glycolysis and carbohydrate metabolism

GLYCOLYSISUniversity of Babylon College of pharmacy3rdclass –second semester Biochemistry -carbohydrate metabolismDr. Abdulhussien M. K. Aljebory2014 –2015 08-Mar-151
GLYCOLYSISThere are two types of metabolism : 1-Aerobic respirationis relate to the structure of a mitochondrion.GlycolysisandKrebscycle synthesis 10% of ATP.Electron transportchain synthesis 90% of ATP.2-an aerobic respiration -formation of lactate .Respiration involves three stages:1.Glycolysisharvests chemical energy by oxidizing glucose to pyruvateand produces about 5% of ATP (in cytoplasm)2.Krebs cyclecompletes the energy-yielding oxidation of organic molecules and produces about 5% of ATP(in mitochondrial matrix)3.Electron transportchain to synthesisATP and produces about 90% of ATP(inner mitochondrial membrane).vCellular respiration generates many ATP molecules. For each sugar molecule it oxidizes (38 ATP molecule).08-Mar-152
Respiration involves glycolysis, the Krebs cycle, and electron transport•Respiration occurs in three metabolic stages: glycolysis, the Krebs cycle, and the electron transport chainand oxidative phosphorylation.•Glycolysisoccurs in the cytoplasm.–It begins catabolism by breaking glucoseinto two molecules of pyruvate.•The Krebs cycle occurs in the mitochondrial matrix.–It degrades pyruvateto carbon dioxide.•Several steps in glycolysisand the Krebs cycle transfer electrons from substrates to NAD+, forming NADH.•NADHpasses these electrons to the electron transport chain.08-Mar-153
Phosphorylation•I-Substrate-level phosphorylation:•Some ATP is generated in glycolysisand in Krebs cycle by Substrate-level phosphorylation. phosphate group is transferred from an organic molecule (the substrate) to ADP, forming 10% ATP (4 ATP).•II-Oxidative phosphorylation:•As electrons passed along the chain, their energy stored in the mitochondrion in a form that can be used to synthesize the rest 90% of the ATPATP(34 ATP).•via Oxidative phosphorylation.•Ultimately 38 ATPare produced per mole of glucose that is degraded to carbon CO2and H2Oby respiration.08-Mar-154 Phosphorylation•I-Substrate-level phosphorylation:•Some ATP is generated in glycolysisand in Krebs cycle by Substrate-level phosphorylation. phosphate group is transferred from an organic molecule (the substrate) to ADP, forming 10% ATP (4 ATP).•II-Oxidative phosphorylation:•As electrons passed along the chain, their energy stored in the mitochondrion in a form that can be used to synthesize the rest 90% of the ATPATP(34 ATP).•via Oxidative phosphorylation.•Ultimately 38 ATPare produced per mole of glucose that is degraded to carbon CO2and H2Oby respiration.08-Mar-154 General aspectA. More than 60% of our foods are carbohydrates. Starch, glycogen, sucrose, lactose and cellulose are the chief carbohydrates in our food. Before intestinal absorption, they are hydrolysedto hexosesugars (glucose, galactoseand fructose).B. A family of a glycosidasesthat degrade carbohydrate into their monohexosecomponents catalyzes hydrolysis of glycocidicbonds. These enzymes are usually specific to the type of bond to be broken.08-Mar-155 Digestion of carbohydrate by salivary ? -amylase (ptylin) in the mouth:A. This enzyme is produced by salivary glands. Its optimum pH is 6.7.B. It is activated by chloride ions (cl-). C. It acts on cooked starch and glycogen breaking ? 1-4 bonds, converting them into maltose [a disaccharide containing two glucose molecules attached by ? 