Metabolism of Carbohydrates PDF

MODULE 3.1 METABOLISM OF CARBOHYDRATES Carbohydrate metabolism is a vital biochemical process that plays a central role in energy production and homeostasis within living organisms. Carbohydrates, including sugars and starches, serve as the primary source of fue...

MODULE 3.1 METABOLISM OF CARBOHYDRATES Carbohydrate metabolism is a vital biochemical process that plays a central role in energy production and homeostasis within living organisms. Carbohydrates, including sugars and starches, serve as the primary source of fuel for cells, particularly in the form of glucose. This complex metabolic pathway involves the breakdown of carbohydrates into simpler molecules through processes like glycolysis, followed by the subsequent utilization of these molecules for energy production or their conversion into storage forms such as glycogen or fat. Additionally, carbohydrate metabolism is intricately linked with the regulation of blood sugar levels and is essential for maintaining the body's energy balance and overall health. DIGESTION AND ABSORPTION OF CARBOHYDRATES Digestion is the biochemical process by which food molecules, through hydrolysis, are broken down into simpler chemical units that can be used by cells for the metabolic needs. Digestion is the first stage in the processing of food products 1. Mouth - enzyme salivary 𝛼-amylase catalyzes the hydrolysis of 𝛼-glycosidic linkages in starch from plants and glycogen from meats to produce smaller polysaccharides and the disaccharide maltose - only a small amount of carbohydrate digestion occurs in the mouth because food is swallowed so quickly 2. Stomach - although the food mass remains longer in the stomach, very little further carbohydrate digestion occurs there either, because salivary 𝛼-amylase is inactivated in the acidic environment of the stomach - do not contain any carbohydrate-digesting enzymes 3. Small Intestine - primary site of carbohydrate digestion - pancreatic 𝛼-amylase breaks down breaks down polysaccharides to maltose (disaccharide) and glucose 4. Outer membranes of Intestinal Mucosal Cells - final step of carbohydrate digestion - enzymes convert disaccharides to monosaccharides  5. Absorption - absorbed in the bloodstream through the intestinal wall - intestinal walls are lined with highly vascularized villi - active transport though protein carriers on the cell membrane known as Glucose Transporters 6. Liver - fructose and galactose are rapidly converted to compounds that are metabolized by the same pathway as glucose FATES OF GLUCOSE Because glucose is the body's preferred source for synthesizing ATP, its use depends on the needs of body cells, which include the following: 1. ATP production. In body cells that require immediate energy, glucose is oxidized to produce ATP. Glucose not needed for immediate ATP production can enter one of several other metabolic pathways. 2. Amino acid synthesis. Cells throughout the body can use glucose to form several amino acids, which then can be incorporated into proteins. 3. Glycogen synthesis. Hepatocytes and muscle fibers can perform glycogenesis, in which hundreds of glucose monomers are combined to form the polysaccharide glycogen. Total storage capacity of glycogen is about 125g in the liver and 375g in skeletal muscles. 4. Triglyceride synthesis. When the glycogen storage areas are filled up, hepatocytes can transform the glucose to glycerol and fatty acids that can be used for lipogenesis, the synthesis of triglycerides. Triglycerides then are deposited in adipose tissue, which has virtually unlimited storage capacity. GLUCOSE CATABOLISM The oxidation of glucose to produce ATP is also known as cellular/aerobic respiration, and it involves four sets of reactions: glycolysis, the formation of acetyl coenzyme A, the Krebs cycle, and the electron transport chain. 1. Glycolysis is a set of reactions in which one glucose molecule is oxidized and two molecules of pyruvic acid are produced. The reactions also produce two molecules of ATP and two energy-containing NADH + H. Because glycolysis does not require oxygen, it is a way to produce ATP anaerobically (without oxygen) and is known as anaerobic cellular respiration. 2. Formation of acetyl coenzyme A is a transition step that prepares pyruvic acid for entrance into the Krebs cycle. This step also produces energy- containing NADH + H' plus carbon dioxide (CO). 