METABOLISM OF FOODSTUFFS Ptns, CHO, lipids carbon compounds CO2 & H2O excretion
Dietary Carbohydrates: Monosaccharides: glucose, fructose and galactosein fruits and honey & obtained by hydrolysis of oligo- & polysacs. Disaccharides: sucrose, lactose, maltose (by hydrolysis of starch). Polysaccharides:starch (in potatoes, rice, corn and wheat)Cellulose (in cell wall of plants)not digested by humans due to absence of cellulase
Digestion of Carbohydrates:In the mouth:Salivary amylase hydrolyzes starch into dextrin +maltoseIn the stomach:due to drop of pH salivary amylase acts for a very short timeIn the small intestines:Pancreatic and intestinal enzymes hydrolyze the oligo- and polysaccharides as follows: Pancreatic amylase Starch maltose + isomaltose Maltase Maltose 2 glucose Lactase Lactose glucose + galactose Sucrase Sucrose glucose + fructose
Absorption of monosaccharides:1. Simple diffusion:Depending on the concn gradient of sugars between intestinal lumen and mucosal cells.e.g. Fructose and pentose2. Facilitated transport:It requires a transporter.e.g. Glucose, Fructose and galactose3. Active transport (cotransport):It needs energy derived from the hydrolysis of ATP.glucose & galactose are actively transported againsttheir concentration gradients by this mechanism.
Fate of absorbedmonosaccharides:In the liver, fructose and galactose are convertedto glucose. Fate of glucose:A. Uptake by different tissues (by facilitated diffusion)B. Utilization by the tissues: in the form of:1. Oxidation to produce energy: - Major pathways (glycolysis & Krebs cycle). - Minor pathways (hexose monophosphate pathway & uronic acid pathway)2. Conversion to other substances:Carbohydrates: ribose (RNA,DNA), galactose (in milk), fructose (semen)Lipids: Glycerol-3 P for formation of triacylglycerols.Proteins: Non-essential amino acids which enter in formation of proteins.C. Storage of excess glucose:as glycogen in liver and muscles,when these reserves are filled it is converted to TAG & deposited in adipose tissue.D. Excretion in urineIf blood glucose exceeds renal threshold (180 mg/dL), it will be excreted in urine.
Glucose OxidationExtracting Energy from Glucose:There are 3 major biochemical processes that occur incells to progressively breakdown glucose with therelease of various packets of energy:Glycolysis (occurs in the cytoplasm and is only moderately efficient).Krebs cycle (takes place in the matrix of the mitochondria and results in a great release of energy).Electron transport chain.
GLYCOLYSISSeries of biochemical reactions by whichglucose is converted to:-Pyruvate (in aerobic conditions)or-Lactate (in anaerobic conditions).Site: cytosol of every cell.Physiologically it occurs in:-muscles during exercise (lack of oxygen)-RBCs (no mitochondria).
Steps:Phase one: 1 molecule of glucose (C6) is converted to 2 molecules of glyceraldehyde 3-phosphate (C3)as follows: ATP ATPGlucose (C6) 2 Glyceraldehyde 3 P (C3)
Phase two: in this phase the 2 molecules ofglyceraldehyde 3-P are converted to 2 molecules ofpyruvate (aerobic)or lactate (anaerobic): 4 ATP2 Glyceraldehyde-3 P (C3) 2 Pyruvic Acid (C3) 2 NADH + 2 H+ 2 NAD+ 2 Lactic Acid
Overall, glycolysis can thus be summarizedas follows:Glucose 2 Pyruvic Acid + 2 net ATP +4 hydrogens (2 NADH2) 2 Lactic Acid + 2 net ATP
Regulation ofGlycolysis:It can be noted that all reactions of glycolysisare reversible except those catalyzed by: Glucokinase (or hexokinase) (GK) Phosphofructokinase (PFK) Pyruvate kinase (PK)Glycolysis is regulated by factors whichcontrol the activity of the key enzymeswhich catalyze the 3 irreversiblereactions.
