Structural Biochemistry/AnabolismLet us make an in-depth study of the metabolism of carbohydrates. The metabolism of carbohydrates is done through two processes: Catabolic Processes and B. The catabolic processes of carbohydrates include: Citric Acid Cycle 3.
Structural Biochemistry/Anabolism - Wikibooks, open books for an open world
Let us make an in-depth study of the metabolism of carbohydrates. The metabolism of carbohydrates is done through two processes: Catabolic Processes and B.
The catabolic processes of carbohydrates include: Citric Acid Cycle 3. The anabolic processes of carbohydrates include: Breakdown of molecules is known as catabolism and synthesis is termed as anabolism. Glycolysis is the breakdown lysis of glucose to pyruvic acid under aerobic conditions and to lactic acid under anaerobic conditions. Anaerobic glycolysis is also termed as Embden-Meyerhof pathway EMP , after the scientists who proposed it.
Glycolysis occurs in the cytosol of the cell and is initiated when the ATP level of the cell is low. In stage one, one molecule of glucose in converted into two molecules of D-glyceraldehydephosphate. The phosphorylation of glucose serves two purposes. First, it makes the glucose molecule more reactive and ready for other reactions.
Second, because phosphorylated compounds cannot pass through the cell membrane, phosphorylation keeps the glucose inside the cell. The six carbons in glucose- 6-phosphate structure need to be rearranged to form fructosephosphate so that it can split into two structures of 3 carbons each. The new compound, fructosephosphate is phosphorylated again so that each of the 2, three carbon units have a phosphate group attached to them. The conversion of fructose- 6-phosphate to fructosedisphosphate via phosphofructokinase is the primary regulation point of glycolysis.
The final step of stage one is the splitting of fructosedisphosphate into 2 molecules of glyceraldehydephosphate. Glyceraldehyde is oxidized, in other words a hydrogen atom is removed from it, and phosphorylated to produce 1,3-diphosphoglycerate. In the next four reactions, four additional ATPs are synthesized two each from both the three carbon compounds , before the final product of glycolysis i.
The three-carbon structure of pyruvate has several fates depending upon the energy state of the cell. It is the main route for glucose metabolism. It occurs in all the cells of the body. Brain and RBC depend only on glucose for oxidation and production of energy. In brain aerobic glycolysis occurs whereas in RBC there is always anaerobic glycolysis due to the absence of mitochondria , leading to the production of lactic acid.
In skeletal muscle aerobic glycolysis occurs in normal conditions but during vigorous muscular contraction, anaerobic glycolysis is the major pathway for energy production. Glycolysis can be initiated via glucose entering the cell from the blood or glucose arising from the breakdown of glycogen.
In human muscle, glycolysis is almost always initiated from the breakdown of glycogen. Since the human brain does not store glycogen, glycolysis is initiated in this tissue from blood glucose. The initiation of glycolysis is regulated by the ATP concentration in the cytoplasm. Erythrocytes metabolize excessive amounts of glucose by the glycolytic pathway.
This generates much ATP which is not required and cannot be used by erythrocyte. Thus if ATP production by substrate phosphorylation is prevented by taking diversion pathway, it will: Hence 1, 3-diphosphoglycerate formed in normal glycolysis is not converted to 3-phosphoglycerate, instead it takes a bypass route through 2,3-diphosphoglycerate, as under—.
Pyruvate is an important regulatory point for energy production. The ultimate fate of pyruvate depends on the energy state of the cell and the degree of oxidative phosphorylation taking place. The pyruvate dehydrogenase complex is one of the most complex proteins in the body and consists of more than 60 subunits. However, if pyruvate is present during the time of high-energy states, such as the liver metabolism of fructose, pyruvate is transformed into acetyl- CoA and is packaged as lipid.
If oxygen to the cell is limiting, such as during intensive exercise, glycolysis proceeds anaerobically and pyruvate is converted to lactate by the lactate dehydrogenase enzyme. Finally, pyruvate can be converted into the amino acid alanine via transamination.
Pyruvate dehydrogenase complex is a multi-enzyme complex made up of 3 enzymes viz: Acetyl-CoA formed in the above reaction may take part either in its oxidation to carbon dioxide and water, through TCA cycle, or formation of lipids, or synthesis of cholesterol etc. Citric acid cycle also known as tricarboxylic acid TCA cycle is named after the scientist Sir Hans Krebs who discovered it. He proposed the key elements of this pathway in and was awarded the Nobel Prize in Medicine for the discovery in This is the first stable tricarboxylic acid in the cycle and hence the name TCA cycle.