3. Krebs cycle reactions oxidize acetyl coenzyme A and produce CO2, ATP energy-containing NADH + H', and FADH 4. Electron transport chain reactions oxidize NADH + H' and FADH, and transfer their electrons through a series of electron carriers. The Krebs cycle and the electron transport chain both require oxygen to produce ATP and are collectively known as aerobic cellular respiration. METABOLIC PATHWAY OF GLUCOSE Glycolysis Glycolysis is the metabolic pathway by which glucose (a C6 molecule) is converted into two molecules of pyruvate (a C3 molecule), chemical energy in the form of ATP is produced, and NADH – reduced coenzymes are produced. It is a linear rather than cyclic pathway that functions in almost all cells. Stages of Glycolysis A. Six-Carbon Stage of Glycolysis - energy consuming stage - intermediates: glucose and fructose derivatives attached to phosphate groups 1. Phosphorylation Using ATP - formation of Glucose 6-Phosphate - enzyme hexokinase, phosphorylates (adds a phosphate group to) glucose in the cell's cytoplasm - requires energy - provides a way of trapping glucose within the cells 2. Isomerization - formation of Fructose 6-Phosphate - Glucose 6-Phosphate is isomerized to fructose 6-phosphate by the enzyme, phosphoglucoisomerase 3. Phosphorylation using ATP - formation of Fructose 1,6-Biphosphate - enzyme phosphofructokinase, uses another ATP molecule to transfer a phosphate group to fructose 6-phosphate to form fructose 1, 6-bisphosphate B. Three-Carbon Stage of Glycolysis - energy-generating stage - intermediates: phosphorylated derivatives of dihydroxyacetone, glyceraldehyde, or pyruvate (derivatives of either glycerol or acetone) 4. Cleavage - formation of two triose phosphates: dihydroxyacetone phosphate and glyceraldehyde phosphate - since Fructose 1,6-Biphosphate is unsymmetrical the two triose are not identical - enzyme aldose, splits fructose 1, 6-bisphosphate into two sugars that are isomers of each other 5. Isomerization - formation of Glyceraldehyde-3-Phosphate - only Glyceraldehyde-3-Phosphate is a glycolysis intermediated, thus, dihydroxyacetone will be converted to Glyceraldehyde-3-Phosphate - reaction is catalyzed by the enzyme triosephosphate isomerase 6. Oxidation and Phosphorylation using P i - formation of 1,3-Bisphosphoglycerates - enzyme glyceraldehyde 3-phospahte dehydrogenase, attaches a phsopahate group to Glyceraldehyde-3-Phosphate to 1,3-Bisphosphoglycerates - the hydrogen of the aldehyde group becomes part of the NAD+ to form cytosolic NADH 7. Phosphorylation of ADP - formation of 3-Phosphoglycerate - enzyme phosphoglycerokinase, transfers a P from 1,3-bisphosphoglycerate to a molecule of ADP to form ATP 8. Isomerization - formation of 2-phosphoglycerate - enzyme phosphoglyceromutase, relocates the P from 3-phosphoglycerate from the third carbon to the second carbon to form 2-phosphoglycerate 9. Dehydration - formation of Phosphoenolpyruvate - enzyme enolase, removes a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvate (PEP) 10. Phosphorylation of ADP - formation of Pyruvate - enzyme pyruvate kinase, transfers a P from PEP to ADP to form pyruvate and ATP ATP molecules are involved in Steps 1, 3, 7, and 10 of glycolysis. Considering these steps collectively shows that there is a net gain of two ATP molecules for every glucose molecule converted into two pyruvates. Though useful, this is a small amount of ATP compared to that generated in oxidative phosphorylation. The net overall equation for the process of glycolysis is: Entry of Galactose and Fructose into Glycolysis The breakdown products of carbohydrate digestion are glucose, galactose, and fructose. Both the galactose and fructose are converted, in the liver, to intermediates that enter into the glycolysis pathway. The entry of galactose into the glycolytic pathway begins with its conversion to glucose 1-phosphate (a four-step sequence), which is then converted to glucose 6- phosphate, a glycolysis intermediate.  The entry of fructose into the glycolytic pathway involves phosphorylation by ATP to produce fructose 1-phosphate, which is then split into two trioses- glyceraldehyde and dihydroxyacetone phosphate. Dihydroxyacetone phosphate enters glycolysis directly; glyceraldehyde must be phosphorylated by ATP to glyceraldehyde 3-phosphate before it enters the pathway. FATES OF PYRUVATE The production of pyruvate from glucose (glycolysis) occurs in a similar manner in most cells. In contrast, the fate of the pyruvate so produced varies with cellular conditions and the nature of the organism. Three common fates for pyruvate, all of importance, exist. They are conversion to acetyl CoA, conversion to lactate, and conversion to ethanol. Acetyl CoA formation requires aerobic (oxygen-rich) conditions, lactate and ethanol formation occur under anaerobic (oxygen-deficient) conditions, and ethanol formation is limited to some microorganisms.  A key concept in considering these fates of pyruvate is the need for a continuous supply of NAD+ for glycolysis. As glucose is oxidized to pyruvate in glycolysis, NAD+ is reduced to NADH. Oxidation to Acetyl CoA - under aerobic respiration - pyruvate formed in the cytosol trough glycolysis crosses the two mitochondrial membranes and enters the mitochondrial matrix, where oxidation takes place - transitional step between glycolysis and the Krebs cycle - the overall reaction, in simplified terms, is: - Steps in the formation of Acetyl Coenzyme A: NOTE: in the oxidation of glucose requires a different enzyme, and often a coenzyme as well. The coenzyme used at this point in cellular respiration is coenzyme A (CoA), which is derived from pantothenic acid, a B vitamin. 