Activity of these enzymes increase duringCHO feeding, and decreases duringstarvation: Regulation according to energy requirements of cell Regulation by hormones
Regulation according toenergy requirements of cell:Each cell regulates glycolysis according tothe rate of utilization of ATP:i) High levels of AMP(indicating high ATP utilization):+++ PFK (i.e. activates glycolysis).ii)High levels of ATP(indicating little utilization of ATP):- - -PFK and PK (i.e. inhibits glycolysis).
Regulation by hormones:Postprandial hyperglycemia causes:+++ of insulin--- glucagon & adrenaline (anti-insulin hormones)i) Insulin:+++ all pathways of glucose utilization.+++ glycolysis by inducing synthesis, activationof all the glycolytic key enzymes (GK, PFK, PK).ii) Glucagon:Inhibits glycolysis by acting asrepressor & inactivator of the glycolytic key enzymes.
Importance ofGlycolysis:1. Glycolysis provides mitochondria with pyruvic a oxaloacetate which is the primer of the Krebs cycle.2. Glycolysis provides dihydroxyacetone P glycerol 3-P that is important for lipogenesis (TAG synthesis)3. Energy production:Glycolysis liberates only a small part of energy from glucose, however:a. Important during severe muscular exercise, where oxygen supply is often insufficient to meet the demands of aerobic metabolism.b. Provides all energy required by the R.B.Cs. (due to lack of mitochondria).
Energy yield ofglycolysis:In absence of oxygen:2 ATP are consumed for conversion of glucose to Fructose 1,6 P.2 ATP are produced during conversion of glyceraldehydes 3-P to pyruvate.Since 1 glucose molecule gives 2 molecules ofG 3-P, then total number of ATP produced is 4.net gain of ATP in absence of oxygen is: 4-2=2 ATP.
Energy yield ofglycolysis:In presence of oxygen:2 ATP are produced directly(as in absence of oxygen),6 ATP are produced indirectly:from oxidation of 2 NADH2 through ETCnet gain of ATP in presence of oxygen is: 2+6= 8 ATP.
Reactions These link glycolysis to the Krebs Cycle Alternate Fates of PyruvateA. Oxidative Decarboxylation B. Carboxylation forms Acetyl CoA forms Oxaloacetate
Oxidative decarboxylation ofpyruvate:Puruvate dehydrodenase complex irreversibly converts pyruvate into acetyl CoA:Pyruvic acid (3C)+NAD++Coenzyme AAcetyl CoA(2C)+CO2+ NADH+ H+Acetyl CoA can also be produced by breakdown of: lipids or certain (ketogenic) amino acids.-NAD+ is converted into NADH+H+.Those hydrogens go through oxidative phosphorylation and produce 3 more ATP.
Oxidative decarboxylation of pyruvate:NADH+H NADH+H 2 CoA
Carboxylation of pyruvate to oxaloacetate:Pyruvate carboxylase convertspyruvate to oxaloacetate.Pyruvic acid (3C) + CO2 + ATPOxaloacetic acid (4C) + ADP + Pi
Finally, comes the KrebsCycle Krebs Cycle (Citric Acid Cycle) (Tricarboxylic Acid Cycle) "TCA"
Site: mitochondria of everycellSeries of biochemical reactions that areresponsible for complete oxidation ofCHO, fats and Ptns to form :CO2 + H2O + Energy
During this process the following isproduced: 3x2=6 NADH+H+ 1x2=2 FADH2 1x2=2 ATP 2x2=4 CO2
Each glucose molecule that goes through Krebs cycle+ the preparatory conversion to Acetyl CoA gives: 8 NADH 2 FADH2 2 ATP 6 CO2 N.B.: glycolysis produced 2 ATP + 2 NADH, so there is a net production of: 4 ATP 10 NADH
Energy yield of Krebs cycle:Glucose 2 puruvate 2 NADH 6 ATP 2 oxaloacetate 6 ATP 6 ATP 4 ATP 6 ATP
Energy yield of Krebscycle:1 mole of acetyl CoA through Krebs cycle produces 12 ATPs:1 ATP (substrate level oxidative phosphorylation).1 FADH2 → 2 ATP (respiratory chain oxidative phosphorylation).3 NADH+H+→9 ATP(respiratory chain oxidative phosphorylation)oxidative decarboxylation of pyruvate gives 1 NADH+H+ → 3 ATPThus net ATP gain is: 12 + 3 = 15 ATPSince 1 glucose molecule by undergoing glycolysis gives 2 pyruvateThus 1 glucose molecule yields 15 × 2 = 30 ATP.
