Metabolism of Carbohydrates PDF

Metabolism of Carbohydrates 1  ATP - Cellular Energy Currency  Catabolism - Degradation (CHO, proteins, & lipids)  Cells use an energy conversion strategy that oxidizes glucose Small amounts of energy are released at several points in this pathway Energy is harvest...

Metabolism of Carbohydrates 1  ATP - Cellular Energy Currency  Catabolism - Degradation (CHO, proteins, & lipids)  Cells use an energy conversion strategy that oxidizes glucose Small amounts of energy are released at several points in this pathway Energy is harvested and stored in bonds of adenosine triphosphate ATP (universal energy 2  ATP - Adenosine Triphosphate  Combustion of a mole of glucose yields 686 kcal  ATP serves as a “go-between” molecules and coupling: exergonic - energy releasing, catabolism reactions endergonic - energy requiring, anabolic reactions  ATP “captures” energy as phospho-anhydride bonds  Anhydride bonds hydrolysis  provides energy for 3  ATP - Molecule structure  ATP is a nucleotide, composed of: Nitrogenous base - (adenine) 5-carbon sugar - (ribose) three phosphoryl groups  Phosphoester bond joins the 1st phosphoryl group to ribose  Phosphoanhydride bonds join 2nd & 3rd groups = 4  Phosphoanhydride bond hydrolysis Hydrolysis of phosphoanhydride bond release large energy  used for cellular process such as: Phosphorylation of glucose or fructose 5  Overview of catabolic processes  CHO, fats & proteins are degraded to release energy  CHO are the most readily used energy source  Saliva - Composition  Digestion of carbohydrates begins in the mouth  Secretions of salivary glands (parotid, submandibular/sublingual)  Major components: 1.Mucus made of Mucins: glycoproteins & mucopolysaccharides (long-unbranched polysaccharides)  serves as a lubricant 2.Amylase  initiates the digestion of starch 3.Lingual lipase  begins digestion of fat 7  Salivary amylase  Attacks α(14) glycosidic bonds of starch (amylose & amylopectin)  Breaks down starch to maltose (glucose α(1→4) disaccharide) & dextrin  Dextrin is a short polymer of glucose linked by α- (1→4) or α-(1→6)  Gastric acids in the stomach inactivate salivary amylase (pH = 5.6 to 7) 8  Lingual lipase  Similar to gastric lipase  involved in 1st phase of fat digestion  Active at alkaline (mouth) and acidic pH (stomach)  Converts TG to monoacylglycerol, Glycerol, & 9  Saliva - Functions  Tasting & moistening, bolus formation, speech & swallowing facilitation  Pellicle - protein that binds to glycoprotein of saliva  prevents deposition of Ca-phosphate. Protects teeth against acids produced by bacteria  Alpha amylase & Lingual lipase digest starch and lipids  Lysozome offers a beneficial antiseptic function in digestion 10   Carbohydrates – Digestion  α-amylase secreted by salivary glands & pancreas breaks α(14) bonds  Isomaltase secreted by the intestine breaks α(16) bonds  Maltase converts Maltose to 2 moles of glucose  Lactase converts Lactose to glucose & galactose 11  Sucrase converts Sucrose to  Carbohydrates – Absorption  D-Glucose, D-Galactose & D-Fructose can passively (but slowly) diffuse through the intestinal villi or via Hexose transporter  Luminal membranes of mucosal cells in the small intestine contain a symport that transports glucose into the cell only if Na+ binds to the protein and is 12 Glucose transport in intestinal cells 13 Process of carbohydrates digestion 14 Hydrolysis of CHO, Proteins, & Lipids 15 Cell membrane transport pathways Malabsorpti on 17  Indigestible material  Not everything can be digested...  