1-4 linkage]. This bond is not attacked by -amylase.•Because both starch and glycogen also contain 1-6 bonds, the resulting digest contains isomaltose[a disaccharide in which two glucose molecules are attached by 1-6 linkage].•E. Because food remains for a short time in the mouth, digestion of starch and glycogen may be incomplete and gives a partial digestion products called: starch dextrins(amylodextrin, erythrodextrinand achrodextrin).•F. Therefore, digestion of starch and glycogen in the mouth gives maltose, isomaltoseand starch dextrins. 08-Mar-156 •III. lnthe stomach: carbohydrate digestion stops temporarily due to the high acidity which inactivates the salivary -amylase.•IV.Digestion of carbohydrate by the pancreatic -amylase small intestine in the small intestine. •A. ?-amylase enzyme is produced by pancreas and acts in small intestine. Its optimum pH is 7.1.•B. It is also activated by chloride ions.•C. It acts on cooked and uncooked starch, hydrolysingthem into maltose and isomaltose.•A. The final digestive processes occur at the small intestine and include the action of several disaccharidases. These enzymes are secreted through and remain associated with the brush border of the intestinal mucosal cells.08-Mar-157 •B. The disaccharidasesinclude:•1. Lactase (?-galactosidase) which hydrolyses lactose into two molecules of glucose and galactose:•Lactase •Lactose Glucose + Galactose•2. Maltase (?-glucosidase), which hydrolyses maltose into two molecules of glucose:•Maltase •Maltose Glucose + Glucose•3. Sucrose (?-fructofuranosidase), which hydrolyses sucrose into two molecules of glucose and fructose:•Sucrose•Sucrose Glucose + Fructose•4. ?-dextrinase(oligo-1,6 glucosidase) which hydrolyze (1 ,6) linkage of isomaltose.•Dextrinase•IsomaltoseGlucose + Glucose 08-Mar-158 •VI. Digestion of cellulose:•A. Cellulose contains ?(1-4) bonds between glucose molecules.•B. In humans, there is no ?(1-4) glucosidasethat can digest •such bonds. So cellulose passes as such in stool.•C. Cellulose helps water retention during the passage of food •along the intestine ?producing larger and softer feces ?•preventing constipation.08-Mar-159 Absorptions: introduction•A.Theendproductsofcarbohydratedigestionaremonosaccharides:glucose,galactoseandfructose.Theyareabsorbedfromthejejunumtoportalveinstotheliver,wherefructoseandgalactosearetransformedintoglucose.•B.Twomechanisms are responsible for absorption of monosaccharides: active transport (against concentration gradient i.e. from low to high concentration) and passive transport (by facilitated diffusion).•C.Foractivetransporttotakeplace,thestructureofsugarshouldhave:1.Hexosering.•2. OH group at position 2 at the right side. Both of which are present •in glucose and galactose. Fructose, which does not contain -OH •group to the right at position 2 is absorbed more slowly than •glucose and galactoseby passive diffusion (slow process).•3. A methyl or a substituted methyl group should be present at •carbon 5.08-Mar-1510 •II. Mechanisms of absorption:•A. Active transport:•1. Mechanism of active transport:•a) In the cell membrane of the intestinal cells, there is a mobile •carrier protein called sodium dependant glucose transporter •(SGL T-1) It transports glucose to inside the cell using •energy. The energy is derived from sodium-potassium •pump. The transporter has 2 separate sites, one for sodium •and the other for glucose. It transports them from the •intestinal lumen across cell membrane to the cytoplasm. •Then