Thus complete oxidation ofglucose (in presence of oxygen)gives: Glycolysis: 8 ATP Total ATP yield = 30+8 = 38 ATP.
Regulation of Krebscycle:1. Regulation according to energy status of the cell:+++NADH/NAD and ATP/ADP (thus no need for further energy production) inhibit the cycle, and vice versa.Krebs cycle is only aerobic, since under anaerobic conditions the respiratory chain is inhibited leading to increased NADH/NAD ratio which inhibits the cycle.2. Regulation according to availability of substrate:+++ acetyl CoA and oxaloacetate +++ cycle.+++ intermediate products of cycle (citrate & succinyl Co A) ---feedback inhibition of the cycle.
Importance of Krebscycle:1. Energy production: 1 acetyl CoA yields 12 ATP.2. It is the final common metabolic pathway for complete oxidation of acetyl CoA which results from the partial oxidation of CHO, fats and proteins (amino acids).3. Interconversion of carbohydrates, fats and proteins (gluconeogenesis, lipogenesis, and formation of non-
Minor Pathways of GlucoseOxidation: Hexose monophosphate pathway (HMP shunt). Uronic acid pathway.
Hexose Monophosphate Pathway(HMP shunt)Pentose Phosphate PathwayPentose Shunt Site: cytoplasm of cells e.g. liver, adipose tissue, adrenals, gonads, RBCs and retina. Steps: Glucose-6-P dehydrogenase G-6-P R-5- P NADP+ CO2 NADPH+H+
Importance of HMP shuntR-5-P Importance NADPH for RBCs
Importance of HMPshunt:2. It is the main source of NADPH:coenzyme for reductases, hydroxylases and NADPH oxidasewhich catalyze several important biochemical reactions, e.g.:i) Fatty acid synthesis lipogenesis:HMP is active in liver, adipose tissue & lactating memory gland.ii) Steroid synthesis:HMP is active in adrenal cortex, testis, ovaries and placenta.iii) Important for vision: NADPHretinal retinol (important for
3) Importance of HMP in RBCs:-H2O2 (powerful oxidant) produces damage of: cellular DNA, Ptns phospholipids of cell membrane.-RBCs are liable to oxidative damage by H2O2 due totheir role in O2 transport.-H2O2 produces oxidative damage in the form of: Oxidation of Fe2+ to Fe3+ (metHb can’t carry O2) Lipid peroxidation which increases cell membrane fragility.
HMP in RBCs produces NADPH, whichprovides reduced GSH to remove H2O2 protects cell from oxidative damageGSH reductase & GSH peroxidase remove
Favism: Genetic condition due to deficiency of (G6PD), There is impaired HMP in the RBCs, and RBC capacity to protect itself from oxidative damage is markedly decreased (--- NADPH) Eating Fava beans (which contain oxidizing agents), or administration of certain drugs (e.g. aspirin, sulfonamides or primaquin) which stimulate production of H2O2, produce lysis of the fragile red cells.
