Cell Energetics
Catabolite Repression This type of control is frequently observed when organisms are grown on glucose or some other rapidly metabolizable energy source. The catabolite repression results in a repression of synthesis of enzymes that would metabolize the added substrate less rapidly than glucose, this is oftentimes referred to as the glucose effect. When the lac system is induced, the rate of synthesis of beta-galactosidase is considerably reduced in cultures growing upon glucose, compared with cells for which some other metabolite is provided as the carbon source. The addition of cAMP to cultures overcomes glucose repression by stimulating transcription of the inducible enzyme, beta-galactosidase. The level of cAMP in the cell varies with conditions of growth and reflects the energetic needs of the cell. The level is low when the available energy exceeds the biosynthetic requirement for energy and the level of cAMP rises when the carbon supply of the organisms is depleted. The molecular aspects of catabolite repression are presented in.
Metabolite Regulation Pasteur Effect In facultative organisms as observed under the microscopes, the fermentative capacity of the cell is blocked in the presence of oxygen, and the energy is supplied almost exclusively by respiration. As a result, less glucose is consumed, and the accumulation of lactate is decreased. This phenomenon, first recognized by Pasteur in fermenting yeast, is known as the Pasteur Effect. The benefits of this effect are obvious in terms of the energy gain realized in switching from an anaerobic metabolism to an aerobic one. Anaerobic glycolysis releases only about eight percent of the energy that is obtained from the complete breakdown of glucose. Therefore, if oxygen is available and the glucose is oxidized to carbon dioxide and water without the accumulation of lactic acid, the energy needs of the cell can be met by the utilization of less glucose.
A number of factors may be responsible for the Pasteur effect, but the major determinant is the key enzyme phosphofructokinase that plays a central role in the regulation of glycolysis. In the generation of ATP via the respiratory pathways, increased levels of ATP relative to ADP inhibit phosphofructokinase, thereby decreasing the flow of glucose into the glycolysis pathway.
In addition to ATP, other products of respiration can modulate glycolytic activity. The need for additional controls stems from the dual function of glycolysis as an amphibolic pathway. The glycolytic rate must respond to the need for supplying synthetic intermediates as well as the need for regenerating ATP.
Phosphofructokinases from various bacterial species exhibit many similar, though not identical, regulatory properties.
Localization of Enzymic Activities In eucaryotic organisms as monitored and studied under the microscopes, the mitochondrion and the chloroplast contain the units that transform oxidative energy into the bond energy of ATP. No mitochondria are found in bacteria, but a functional equivalent is present. In such cells, either the cell membrane itself or extensions of the membrane contain the subunits that earn out energy transductions. Although bacteria have essentially a single membrane system, this system is a composite of almost all the membrane systems found in the more complex forms of life. Since the small size of the bacterial cell and the nature of the open membrane system of which the respiratory apparatus is an integral part, there is in bacteria direct interaction between the membranes carrying the respiratory chain and the cytoplasm, which is the source of coenzymes, substrates, and ADP. The metabolites that accumulate in the cytoplasm as a result of glycolysis are rapidly oxidized by the enzymes in the membranes.