both glucose and sodium are released into the •cytoplasm allowing the carrier to return for more transport •of glucose and sodium.08-Mar-1511 •b) The sodium is transported from high to low concentration •(with concentration gradient) and at the same time causes the •carrier to transport glucose against its concentration gradient. •The Na+ is expelled outside the cell by sodium pump. Which •needs ATP as a source of energy. The reaction is catalyzed by •an enzyme called "Adenosine triphosphatase(ATPase)". •Active transport is much more faster than passive transport.•c) Insulin increases the number of glucose transporters in •tissues containing insulin receptors e.g. muscles and adipose •tissue. 08-Mar-1512 •2. Inhibitors of active transport:•a) Ouabin(cardiac glycoside): Inhibits adenosine triphosphatase•(ATPase) necessary for hydrolysis of ATP that produces energy •of sodium pump. •b) Phlorhizin; Inhibits the binding of sodium in the carrier protein.•B. Passive transport (facilitated diffusion):•Sugars pass with concentration gradient i.e. from high to low •concentration. It needs no energy. It occurs by means of a sodium •independent facilitative transporter (GLUT -5). Fructose and •pentosesare absorbed by this mechanism. Glucose and •galactosecan also use the same transporter if the concentration •gradient is favorable.•C.There is also sodium–independent transporter (GLUT-2), that •is facilitates transport of sugars out of the cell i.e. to circulation.08-Mar-1513 •III. Defects of carbohydrate digestion and absorption:•A. Lactase deficiency = lactose intolerance:•1. Definition: •a) This is a deficiency of lactase enzyme which digest lactose into •glucose and galactose•b) It may be:•(i) Congenital: which occurs very soon after birth (rare).•(ii) Acquired: which occurs later on in life (common).•2. Effect:The presence of lactose in intestine causes:•a) Increased osmotic pressure: So water will be drawn from the tissue •(causing dehydration) into the large intestine (causing diarrhea).•b) Increased fermentation of lactose by bacteria: Intestinal bacteria •ferment lactose with subsequent production of CO2 gas. This causes •distention and abdominal cramps.•c) Treatment: Treatment of this disorder is simply by removing lactose •(milk) from diet.08-Mar-1514 •B. Sucrose deficiency:•A rare condition, showing the signs and symptoms of lactase deficiency. It •occurs early in childhood.•C. Monosaccharide malabsorption:•This is a congenital condition in which glucose and galactoseare absorbed •only slowly due to defect in the carrier mechanism. Because fructose is not •absorbed by the carrier system, its absorption is normal.•IV. Fate of absorbed sugars:•Monosaccharides(glucose, galactoseand fructose) resulting from carbohydrate digestion are absorbed and undergo the following:•A. Uptake by tissues (liver):•After absorption the liver takes up sugars, where galactoseand fructose are converted into glucose.•B. Glucose utilization by tissues: •Glucose may undergo one of the following fate:08-Mar-1515 •1. Oxidation:through•a) Major pathways (glycolysisand Krebs cycle) for production of energy.•b) Hexosemonophosphatepathway: for production of ribose, deoxyribose•and NADPH + H+ •c) Uronicacid pathway, for production of glucuronicacid, which is used in •detoxicationand enters in the formation of mucopolysaccharide.•2. Storage:in the form of:•a) Glycogen: glycogenesis.•b) Fat: lipogenesis.•3.Conversion:to substances of biological importance:•a) Ribose, deoxyribose?RNA and DNA.