Lecture 2-Glycolysis PDF

Digestion& absorption of carbohydrates 1 Oiges†ion in Mou†h Diges†ion of corbohydro†e s†or†s o† †he mou†h. In mou†h,food undergoes mm†ico†ion. During mm†ico†ion, food comeś in con†oc† wi†h Solivo(5ecre†ed by Solivory glond). Saliva con†oin solivory omylase (ptyolin)....

Digestion& absorption of carbohydrates 1 Oiges†ion in Mou†h Diges†ion of corbohydro†e s†or†s o† †he mou†h. In mou†h,food undergoes mm†ico†ion. During mm†ico†ion, food comeś in con†oc† wi†h Solivo(5ecre†ed by Solivory glond). Saliva con†oin solivory omylase (ptyolin). e»yw- ft requireCion foroc†ivo†ion and PN 6.7. The enzyme hydrolyze5a-(1-4) glycosidic bonds a† random deep inside polysocchoride (s†orch, glycogen). Producing dextrins, mol†ose, moI†o†riose, glucose. 2 Glycogen molecule 3  Diges†ior of carbohydra†e †emporariiy s†ops in the stomach. The ac†ionofsaiivary amyime s†ops in s†omah because of high acidi@of stomach. No carbohydra†e spii††ing enzymes avaiiabie in gastric juice. 4 Oiges†ion in Zn†es†ine Fur†her digm†ion of carbohydra†e occurs in smaii in†m†ine by pancrea†ic erzymes. Food boius reaches †he smaii in†es†ine from stomach where it meets the pancreatic juice. Pancrea†ic juice con†ain enzyme caiied pancrea†ic amyime(amyiopsin) simiiar to S. amyime. There are †wo phase of in†m†inai digestion.... Digestion due to pancreatic omylose Digestion due to intestinal brush border enzyme. 5  Ac†ion of pancrea†ic amylase- It hydrolyzes the dextrins to mixture of maltose, isomaI†ose, limi† dex†rin. (olgosocc ‹ori ) 6  Ac†ion of iu†es†inol bvsh border enzyme- These enzymes are responsible for final digestion of carbohydrate. The enzymes4 their reactions are os follows...... Mal†ase ZsOmaI†ose! G lucose 4 Glucose Sucrase Lactase Dextrinase L?mi† dextrin Glucose 1 Md†ose $ Maho†riose 7 8 9 Why ceiluiose is no† digested by humans1 Ceiiulose is polysaccharidefound in plan†s. s†ruc†ure. Humacs cannot synthesize the enzyme which can break glycosidic bond. So, cellulose is not digested by humans. Bu†, although is no† digested i† is one of †he imp comporen† in †he die†. Because, undiges†ed celluiose provide bulk os fibre in †he die†. Fibre helps in in†es†inal mo†iii@& asa s†ooI 10 softer.  Absorption of carbohydrates o corbohydro†e digestion ore... Glucose Fructose goloctose ore reodily absorbed through †he in†e5†inol muco5ol cells in†o †he blood 5†reom. Two mechonism ore responsiblefor †he obsorp†ion of †hese sugors.... Facilitated transport, with conc gradient.. Active transport, against conc. gradient. 11 Passive transport: transport: Transport of molecules with conc. gradient from higher (intestinal lumen) to to lower (mucosal cells) conc. across membranes. It is isa a bidirectional transport needs no no ATP. A- diffusion: Molecules move across A- Simple diffusion: membranes without the assistance of membrane proteins B- B- Facilitated diffusion: diffusion: The The membrane allows only selective molecules and ions to pass through it by by the the assistance of membrane proteins. AllAll monosaccharides areabsorbed are absorbed by by facilitated diffusion 12  te tr«wport of gluco€e ond Golacto6e ocro6s the bru3h boorder of mucosol ceII6 occur by ac†ive †rampor†. I† is a enerp requiring proc&s †ho† require trowport protein ond presence of sodium iow. A śodium dependent glucose tromporter (SCT- 1) binds both gluco6e ond sodium at seporate sitm ond †rompor†6 †hem in†o †he cell. te śodium †ronspor† down conc. grodien†6 glucose trQnsported QgQinst cohc. grQdient. 1is process is called cotronsport or symport. te energy for †his rmc†ion is provided by AO 13 linked to 6odium pump. Absorption of Carbohydrates 14  Disorders of digestion& absorption of carbohydrates Lactose intolerance S Deficiency of lactase enzyme (lactose — glu+ glu + gal) S Lactose accumulates in in the gut, fermented by by bacteria in large intestine and and produces H H22,, CO CO22,, low lowM.Wt M.Wt acids (acetic, propionic, butyric) S The The acids produced osmotically draw water from intestinal cells into lumen of ofGIT GIT causes diarrhea and and dehydration Sucrase deficiency R It looks like lactose intolerance in symptoms 15  Fate of of absorbed sugars su9ars I- Uptake by