Cellulose not digested at all  Keratins and some plant proteins are difficult to digest  95% of fat; 70-90 % of starch & protein is digested  92% to 97% of the diet is absorbed  Indigestible material (fiber) helps movement through GIT  Anaerobic fermentation by intestinal flora produces gas (hydrogen, CO2, CH4) 18  Malabsorption Absorption depends on:  Presence of substance in absorbable form  Integrity and normal structure of the absorptive area (no atrophy)  Normal ratio of speed of absorption to speed of passage through GIT 19  Malabsorption - Causes  Pancreatic insufficiency in chronic pancreatitis  Coeliac disease - flattening of the villi  inadequate mucosal surface  Crohn’s disease - inflammation, thickening of villi surface  GIT thickening  Acute infection - salmonella  food poisoning  inflammatory response  Chronic infection - tropical sprue – infection causing 20  Malabsorption - Consequences  Malabsorption results in: weight loss, weakness, failure to thrive (infants), various disorders due to vitamin deficiencies  Malabsorption is accompanied by: steathorrhoea (visible fat in feces), intestinal discomfort, diarrhea  Malabsorption is investigated by: Endoscopy – investigates atrophy, gastritis, ulcers Biopsy – investigates villi 21  Malabsorption of Carbohydrates  Lactase deficiency: Sugar of milk - Lactase deficiency results in intolerance Lactose fermented by gut bacteria  gas Lactose raises osmotic pressure in lumen, preventing water resorption diarrhea  Sucrase-isomaltase deficiency: Oral tolerance test, oral administration of sucrose  sucrose appearance 22  Stage 1: Hydrolysis of dietary macromolecules  Polysaccharides to monosaccharides - Begins in the mouth with amylase action on starch & continues in small intestine with pancreatic amylase  monosaccharides  Proteins to AA - Begins in the stomach with acid hydrolysis & serine proteases act in the small intestine 23 Stage 2: Conversion of monomers to oxidized forms Small subunits go to pathways of energy metabolism (major pathways)  Glycolysis CHO enter as glucose or fructose CHO converted to acetyl CoA  enter TCA  Citric acid cycle CHO enter as acetyl CoA Proteins enter as the carbon skeleton of AA FA enter as acetyl CoA 24  Stage 3: Complete oxidation & ATP production  Acetyl CoA carries 2-C fragments of the nutrients  Acetyl CoA enters TCA Acetyl group is oxidized to produce CO2 Electrons & hydrogen are harvested & used to 25 Glycolysis 26 Glycolysis: Embden-Meyerhof pathway Glycolysis  Pathway for CHO catabolism that requires no oxygen  10 step pathway catalyzed by enzymes in the cytoplasm  D-glucose - substrate (used by all almost organisms for energy) Products  2 Pyruvates (fate depends on cellular conditions)  4 ATP – substrate level phosphorylation (2 27 Glycolysis - Overview 28  Step 1 - Hexokinase  The conversion of D-glucose into glucose-6- phosphate (G6P)  The enzyme that catalyzes this phosphorylation reaction is hexokinase  At this point in glycolysis, 1 ATP molecule has been consumed 29  Step 2 - Phosphoglucose isomerase  The G6P is rearranged (isomerization) into fructose 6-phosphate (F6P) by phosphoglucose Isomerase (six- to five-membered ring)  The six-membered ring opens and then closes in such a way that the first carbon becomes now external to the ring 30  Step 3 - Phosphofructokinase (PFK)  The fructose 6-phosphate (F6P) is converted into fructose 1,6-bisphosphate with a phosphofructokinase  Similar to the 1st reaction, 1 ATP molecule has been consumed providing the P-group that is added on to the F6P 31  Step 4 - Aldolase  The fructose 1,6-bisphosphate is split into two sugars that are isomers of each other with an aldolase  These two sugars (3-C molecules) are dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP) 32  Step 5 - Triphosphate isomerase  The enzyme triophosphate isomerase rapidly inter-converts the molecules DHAP and GAP  GAP is removed/used in next step of glycolysis and is the only molecule that continues in the glycolytic pathway 33  Step 6 - Glyceraldehyde-3-P dehydrogenase “GAPDH” GAPDH dehydrogenates and adds an inorganic phosphate to GAP, producing 1,3- bisphosphoglycerate (1,3BPG) in 2 steps:  GAP is oxidized by the coenzyme NAD  The molecule is phosphorylated by addition of a free P-group 34  Step 7 - Phosphoglycerate kinase (PGK)  Phosphoglycerate kinase transfers a P-group from 1,3BPG to ADP to form ATP and 