•b) Lactose ?milk.•c) Glucosamine, galactosamine?mucopolysaccharides.•d) Glucoronicacid ?mucopolysaccharides.•e) Fructose ?in semen.08-Mar-1516 Glucose Oxidation•I. Glycolysis(EmbdenMeyerhof Pathway):•A. Definition:•1. Glycolysismeans oxidation of glucose to give pyruvate(in the •presence of oxygen) or lactate (in the absence of oxygen).•B. Site:•cytoplasm of all tissue cells, but it is of physiological importance in:•1. Tissues with no mitochondria: mature RBCs, cornea and lens.•2. Tissues with few mitochondria: Testis, leucocytes, medulla of the •kidney, retina, skin and gastrointestinal tract. •3. Tissues undergo frequent oxygen lack: skeletal muscles especially •during exercise.08-Mar-1517 •C. Steps:•Stages of glycolysis•1. Stage one(the energy requiring stage):•a) One molecule of glucose is converted into two molecules of •glycerosldhyde-3-phosphate.•b) These steps requires 2 molecules of ATP (energy loss) •2. Stage two(the energy producing stage(:•a) The 2 molecules of glyceroaldehyde-3-phosphate are converted into •pyruvate(aerobic glycolysis) or lactate (anaerobic glycolysis(.•b) These steps produce ATP molecules (energy production).•D. Energy(ATP) production of glycolysis:•ATP production = ATP produced -ATP utilized •08-Mar-1518 Glycolysistakes place in the cytosolof cells.Glucose enters the Glycolysis pathway by conversion to glucose-6-phosphate.Initially there is energy input corresponding to cleavage of two ~P bonds of ATP. HOOHHOHHOHCH2OPO32?HOHH165432glucose-6-phosphate 08-Mar-1519 HOOHHOHHOHCH2OHHOHHHOOHHOHHOHCH2OPO32?HOHH234561165432ATP ADPMg2+glucose glucose-6-phosphate Hexokinase 1.Hexokinasecatalyzes:Glucose+ATPàglucose-6-P+ADPThe reaction involves nucleophilic attack of the C6 hydroxyl O of glucose on P of the terminal phosphate of ATP. ATP binds to the enzyme as a complex with Mg++.08-Mar-1520 First Phase of GlycolysisThe first reaction -phosphorylationof glucose•Hexokinaseor glucokinase•This is a priming reaction -ATP is consumed here in order to get more later •ATP makes the phosphorylationof glucose spontaneous •Be SURE you can interconvert Keqand standard state free energy change •Be SURE you can use Eq. 3.12 to generate far right column of Table 19.108-Mar-1521 Hexokinase1st step in glycolysis; ?G large, negative•Hexokinase(and glucokinase) act to phosphorylateglucose and keep it in the cell •Kmfor glucose is 0.1 mM; cell has 4 mm glucose •So hexokinaseis normally active! •Glucokinase(Kmglucose= 10 mM) only turns on when cell is rich in glucose•Hexokinaseis regulated -allostericallyinhibited by (product) glucose-6-P -but is not the most important site of regulation of glycolysis-Why?08-Mar-1522 Energy production of glycolysis: Net energyATP utilizedATP produced2 ATP2ATPFrom glucose to glucose -6-p.From fructose -6-p to fructose 1,6 p.4 ATP (Substrate level phosphorylation) 2ATP from 1,3 DPG.2ATP from phosphoenolpyruvateIn absence of oxygen (anaerobic glycolysis)6 ATPOr8 ATP2ATP-From glucose to glucose -6-p.From fructose -6-p to fructose 1,6 p.4 ATP (substrate level phosphorylation) 2ATP from 1,3 BPG.2ATP from phosphoenol pyruvate.In presence of oxygen (aerobic glycolysis)+ 4ATP or 6ATP(from oxidation of 2 NADH + H in mitochondria).08-Mar-1523 •E. oxidation of extramitochondrialNADH+H+:•1. cytoplasmicNADH+H+ cannot penetrate mitochondrial membrane, •however, it can be used to produce energy (4 or 6 ATP) by respiratory •chain phosphorylationin the mitochondria.