1. Decarboxylation - enzyme pyruvate dehydrogenase, which is located exclusively in the mitochondrial matrix, converts pyruvic acid to a two-carbon fragment called an acetyl group by removing a molecule of carbon dioxide - first reaction in cellular respiration that releases CO2  2. Oxidation - oxidation of the pyruvate is taking place during decarboxylation - each pyruvic acid loses two hydrogen atoms in the form of one hydride ion (H-) plus one hydrogen ion (H +) - coenzyme NAD+ is reduced as it picks up the H - from pyruvic acid - H+ is released into the mitochondrial matrix - since the oxidation of one glucose molecule produces two molecules of pyruvic acid, so for each molecule of glucose, two molecules of carbon dioxide are lost and two NADH and H+ are produced - the acetyl group attaches to coenzyme A, producing a molecule called acetyl coenzyme A (acetyl CoA) THE KREBS CYCLE - named for the biochemist Hans Krebs, who described these reactions in the 1930s - also known as the citric acid cycle, for the first molecule formed when an acetyl group joins the cycle - occur in the matrix of mitochondria - consist of a series of oxidation–reduction reactions and decarboxylation reactions The eight reactions of the Krebs cycle 1. Formation of Citrate - enzyme citrate synthase, catalyzes the fusion of the acetyl group of acetyl- CoA with oxaloacetate - the chemical bond that attaches the acetyl group to coenzyme A (CoA) breaks, and the two-carbon acetyl group attaches to a four-carbon molecule of oxaloacetic acid to form a six-carbon molecule called citric acid - CoA is free to combine with another acetyl group from pyruvic acid and repeat the process 2. Formation of Isocitrate - enzyme aconitase, isomerizes citric acid to isocitric acid through a series of dehydration and rehydration reactions - notice the hydroxyl group (-OH) attached to a different carbon 3. Oxidative decarboxylation of Isocitrate - enzyme isocitrate dehydrogenase, mediates the conversion of isocitric acid to alpha-ketoglutaric acid - isocitric is oxidized and loses a molecule of CO2 - H- from the oxidation is passed on to NAD+, which is reduced to NADH and H+ 4. Oxidative decarboxylation of 𝜶-ketoglutaric acid - α-ketoglutarate dehydrogenase, converts α-ketoglutarate is converted to succinyl CoA - alpha-ketoglutaric acid is oxidized, loses a molecule of CO2, and picks up CoA to form succinyl CoA - NAD+ molecule is reduced to NADH + H+ 5. Thiester Bond Cleavage in Succinyl CoA and Phosphorylation of GDP - enzyme succinyl-coenzyme A synthetase (SCS), catalyzes the phosphorylation of GDP using a succinyl-coenzyme A substrate to produce succinate and GTP - CoA is displaced by a phosphate group, which is then transferred to guanosine diphosphate (GDP) to form guanosine triphosphate (GTP) - GTP can donate a phosphate group to ADP to form ATP 6. Oxidation of Succinate - enzyme succinate dehydrogenase, oxidizes succinic acid to fumaric acid - two of its hydrogen atoms are transferred to the coenzyme flavin adenine dinucleotide (FAD), which is reduced to FADH2 7. Hydration of Fumarate - enzyme fumarate hydratase, converts fumaric acid to malic acid by the addition of a molecule of water  8. Oxidation of L-Malate to Regenerate Oxaloacetate - enzyme malate dehydrogenase, oxidizes malic acid to reform oxaloacetic acid - two hydrogen atoms are removed and one is transferred to NAD+, which is reduced to NADH + H+ - regenerated oxaloacetic acid can combine with another molecule of acetyl CoA, beginning a new cycle Summary of the Citric Acid Cycle Regulation of the Citric Acid Cycle The rate at which the citric acid cycle operates is controlled by the body's need for energy (ATP). When the body's ATP supply is high, the ATP present inhibits the activity of citrate synthase, the enzyme in Step 1 of the cycle. When energy is being used at a high rate, a state of low ATP and high ADP concentrations, the ADP activates citrate synthase and the cycle speeds up. A similar control mechanism exists at Step 3, which involves isocitrate dehydrogenase; here NADH acts as an inhibitor and ADP as an activator THE ELECTRON TRANSPORT CHAIN - a series of biochemical reactions in which electrons and hydrogen ions from NADH and FADH2 are passed to intermediate carriers and then ultimately react with molecular oxygen to produce water - NADH and FADH2 are oxidized - electrons that pass through the various steps of the ETC lose some energy with each transfer along the chain - some of this lost energy are used to make ATP from ADP - enzymes and electron