3- phosphoglycerate  Since two molecules of 1,3BPG used, 2 ATP are synthetized  the 2 ATP molecules used (Steps 1&3) are now cancelled 35  Step 8 - Phosphoglycerate mutase (PGM)  The P-group fom is relocated from the 3rd to the 2nd carbon to form 2-phosphoglycerate with phosphoglycero mutase  A rearrangement by first adding an additional P-group to the 2′ position. The enzyme then removes the P from the 3′ position 36  Step 9 - Enolase  A molecule of water is removed from 2- phosphoglycerate to form phosphoenolpyruvic acid (PEP) by the enolase  Enolase works by removing a water group, or dehydrating the 2 phosphoglycerate 37  Step 10 - Pyruvate kinase  A P-group is transferred from PEP to ADP to form pyruvic acid and ATP with a pyruvate kinase  Again, since there are two molecules of PEP, here we actually generate 2 ATP molecules 38  Regulation of Glycolysis Negative: Glu-6-P, ATP Positive: Fru-2,6-BisP, AMP Negative: ATP, Citrate Positive: Fru-1,6-BisP, AMP Negative: ATP, Acetyl CoA 39  Fermentation  Pyruvate must be degraded & NADH re- oxidized Aerobically - occurs in cellular respiration Anaerobically - occurs in fermentation  Fermentation is catabolic with no net oxidation using 2 pathways: Lactate fermentation 40  Lactate fermentation  Lactate dehydrogenase anaerobically degrades pyruvate to lactate and produces NAD+  Occurs in exercising muscle & bacteria (produce yogurt & cheese)  Alcohol fermentation Yeasts ferment anaerobically sugars of fruit & grains (pyruvate) 1. Pyruvate is decarboxylated to acetaldehyde with CO2 removed using pyruvate decarboxylase 2. Acetaldehyde reduced to ethanol with NAD+ produced using alcohol dehydrogenase Gluconeoge nesis 43  Gluconeogenesis It occurs primarily in the liver. It is the metabolic process by which organisms produce sugars (glucose) for catabolic reactions from non- carbohydrate precursors: lactate, glycerol, and amino acids (2 latter are used only in starvation) 44  Comparison of glycolysis & gluconeogenesis  Basically opposite processes but not reversal of each other  The 3 nonreversible steps of glycolysis are: Glucose-6-phosphate  Glucose Fructose-1,6-bisphosphate  Fructose-6- 45  Comparison of glycolysis & gluconeogenesis  Catalyzed by PFK  Stimulated by: high AMP, ADP, Pi  Inhibited by high ATP HOWEVER  Reversed in gluconeogenesis  Fructose-1,6- bisphosphatase  stimulated by high ATP 46  Cori cycle  When anaerobic conditions occur in an active muscle, glycolysis produces lactate  The lactate moves through the bloodstream to the liver, where it is oxidized back to pyruvate  Gluconeogenesis converts pyruvate to glucose, which is carried back to the muscles  The Cori cycle is the flow of lactate and glucose Glycogenoly sis 48  Glycogen synthesis & degradation  Glucose is the sole source of energy for RBCs & major source for brain  Glucose supplied: Diet Gluconeogenesis Glycogenolysis - biochemical breakdown of glycogen to glucose 49  Glycogen structure  Polymer of glucose  Highly branched α(14) α(16)  Stored as granules in liver &  Glycogenolysis  Is the process of glycogen degradation, which is controlled by: Glucagon - Pancreas responds to low blood sugar Epinephrine - Adrenal gland to stress/threat  Step 1: An end glucose removed as G1P with glycogen phosphorylase  Step 2: Removal of the last glucose at an α(16) 51  Glycogen phosphorylase  It catalyzes the reaction in which an α(14) linkage, between two glucose residues at a nonreducing end, undergoes attack by inorganic P (Pi)  Removes the terminal glucose residue as G-1P 52  Debranching enzyme  Glucose residues are removed from branches until 4 residues before a glucose that is branched with a α(1→6) linkage  Glycogen debranching enzyme then transfers three of the remaining 4 glucose units to the end of another glycogen branch  This exposes the α(1→6) branching point, which is hydrolyzed by its α(1→6) glucosidase activity, 53 removing the final glucose residue of the branch as  Debranching enzyme Debranching