•2. This can be done by using special carriers for hydrogen of NADH+H+ •These carriers are either dihydroxyacetonephosphate (Glycerophosphate•shuttle) or oxaloacetate(aspartatemalateshuttle).•a) Glycerophosphateshuttle:•1) It is important in certain muscle and nerve cells.•2) The final energy produced is 4 ATP.•3) Mechanism:•-The coenzyme of cytoplasmicglycerol-3-phosphate dehydrogenase•is NAD+.•-The coenzyme of mitochodrialglycerol-3-phosphate dehydogenaseis •FAD.•-Oxidation of FADH, in respiratory chain gives 2 ATP. As glycolysis•gives 2 cytoplasmicNADH + H+ ?2 mitochondrial FADH, 2 x 2 •ATP ?= 4 ATP.•b) Malate–aspartateshuttle:•1) It is important in other tissues patricularyliver and heart.•2) The final energy produced is 6 ATP.08-Mar-1524 Differences between aerobic and anaerobic glycolysis:AnaerobicAerobicLactatePyruvate 1. End product2 ATP 6 or 8 ATP2 .energyThrough Lactate formation Through respiration chain in mitochondria3. Regeneration of NAD+Not available as lactate is cytoplasmic substrateAvailable and 2 Pyruvate can oxidize to give 30 ATP4. Availability to TCA in mitochondria08-Mar-1525 2. PhosphoglucoseIsomerasecatalyzes: glucose-6-P(aldose) ?àfructose-6-P(ketose) The mechanism involves acid/base catalysis, with ring opening, isomerizationvia an enediolateintermediate, and then ring closure. A similar reaction catalyzed by TriosephosphateIsomerasewill be presented in detail. HOOHHOHHOHCH2OPO32?HOHH165432CH2OPO32?OHCH2OHHOHHHHOO654321glucose-6-phosphate fructose-6-phosphate Phosphoglucose Isomerase 08-Mar-1526 3.Phosphofructokinasecatalyzes:fructose-6-P+ATPàfructose-1,6-bisP+ADPThis highly spontaneousreaction has a mechanism similar to that of Hexokinase. The Phosphofructokinasereaction is the rate-limiting stepof Glycolysis. The enzyme is highly regulated, as will be discussed later. CH2OPO32?OHCH2OHHOHHHHOO654321CH2OPO32?OHCH2OPO32?HOHHHHOO654321ATP ADPMg2+ fructose-6-phosphate fructose-1,6-bisphosphate Phosphofructokinase 08-Mar-1527 PhosphoglucoisomeraseGlucose-6-P to Fructose-6-P •Why does this reaction occur?? –next step (phosphorylationat C-1) would be tough for hemiacetal-OH, but easy for primary -OH –isomerizationactivates C-3 for cleavage in aldolasereaction •Ene-diolintermediate in this reaction •Be able to write a mechanism!08-Mar-1528 4.Aldolasecatalyzes:fructose-1,6-bisphosphate?àdihydroxyacetone-P + glyceraldehyde-3-PThe reaction is an aldolcleavage, the reverse of an aldolcondensation. Note that C atoms are renumbered in products of Aldolase.654321CH2OPO32?CCCCCH2OPO32?OHOHHOHHOH321CH2OPO32?CCH2OHOCCCH2OPO32?HOHOH+123 fructose-1,6- bisphosphate Aldolase dihydroxyacetone glyceraldehyde-3- phosphate phosphate Triosephosphate Isomerase 08-Mar-1529 A lysineresidue at the active site functions in catalysis. The ketogroup of fructose-1,6-bisphosphate reacts with the e-amino group of the active site lysine, to form a protonated Schiff baseintermediate.Cleavage of the bond between C3 & C4 follows. CH2OPO32?CCHCCCH2OPO32?NHHOHOHHOH(CH2)4Enzyme654321+Schiff base intermediate of Aldolase reaction H3N+CCOO?CH2CH2CH2CH2NH3H? lysine 08-Mar-1530 5.TriosePhosphateIsomerase(TIM)catalyzes:dihydroxyacetone-P?àglyceraldehyde-3-PGlycolysiscontinues from glyceraldehyde-3-P. TIM s Keqfavors dihydroxyacetone-P. Removal of glyceraldehyde-3-P by a subsequent spontaneous reaction allows throughput. 