A single enzyme, aconitase performs this two-step process. Oxidative decarboxylation takes place in the next reaction. The reaction is catalysed by the enzyme isocitrate dehydrogenase. Succinyl-CoA is a high potential energy molecule. The first step in the conversion is the dehydrogenation of succinate to yield fumarate facilitated by the enzyme succinate dehydrogenase. These reducing equivalents are oxidized through the electron transport chain.
The regulation of the TCA cycle is largely determined by substrate availability and product inhibition. Succinyl-CoA inhibits succinyl-CoA synthase and citrate synthase.
This increases the reaction rate of many of the steps in the cycle, and therefore increases flux throughout the pathway. TCA cycle is the common pathway for the oxidation of carbohydrates, fats and proteins catabolic role.
The anabolic role is synthesis of various carbohydrates, amino acids and fats. As it takes part both in anabolism and catabolism, it is said to be amphibolic pathway of metabolism. It is the replenishment of the depleted intermediates of TCA cycle.
As the TCA cycle takes part in the anabolic reactions, the intermediates of TCA cycle are utilized for the synthesis of various compounds. This results in the deficiency of one or more of the TCA cycle intermediates. In order to continue the TCA cycle, those intermediates, which are deficient, must be filled up by some other process and this process is known as anaplerosis.
For example oxaloacetate is utilized for the synthesis of the amino acid aspartic acid and oxaloacetate is replaced via anaplerosis by carboxylation of pyruvate to oxaloacetate by the enzyme pyruvate carboxykinase.
There are some reactions that take place in the cytosol which produce NADH. NADH is not permeable to the mitochondrial membrane; therefore shuttle systems operate for its transport. Glycogen is a polysaccharide made up of glucose. It is the storage form of glucose in the body.
Glucose requires more water for storage, but glycogen can be stored with much less amount of water hence glucose is stored as glycogen in the cell. The largest amount of glycogen is stored in the liver and muscle. Liver glycogen provides glucose to other cells and maintains the blood glucose level in normal amounts.
Muscle glycogen serves as readily available source of glucose during vigorous exercise, for glycolysis in the muscle itself. Glycogen phosphorylase is the key enzyme of glycogenolysis.
Glycogen metabolism is reciprocally regulated, mainly by the action of hormones. At the time of shock and excitement, epinephrine stimulates glycogenolysis, both in muscle and liver, whereas glucagon stimulates glycogenolysis only in the liver under hypoglycemic conditions.
Insulin inhibits glycogenolysis and promotes glycogenesis. Glycogen storage diseases are a group of inherited disorders characterized by deficient mobilization of glycogen and deposition of abnormal forms of glycogen. Hexose monophosphate shunt pathway or the HMP pathway is an alternative pathway for glucose oxidation. It neither utilizes nor produces ATP. The organs in which HMP pathway occurs are those which are actively concerned with lipid synthesis, like the adipose tissue, kidney, lactating mammary gland, liver, RBC, thyroid and gonads.
It takes place in the cytosol. Transfer of 2-carbon moiety i. It is catalysed by the enzyme transketolase and the coenzyme is Thaimine pyrophosphate TPP. In thiamine deficiency also in pernicious anemia transketolase activity is decreased in blood. Transfer of 3-carbon moiety i. It is catalysed by the enzyme transaldolase. It produces glucuronic acid which takes part in detoxification of bile pigments, phenols, aromatic acids and steroid hormones. In the diet fructose is obtained from fruits, honey and table sugar sucrose.
In human body it is the sugar of the semen and amniotic fluid. It is a genetic defect in which there is excretion of fructose in the urine due to the lack of the enzyme fructokinase.
A person shows disliking towards fruits and fructose rich diets due to the deficiency of the enzyme aldolase-B. In the diet, galactose is mainly derived from the milk sugar lactose. In the body it is converted to glycogen or may take part in the synthesis of the milk sugar lactose in lactating mammary gland. This is due to the deficiency of the enzyme galactosephosphate uridyl transferase, which results in the accumulation of galactose in the blood i.
Such infants are intolerant to lactose and hence to milk. They show symptoms like diarrhoea and vomiting on giving milk.
Lactose free milk is the only remedy. Synthesis of glycogen from glucose is known as glycogenesis. At this time another enzyme i. The process of addition of glucose by glycogen synthase to the linear chain and branching enzyme creating the branching points is repeated and thus glycogenesis is completed.
Gluconeogenesis is the formation of glucose from non-carbohydrate sources.