carriers needed for the ETC are located along the inner mitochondrial membrane - within this membrane are four distinct protein complexes, each containing some of the molecules needed for the ETC process to occur - electron carriers which serves as mobile electron carriers that shuttle electrons between the various complexes include coenzyme Q and cytochrome c A. Complex I: NADH- Coenzyme Q Reductase - receives electrons from NADH (Nicotinamide adenine dinucleotide), which is produced during glycolysis and the citric acid cycle - contains 40 subunits, including B vitamin-containing Flavin mononucleotide (FMN) and serverliron-sulfur proteins (FeSP) - transfer of electrons from NADH to Coenzyme Q thru different intermediate carriers 1. Redox reaction of NADH to FMNH2 - NADH is oxidized to NAD+ - two hydrogen ions and electrons are passed to FMN, which is reduced to FMNH2 2. Redox reaction of FMNH2 to 2Fe(II) SP - Fe3+ is reduced to Fe2+ - the two H atoms of FMNH2 are released as two H+ ions - two FeSP molecules are needed to accommodate the two electrons released by FeMNH2 because an Fe3+/Fe2+ reduction involves only one electron 3. Redox reaction of 2Fe(II) SP to CoQH2 - Fe(II) SP is reconverted into Fe (III) SP - the two Fe(II) SP units passes an electron to CoQ, which is then reduced to CoQH2 4. Coenzyme Q shuttle its newly acquired electrons to complex III, where it becomes the initial substrate for reactions at the complex B. Complex II: Succinate-coenzyme Q reductase - smaller than complex I - contains only four subunits - used to process the FADH2, generated in the TCA cycle 1. Redox reaction of FADH2 to 2Fe(II) SP 2. Redox reaction of 2Fe(II) SP to CoQH2 3. Like complex I, Coenzyme Q shuttle its newly acquired electrons to complex III C. Complex III: Coenzyme Q-cytochrome c reductase - contains 11 subunits - electron carriers include several iron-sulfur proteins and cytochromes - receives the CoQH2 from complex I and II - electron transfer process proceeds from CoQH 2 to an FeSP, then to cyt b, then to another FeSP, then to cyt c1, then to cyt c - cytc then delivers its electrons to complex IV D. Complex IV: Cytochrome c oxidase - contains 13 subunits, including two cytochromes - electrons received from cyt c moves to cyt a to cyt a 3 in complex IV - finally, the electrons from cyt a3 and hydrogen ions from cellular solution combine with oxygen to form water - 95% of the oxygen used by cells serves as the final electron acceptor for the ETC OXIDATIVE PHOSPHORYLATION - the biochemical process by which ATP is synthesized from ADP as a result of the transfer of electrons and hydrogen ions from NADH or FADH2 to O2 through the electron carriers involved in the electron transport chain - undergoes coupled reaction with the oxidation reactions of the ETC - complex I, III, and IV can also serve as proton pumps transferring protons from the matrix side of the inner mitochondrial membrane to the intermembrane space - for every two electrons that passed through the ETC, four protons cross the inner mitochondrial membrane through complex I, four through complex III, and two or more for complex IV. - the proton flow causes a buildup of H+ ions in the intermembrane space which becomes a basis for the ATP synthesis - Chemiosmotic coupling – explanation of the coupling of ATP synthesis with ETC reactions that require a proton gradient across the inner mitochondrial membrane. 1. The result of the pumping of protons from the mitochondrial matrix across the inner mitochondrial membrane is a higher concentration of protons in the intermembrane space than in the matrix. This concentration difference constitutes an electrochemical (proton) gradient. A chemical gradient exists when- ever a substance has a higher concentration in one region than in another. Because the proton has an electrical charge (H+ ion), an electrical gradient also exists. Potential energy is always associated with an electro- chemical gradient. 2. A spontaneous flow of protons from the region of high concentration to the region of low concentration occurs because of the electrochemical gradient. This proton flow is not through the membrane itself (it is not permeable to H+ ions) but rather through enzyme complexes called ATP  synthases located on the inner mitochondrial membrane. This proton flow through the ATP synthases "powers" the synthesis of ATP. ATP synthases are thus the coupling factors that link the processes of oxidative phosphorylation and the electron transport chain. 3. ATP synthase has two subunits, the F 0 and F1 subunits. The F0 part of the synthase is the channel for proton flow, whereas the formation of ATP takes place in the F1 subunit. As protons return to the mitochondrial matrix through the F0 subunit, the potential energy associated with the electrochemical gradient is released and used in the F1 subunit for the synthesis of ATP.