654321CH2OPO32?CCCCCH2OPO32?OHOHHOHHOH321CH2OPO32?CCH2OHOCCCH2OPO32?HOHOH+123 fructose-1,6- bisphosphate Aldolase dihydroxyacetone glyceraldehyde-3- phosphate phosphate Triosephosphate Isomerase 08-Mar-1531 The ketose/aldose conversion involves acid/base catalysis, and is thought to proceed via an enediolintermediate, as with Phosphoglucose Isomerase. Active site Glu and His residues are thought to extract and donate protons during catalysis. CCCH2OPO32?OCCCH2OPO32?HOHOHCCCH2OPO32?HOHOHHHOHH+H+H+H+dihydroxyacetone enediol glyceraldehyde- phosphate intermediate 3-phosphate Triosephosphate Isomerase 08-Mar-1532 Hexokinase Phosphofructokinase glucose Glycolysis ATP ADP glucose-6-phosphate Phosphoglucose Isomerase fructose-6-phosphate ATP ADP fructose-1,6-bisphosphate Aldolase glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate Triosephosphate Isomerase Glycolysis continued 08-Mar-1533 Glyceraldehyde-3-phosphate Dehydrogenase Phosphoglycerate Kinase Enolase Pyruvate Kinase glyceraldehyde-3-phosphate NAD+ + Pi NADH + H+ 1,3-bisphosphoglycerate ADP ATP 3-phosphoglycerate Phosphoglycerate Mutase 2-phosphoglycerate H2O phosphoenolpyruvate ADP ATP pyruvate Glycolysis continued.Recall that there are 2 GAP per glucose.08-Mar-1534 Balancesheetfor~PbondsofATP:wHow many ATP ~P bonds expended? ________wHow many ~P bonds of ATP produced? (Remember there are two 3C fragments from glucose.) ________ wNet production of ~Pbonds of ATP per glucose: ________ 08-Mar-1535 They must reoxidizeNADHproduced in Glycolysisthrough some other reaction, because NAD+is needed for the Glyceraldehyde-3-phosphate Dehydrogenasereaction. Usually NADH is reoxidizedas pyruvateis converted to a more reducedcompound. The complete pathway, including Glycolysisand the reoxidationof NADH, is called fermentation. CCCH2OPO32?HOHOHCCCH2OPO32?OOPO32?HOH+ Pi + H+NAD+ NADH123231glyceraldehyde- 1,3-bisphospho- 3-phosphate glycerate Glyceraldehyde-3-phosphate Dehydrogenase Fermentation:Anaerobic organismslack a respiratory chain.08-Mar-1536 CCCH3O?OOCHCCH3O?OHONADH + H+ NAD+Lactate Dehydrogenase pyruvate lactate E.g., Lactate Dehydrogenasecatalyzes reductionof the ketoin pyruvateto a hydroxyl, yielding lactate, as NADH is oxidized to NAD+. Lactate, in addition to being an end-product of fermentation, serves as a mobileform of nutrient energy, & possibly as a signalmolecule in mammalian organisms.Cell membranes contain carrierproteins that facilitate transport of lactate.08-Mar-1537 CCCH3O?OOCHCCH3O?OHONADH + H+ NAD+Lactate Dehydrogenase pyruvate lactate Skeletal musclesferment glucose to lactateduring exercise, when the exertion is brief and intense. Lactatereleased to the bloodmay be taken up by other tissues, or by skeletal muscle after exercise, and converted via Lactate Dehydrogenaseback topyruvate, which may be oxidized in Krebs Cycleor (in liver) converted to back to glucosevia gluconeogenesis08-Mar-1538 •Lactateserves as a fuelsource for cardiac muscleas well as brain neurons. •Astrocytes, which surround and protect neurons in the brain, ferment glucoseto lactateand release it. •Lactatetaken up by adjacent neurons is converted to pyruvatethat is oxidized via Krebs Cycle. Some anaerobic organisms metabolize pyruvateto ethanol, which is excreted as a waste product. NADHis converted to NAD+in the reaction catalyzed by Alcohol Dehydrogenase. Glycolysis, omitting H+: glucose + 2 NAD++ 2 ADP + 2 Pià2 pyruvate+ 2 NADH + 2 ATPFermentation, from glucose to lactate:glucose+2ADP+2Pià2lactate+2ATPAnaerobic catabolismof glucose yields only 2 “high energy” bonds of ATP. 08-Mar-1539 •Substrate level phosphorylation:•This means phosphorylationof ADP to ATP at the reaction itself .in•glycolysisthere are 2 examples:•-1.3 Bisphosphoglycerate+ ADP 3 Phosphoglycerate+ ATP •-Phospho-enolpyruvate+ ADP Enolpyruvate+ ATP•I. Special features of glycolysisin RBCs:•1. Mature RBCs contain no mitochondria, thus:•a) They depend only upon glycolysisfor energy production (=2 ATP).