enzyme transfers the as whole from the branch to the main chain using its transferase activity, then it will hydrolyze the from glycogen using its α(16) 54  Phosphoglucomutase  Phosphoglucomutase uses its Ser residue “phosphorylated” to convert G-1-P to G-6-P  G-6-P formed in the muscle enter glycolysis (energy source)  G-6-P formed in the liver will not enter glycolysis. But it is transported into the lumen of 55 the ER, where it will be Glycogenesi s 56  Glycogenesis: Glycogen synthesis  It is glycogen synthesis  glucose molecules are added to chains of glycogen for storage (could be broken down to glucose when needed)  Once there is a chain consisting of 8 to 10 glycosidic residues in the glycogen fragment, branching begins by 1→6 linkages  Insulin stimulates glycogenesis via dephosphorylation and thus activation of glycogen 57  Glycogenesis: Glycogen synthesis Glucose is converted into G-6-P by a glucokinase (liver)/hexokinase (muscle) with conversion of ATP to ADP G-6-P is converted into G-1-P by a phosphoglucomutase, passing through the obligatory intermediate G-1,6-bisP Glycogenesis: UDP-glucose pyrophosphorylase Glucose-1-phospahate is converted into UDP- glucose by UDP-glucose pyrophosphorylase, & Pyrophosphate is formed Glycogenesis: Glycogenin  Glycogenin primes glycogen synthesis  First 8 UDP-glucose molecules are added by glycogenin, forming α(1→4)  Glycogenesis: Glycogen synthase Glycogen synthase binds to the growing glycogen chain and adds UDP-glucose to the end of the glycogen chain, forming more α(1→4) bonds  Glycogenesis: Branching enzyme Branches are made by branching enzyme, which transfers the end of the chain onto an earlier part via α-1:6 glycosidic bond, forming branches, which further grow by addition 62 Reactions of Glycogenesis 63  Glycogenesis vs. Glycogenolysis  In case of high blood sugar (hyperglycemia), insulin will: stimulate glucokinase stimulate glycogen synthetase inhibit glycogen phosphorylase  In case of low blood sugar (hypoglycemia), glucagon will: stimulate glycogen phosphorylase 64  Opposing effects of insulin & glucagon on glycogen metabolism In high blood sugar (hyperglycemia), In low blood sugar (hypoglycemia), insulin: glucagon:  stimulates glucokinase  inhibits glycogen synthetase  stimulates glycogen synthetase  stimulates glycogen phosphorylase  inhibits glycogen phosphorylase 65  Aerobic respiration & Energy 66  The mitochondria  “Shoe-shaped” organelle with a size of bacterial cell  Dual mitochondrial membranes structure with intermembrane space: Outer membrane has pores  passage of small molecules Inner membrane:  Highly folded “christae”  increase surface 67 The mitochondria Conversion of pyruvate to acetyl CoA In aerobic conditions,  Pyruvate produced from glycolysis enters the mitochondria  Pyruvate must be decarboxylated to acetyl CoA 69  Decarboxylation & oxidation of pyruvate Pyruvate dehydrogenase is responsible for: 1. Decarboxylation: loss of CO2 2. Oxidation: NAD+ accepts hydride anion  NADH 3. Acetyl linked to coenzyme A via a high-energy 70  Pyruvate dehydrogenase complex Contains 3 enzymes & 5 coenzymes (4 vitamin- derived) 1.Thiamine pyrophosphate  thiamine 2.FAD  riboflavin 3.NAD+  niacin 4.Coenzyme A  pantothenic acid 5.Lipoamide Pyruvate dehydrogenase complex  Role of acetyl CoA in metabolism  Acetyl CoA is central in cellular metabolism  Produced by degrading: Glucose FA/ketone bodies Amino Acids  Functions of Acetyl CoA: Carry acetyl to TCAATP Produce cholesterol & FA Eicosanoids  Aerobic respiration  Breakdown of food in presence of O2 to produce ATP  Called oxidative phosphorylation  energy from oxidative reactions is used to phosphorylate ADP making ATP  Enzymes are located in the mitochondrial matrix  3 oxidations  electrons passed from NAD+ or FAD to the electron transport system “ETC” & then O2  Protons transferred to intermembrane space 74  Citric acid cycle - Overview  Final stage in the breakdown of nutrients  Acetyl CoA & oxaloacetate feed the cycle  The acetyl group oxidized to 2 CO2 & high energy electrons are transferred to NAD+ & FAD 75 Citric acid cycle - Overview 76  CTA - 1 Reaction - Condensation st Citrate synthase - acetyl CoA joins with a four- carbons molecule, oxaloacetate, releasing the CoA group and forming a six-carbon molecule called citrate  CTA - 2nd Reaction – Dehydration/Hydration Aconitase - citrate is 1st converted to cis-Aconitate by removal of water molecule. Then cis-Aconitate is hydrated by addition of water to form Isocitrate (citrate isomer) 1 2  CTA - 3rd Reaction - Oxidative decarboxylation  Isocitrate is oxidized and releases a CO2 molecule  α-ketoglutarate, a five-carbon molecule is the product  During this step, NAD+ is reduced to form NADH  Isocitrate dehydrogenase - important to regulate the cycle speed  CTA - 4th Reaction - Oxidative decarboxylation  The α-ketoglutarate is oxidized and releases a CO2 molecule  NAD+ is reduced to form NADH  The remaining 4-C molecule picks up CoA, forming the succinyl CoA  α-ketoglutarate dehydrogenase - important to regulate the cycle speed  CTA - 5th Reaction - Substrate phosphorylation  Succinyl CoA synthase – the CoA of succinyl CoA is replaced by a phosphate group, which is then transferred to GDP to make GTP  The four-carbon molecule produced in this step is called succinate CTA - 6 th Reaction - Dehydrogenation  Succinate is oxidized, forming another 4-C molecule called fumarate  Two hydrogen atoms are transferred to FAD producing FADH2  ​The enzyme, succinate dehydrogenase, is embedded in the inner membrane of the mitochondrion, so FADH2 can transfer its electrons directly into the electron transport system  CTA - 7 th Reaction - Hydration  Water is added to the 4-C molecule fumarate, converting it into another 4-C molecule called malate  Fumarase reduces the double bond of fumarate by hydration CTA - 8 th Reaction - Dehydrogenation  In the last step catalyzed by malate dehydrogenase, oxaloacetate, the starting 4-C compound, is regenerated by oxidation of malate  Another molecule of NAD+ is reduced to form NADH in the process CTA - Wrap Up 85  TCA - Products  In a single turn of the cycle, 2 carbons enter from acetyl CoA and 2 CO2 are released 1 FADH2 and 3 NADH molecules are generated 1 ATP (GTP) molecule is produced  These figures are for one turn of the cycle, corresponding to one molecule of acetyl CoA  Each glucose produces two acetyl CoA molecules, so we need to multiply these numbers by 2 if we want 86 Allosterically regulated reactions 87  ETC & Oxidative phosphorylation  The ETC, also called the respiratory electron transport system occurs in inner mitochondrial membrane  It is made of series of electron carriers passing electrons from one carrier to the next (Coenzymes & Cytochromes)  Enzyme ATP synthase NADH provides 3 ATPs 88  The hydrogen ion gradient  Protons (H+) “high-energy reservoir” are pumped from the matrix to the intermembrane space at 3 sites in the ETC  Each site pumps sufficient protons to produce 1 ATP NADH dehydrogenase passes electrons along all 3 sites FADH2 oxidation passes electrons along only 2 sites  Final component for oxidative phosphorylation is 89 Electron flow through electron carriers 90  ATP synthase & ATP production  Electrons reach last carrier to be placed with an electron acceptor: O2 ½ O2 + 2 H +  H2O  The protons in the intermembrane space flow back through ATP synthase F0 channel 91 ATP synthase 92  Uncoupling proteins (UCPs)  UCPs occur in the inner mitoch. mbne of mammals, including humans  These carrier proteins act as “proton leak”  permit protons to re-enter the mitochondrial matrix without energy being captured as ATP  Energy is released as heat. UCP1 “thermogenin” activated by FA, is responsible for heat production in the brown adipocytes of mammals  Brown fat, unlike white fat, uses almost 90% of its 93 ATP synthase vs. Uncoupling proteins 94 Energy outcome from one glucose molecule 95  The End… 96