tissues II- Utilization by by tissues: A- Oxidation: Glycolysis, TCA A- Oxidation: TCA (give ATP), HMP, Uronic B- B- Conversion into biolog. Subst. (ribose, fructose, galactose, glucuronic acid, amio sugars, amino acids) C- Storage: Glycogen, TAG C- TAG 16 Glycolysis 17  Glycolysis It is isa a ten ten steps metabolic pathway occurs in cytosol to convert glucose into two two molecules of pyruvate and two and two molecules each of NJlA7Zand NADH and ATP. Allcarbohydrates All carbohydrates to be be catabolized must enter the glycolytic pathway. ce’**" Pyruvate c 0 C O û o I I I CHU CHU cHm ûoA-SH ” "“” co,+ Acealdehyde C o I 1C«A CHt o c o L. tate I cn, " C -O Æetyl-CoA ”ö NAg HO — C H Ethanol I Citric acid cycle and H — C — OH CHU Oxidative phosphorylation I CHj Depending on the of oxygen, the pyruvate will either enter the presence of enteraa fermentation process (lactic acid or alcohol) or proceed towards the 30 Oebs the Krebs Cvcle. Cycle.  Glyceraldehyde-3-phosphate Fermentation: Dehydrogenase Fermentation: HH OO ++ H+ O H” OPO32—2 x /y NAD”+ NADH 1C NAD Anaerobic 11CC I 1C I ++ PP;i lackaa organisms lack H H 2C C — 2 OH OH H H —2 2jC C— OH OH 2— 2— respiratory chain. 33 CH CH 2 2 OPO OPO 3 3 2 33CH CH22OPO OPO3 3 2 glyceraldehyde- 1,3-bisphospho- 3-phosphate glycerate They must reoxidize NADH produced inGlycolysis in Glycolysis through some other reaction, because NAD+ NAD+ is needed for for theGlyceraldehyde-3-phosphate the Glyceraldehyde-3-phosphate Dehydrogenase reaction. Usually NADH is reoxidized as pyruvate is converted to aa more reduced compound (lactate). (lactate). The complete pathway, including Glycolysis and The and the 31 reoxidation of NADH, is called fermentation. fermentation.  Lactate Dehydrogenase O O O O C NADH + H+ NAD+ C C O HC OH CH33 CH CH33 CH pyruvate lactate Lactate Dehydrogenase catalyzes reduction of the the keto in pyruvate to toaa hydroxyl, yielding lactate, as NADH is NAD+. oxidized to NAD+. Lactate, in addition to being an end-product of fermentation, fermentation, asaa mobile form of serves as of nutrient energy; possibly asa as a signal molecule in inmammalian mammalian organisms. Cell membranes contain carrier proteins allow transport of lactate to be be used for forenergy energy production after oxid oxid^n to pyi'üvate pyruvate 32  Lactate Dehydrogenase OO OO OO 0O + **c* C NADH++ H H” NAD”+ **c* NAD C CC OO HC — OH HC OH CH CH33 CH CH33 pyruvate lactate Skeletal muscles ferment glucose to lactate during exercise, when the the need of ofEE is brief and intense. brief and Lactate released to the blood may may be taken upup by by other tissues, or or by by skeletal muscle after exercise, and and converted via Lactate Dehydrogenase back topyruvate, to pyruvate, which may may be oxidized in Kreb's or (in liver) converted back to Kreb’s Cycle or to glucose via gluconeogenesis 33  Lactate Dehydrogenase O O* O O O + C NADH++ H H” NAD”+ NAD C CC 0O HC — OH HC OH CH CH33 CH CH33 pyruvate lactate Lactate serves as asaa fuel source for for cardiac muscle as as well as brain neurons. Astrocytes, which surround and and protect neurons ininthe the brain, ferment glucose to lactate and and release it. Lactate taken up up by by adjacent neurons is converted to pymvate pyruvate that is oxidized via Kreb's Kreb’s Cycle. 34  Pyruvate Alcohol Decarboxylase Dehydrogenase OO OO CO22 HH CO NADH + H+ NAD +H” NAD”+ HH **c* C O C O C H —CC — OH OH CH CH33 CH33 CH CH CH33 pyruvate acetaldehyde ethanol Some anaerobic organisms metabolize pywvate pyruvate to to ethanol, which is excreted as asaa waste product. NADH is converted to NAD+ inthe NAD+ in the reaction catalyzed by by Alcohol Dehydrogenase. P;yzurste cszbox;yJst?on, decarboxylation Pyruvate carboxylation, decszbox;yJst?on shares zhszez ›”n cszboh;ydzste metabolism in carbohydrate metsboJ?zm 35  Importance of 2,3 2, 3 BPG BPG cycle 1/2 Glucose- - - GIyceraIdehyde-3-phosphate 36 Pyruvate co right 1996 M W. King  B2PG 3 BPG decreases affinity of Hb 2,3 Hb toO to O22,so , soOO22 can can g gooeasy to tissues. This pathway is increased underhhypoxic oxic conditions. condi ions. When the the need for forATP ATP inRBCs in RBCs is minimal, glycolysis takes this