•b) Lactate is always the end product.•2. Glucose uptake by RBCs is independent on insulin hormone.•3. Reduction of met-hemoglobin: Glycolysisproduces NADH+H+, which •used for reduction of met-hemoglobin in red cells.08-Mar-1540 •Biological importance (functions) of glycolysis:•1. Energy production:•a) anaerobic glycolysisgives 2 ATP.•b) aerobic glycolysisgives 8 ATP.•2. Oxygenation of tissues:•Through formation of 2,3 bisphosphoglycerate, which decreases the •affinity of Hemoglobin to O2.•3. Provides important intermediates:•a) Dihydroxyacetonephosphate: can give glycerol-3phosphate, which is •used for synthesis of triacylglycerolsand phospholipids (lipogenesis).•b) 3 Phosphoglycerate: which can be used for synthesis of amino acid •serine.•c) Pyruvate: which can be used in synthesis of amino acid alanine.•4. Aerobic glycolysisprovides the mitochondria with pyruvate, which gives •acetyl CoAKrebs cycle.08-Mar-1541 •Reversibility of glycolysis(Gluconeoqenesis):•1. Reversible reaction means that the same enzyme can catalyzes the •reaction in both directions.•2. all reactions of glycolysis-except 3-are reversible.•3. The 3 irreversible reactions (those catalyzed by kinaseenzymes) can be •reversed by using other enzymes.•Glucose-6-p ?Glucose •F1, 6 Bisphosphate?Fructose-6-p•Pyruvate?Phosphoenolpyruvate•4. During fasting, glycolysisis reversed for synthesis of glucose from non-•carbohydrate sources e.g. lactate. This mechanism is called: •gluconeogenesis.08-Mar-1542 Comparison between glucokinaseand hexokinaseenzymes:HexokinaseGlucokinaaseAll tissue cellsLiver only1. SiteHigh affinity (low km) i.e. it acts even in the presence of low blood glucose concentration.Low affinity (high km) i.e. it acts only in the presence of high blood glucose concentration.2. Affinity to glucoseGlucose, galactose and fructoseGlucose only3. Substrate No effect Induces synthesis of glucokinase.4. Effect of insulinAllosterically inhibits hexokinaseNo effect5.Effect of glucose-6-pIt phosphorylates glucose inside the body cells. This makes glucose concentration more in blood than inside the cells. This leads to continuous supply of glucose for the tissues even in the presence of low blood glucose concentration.Acts in liver after meals. It removes glucose coming in portal circulation, converting it into glucose -6-phosphate.6. Function08-Mar-1543 Importance of lactate production in anerobicglycolysis:1. In absence of oxygen, lactate is the end product of glycolysis: Glucose ?Pyruvate?Lactate2. In absence of oxygen, NADH +H+is not oxidized by the respiratory chain.3. The conversion of pyruvateto lactate is the mechanism for regeneration of NAD+.4. This helps continuity of glycolysis, as the generated NAD+ will be used once more for oxidation of another glucose molecule. 08-Mar-1544 •As pyruvateenters the mitochondrion, a multienzymecomplex modifies pyruvateto acetyl CoAwhich enters the Krebs cycle in the matrix.–A carboxyl group is removed as CO2.–A pair of electrons is transferred from the remaining two-carbon fragment to NAD+to form NADH.–The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA.08-Mar-1545