Catabolism of glucose is most common metabolic pathway in cells. Animals are consumed fats and protein which may be used to harvest energy.
Metabolism of Fats:
Fats are built from long chain fatty acids and glycerols are triglycerides. Initial catabolism of fat begins with the digestion of triglycerides by way of an enzyme called lipase to glycerol and three fatty acid molecules.
Glycerol is phosphorylated and can enter glycolytic pathway at the level of glycerol dehydes 3 – phosphate free fatty acids move into mitochondrion where their carbons are removed, two at a time to form acetyl coenzyme A plus additional NADH and FADH2. Acetyl coenzyme A is then oxidized by Krebs’ cycle and NADH and FADH2 that are produced are oxidized via electron transport chain. One gram of fat provides about 2.5 times more ATP energy than does either 1g of carbohydrate of protein because the number of hydrogen atoms per unit weight of fat is greater than in carbohydrates of protein. This is why many animals store energy in the form of fat in adipose tissue.
Metabolism of Proteins:
Animals initially digest proteins to yield individual amino acids. Some of these are distributed throughout the body and used to synthesize new proteins. Other amino acids are transported in the blood or extra cellular fluid and comprise the amino acid pool. If needed for fuel these amino acids can be further degraded by removal of amino group to yield ammonia. This process is called deamination reaction.
R – OH – COOH + H2O -------> R – C – COOH + NH2 + H2
| | NH2 |
In deamination, an oxygen atom replaces an amino group to form keto acid. Keto acid can then enter into kreb’s cycle. Finally the carbon skeleton of amino acid is dismantled and oxidized to CO2. Ammonia produced from complete catabolism of amino acid is highly toxic and must be excreted. On the average one gram of protein yields about same amount of energy that is 4 K. Cal as does 1g of glucose.
Control of Metabolism:
Cells are efficient and do not waste energy by making surplus substances they do not need or require in lesser amount. If certain amino acid is over abundant in amino acid pool, the anaerobic pathway that synthesizes that amino acid from an intermediate in kreb’s cycle is turned off. Most common mechanism for this control uses and product (feed back) inhibition. In end product inhibition, the end product of anaerobic pathway inhibits the enzyme that catalyzes key step in the pathway.
Controlling the catabolic activities ca control the activities of cell and the organism. Supposing if a muscle cell is working very hard and its ATP concentration begins to decrease, aerobic respiration increases. When ATP is sufficient to meet demand, aerobic respiration slows, sparing valuable organic molecules for other necessary functions. As with anabolism, control is based on regulating enzyme activity at strategic points in the catabolic pathway. As a result cells are thrifty, expedient and responsive in their metabolism.
Control of Glycolysis through Phosphofructokinase:
During glycolysis, fructose – 6 – phosphate is converted to fructose diphosphate by enzyme phosphofructokinase. It is sensitive to energy needs of cell and the ratio of ATP to ADP or AMP. It has receptor sites for specific inhibitors and activators.
Metabolic Pool:
Catabolic chemical reaction of glycolysis and Krebs cycle not only provide ATP but also make available metabolic pool of material that can be consumed for the synthesis or anabolism reactions of many important cellular components. The balance between catabolism and anabolism maintains homeostasis in the cell as well as the whole animal. The chemical reaction that takes place in the body may be divided into two categories on the basis components can gain lateral entry or not in main system.
(1) Open system: The system of metabolic reaction in which a number of reactants from different sources can enter and participate in the system can be further processed is known as open system. Open system has two way flow of material into and out of it. Various compounds enter the pathways at different points so that carbohydrates, fats and proteins can all be oxidized. At the same time some of the intermediates of these pathways can be withdrawn from the energy harvesting machinery and used in synthesis reactions. Glycolysis and Krebs cycle are examples of open system and the products of glycolysis and Krebs cycle are all part of metabolic pool whereby material is added withdrawn.
(2) Closed system: In some of the systems the chemical reactions take place completely in the closed environment and no reactant or substance can enter into the system till the final metabolic product is obtained, such a system is known as closed system.
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