pathway as as when glucose is oxidized by by this pathway theerythrocyte the erythrocyte loses the ability to gain22 moles of ofATP ATP from glycolytic oxidation of 1,3-BPG into 3-PG via via thephosphoglycerate the phosphoglycerate kinase reaction so there is no no net net production of ATP ATP during this step. 37  In In vitro inhibition of Glycolysis:- Arsenate: It competes with phosphate in in reaction No No 6, in which glyceraldhyde-3-P is converted into 1, 1,3 3 bisphosphoglycerate lodoacetate: Iodoacetate: It inhibits glyceraldhyde-3-PP dehydrogenase. Fluoride: Fluoride: It inhibits enolase forming magnesium fluorophosphate blocking enzyme active site, soso used forfordetermination determination of blood glucose in the laboratory. laboratory. It is added toblood to blood samples to to stop glycolysis in RBCs toto protect glucose level from decrease during storage. 38  Regulation of Glycolysis:- The rate of glycolysis is regulated by the control of the 3 irreversible enzymes (key enzymes): Glucokinase/Hexokinase Phosphofructokinase-1 Pyruvate kinase. 39 1- Hormonal regulation: a- Insulin stimulates the biosynthesis of the last three enzymes, stimulating glycolysis. b- Adrenaline, glucagon inhibit pyruvate kinase, inhibiting glycolysis. Also they inhibit formation of F-2,6-BP so inhibiting PFK-1 and glycolysis 2- Effect of substrates: a- G-6-P inhibits hexokinase (not glucokinase). b- Citrate inhibits phosphofructokinase. c- Fructose 2, 6 BP activates PFK-1 40 3- Role of energy in regulation: a- High levels of ADP, AMP means low energy concentration in the cells, so stimulate phosphofructokinase. b- High levels of ATP means high energy so it inhibits phosphofructokinase, pyruvate kinase and glycolysis 41 Insulin Glucagon Glycolysis pacemaker Glucose-6P is not the only glycolytic intermediate but it enters glycogenesis, HMP for production of reduced NADP. While, the unique reaction for glycolysis is the phosphorylation of fructose-6P by phosphfructokinase-1 into fructose 1,6 bisphosphate. Ɵ Ɵ 42 Ɵ Ɵ Insulin Glucagon Fructose 2, 6 bisphosphate stimulates glycolysis by allosteric stimulation of phosphofructokinase-1 (PFK-1) and inhibits gluconeogenesis by inhibiting fructose 1, 6 bisphosphatase enzyme. PFK-2 has a bi-functional role (kinase and phosphatase). Ɵ Ɵ 43 Ɵ Ɵ Sources of lactate: Skeletaİ muscles ferment glucose to 1- Skeletal to lactate exercise, when theexertion during exercise, and the exertion is brief and intense. intense. 2- Astrocytes, Astrocytes, which surround andand protect neurons in the the brain, ferment glucose toto lactate and release it. and it. 3- RBCs, since no 3- no way way to regenerate NAD NAD except by reducing pyruvate into lactate by by LDH (no LDH (no mitochondria in RBCs) 44 -Lactate, in addition to being an -Lactate, an end-product of of fermentation, fermentation, serves asasaa mobile form of of nutrient energy. Cell membranes contain carrier proteins that facilitate transport of lactate. lactate. reİeased to the blood may -Lactate released may be be taken up by other tissues, up by skeletal muscle after tissues, or by exercise, and exercise, and converted by LDHLDH back into pyruvate, which may may be oxidized in Kreb’s Kreb's or converted back toglucose Cycle or to glucose via via gÍuconeogenesis in the gluconeogenesis the liver (Cori cycle). 45  Skeletal Muscle Blood Liver.û æ a ğ Glucose Glucose Gluconeogenesis NADH 46  Example :- Covalent modification of Pyruvate kinase: cAMP Glucagon | ”(+) Protein kinaseA Protein phosphatase I Insulin 47 Pyruvate kinase deficiency -Pyruvate Pyruvate kinase deficiency in RBCs causes hemolytic anemia. -RBCs have no mitochondria so so completely depends on glycolysis for production of ATP. Deficiency of pyruvate kinase will lead to premature death and and lysis İysis of RBC RBC (why?). -The mismatch between the red cell's the red cells energy requirements and and its ATP-generating capacity damages themembrane irreversibly, distorting the membrane irreversibly, and dehydrating the cell and and and affecting thethe rigidity. As rigidity. Asa a damaged cell,cell, it is destroyed 48 3rematurel:/ by prematurely bi/ the s3leen and the spleen and liver. liver.