis and gluconeogenesis), there are at least some enzymes that are different, some regulation systems that are opposed to each other, and some metabolites that vary between the two. Pathways may even occur in separate cellular compartments. For example, fatty acid synthesis takes place in cytosol, whereas fatty acid oxidation takes place in mitochondria.

The existence of separate pathways is important for two reasons. First, to proceed in a particular direction, a pathway must be exergonic in that direction. If a pathway is strongly exergonic overall, then simple reversal of that pathway is just as strongly endergonic under the same conditions.

Second, and equally important, is the need to control the flow of metabolites in relation to the bioenergetic status of a cell. When ATP levels are high, there is less need for carbon to be oxidized in the citric acid cycle. At such times the cell can store carbon as fats and carbohydrates, so fatty acid synthesis, gluconeogenesis, and related pathways come into play. When ATP levels are low, the cell must mobilize stored carbon to generate substrates for the citric acid cycle, so carbohydrate and fat breakdown must occur. Using separate pathways for the biosynthetic and degradative processes is crucial for control, so conditions that activate one pathway tend to inhibit the opposite pathway and vice versa.

Biosynthetic pathways require energy to operate and degradative pathways yield less energy than biosynthetic pathways require. Thus, if both pathways are running at the same time and place, they will go in circles, producing no useful metabolites while consuming energy. This is referred to as a futile cycle. Instead, cells have control systems to activate one member of a catabolic/anabolic pair, while inhibiting the other. Thus, futile cycles are usually avoided.

See also: Central Pathways of Energy Metabolism, Metabolic Control Mechanisms