Regulation of HMP: NADPH produces feedback (-) G6PD. Insulin produces (+) G6PD.N.B:Insulin produced in response tohyperglycemiaincrease glucose oxidation by HMP( acts as inducer of synthesis of
Uronic Acid PathwayThis pathway converts glucose to glucuronic acid.Site: cytosol of liver cells.Importance of Uronic Acid Pathway:enters in different biological reactions, e.g.:1. Synthesis of glycosaminoglycans (GAGs).2. Conjugation with certain compounds rendering them more water soluble, thus helping in their excretion, e.g.: Steroid hormones. Bilirubin, which is excreted in bile in the form of bilirubin diglucuronide.
Glycogen Metabolism1. Liver glycogen:-Forms 8-10% of the wet weight of the liver.-Maintains blood glucose (especially between meals).-Liver glycogen is depleted after 12-18 hours fasting.2. Muscle glycogen:-Forms 2% of the wet weight of muscle.-Supplies glucose within muscles during contraction.-Muscle glycogen is only depleted after prolonged exercise.
Glycogen metabolism includes: Glycogenesis: synthesis of glycogen from glucose. Glycogenolysis: breakdown of glycogen to glucose-1-phosphate. Gluconeogenesis: synthesis of glucose or glycogen from non-CHO precursors.
Glycogenesis &GlycogenolysisSite: cytoplasm of liver and muscles.The key enzyme of glycogenesis is glycogen synthase.The key enzyme of glycogenolysis is glycogen phosphorylase.In muscles: G-6-P is oxidized by glycolysis to provide energyduring muscle contraction.In liver: G-6-PhosphataseG-6-P Glucose + Pi Blood GN.B: Muscles cannot supply blood glucose due to their lack of the enzyme G-6-phosphatase.
Mechanism of glycogensynthesis (glycogenesis):A. Synthesis of UDP-glucose.B. Synthesis of a primer to initiate glycogen synthesis:A fragment of glycogen (present in cells whose glycogen stores are not totally depleted) can serve as a primer.C. Elongation of glycogen chains by glycogen synthase:-Glycogen synthase uses UDP-glucose to add glucose to glycogen primer (1,4 link), and the process is repeated.D. Formation of branches in glycogen:-When the chain becomes about 6-11 glucose units long, the branching enzyme transfers 5-8 glucosyl residues of α-1,4-chain to a neighboring chain attaching it by α- 1,6- glucosidic linkage
Mechanism of glycogendegradation(glycogenolysis)A. Shortening of chains:Glycogen phosphorylase acts on the 1,4-glucosidic linkage of glycogen G-1-P residuesuntil each branch contains only 4 glucose units.B. Removal of branches:-The transferring enzyme transfers 3 glucose units from one endof the short branch to the end of another branch.-The debranching enzyme cleaves 1,6-glucosidic linkage releasing free G , and the process is repeated.C. Conversion of G-1-P to G-6-P:This is done by phosphoglucomutase enzyme.
Regulation of GlycogenSynthesis vs.DegradationGlycogen synthase & glycogen phosphorylase: key enzymesRegulation of these enzymes occurs via: Covalent modification (phosphorylation & dephosphn.) Allosterics Hormones-Reciprocal control of the two pathways is hormonally mediated through phosphorylation and dephosphorylation of synthase and phosphorylase.-Phosphorylation of enzymes :turns synthesis off (--- glycogenesis), andturns degradation on (+++ glycogenolysis).
Covalent modification :Phosphorylation/dephosphorylationI. Glycogen synthase is present in two forms:a-form: it is the active form and it is dephosphorylated.b-form: it is the inactive form and it is phosphorylated.-Conversion of a- to b-form by protein kinase: ++ by c- AMP-Conversion of b- to a-form by protein phosphatase.II. Glycogen phosphorylase is present in two forms:a-form: it is the active form and it is phosphorylated.b-form: it is the inactive form and it is dephosphorylated.-Conversion of a- to b-form by the enzyme protein phosphatase.-Conversion of b- to a-form by phosphorylase kinase:+by c-AMP
Allosteric regulation:Conformational changes in the enzyme ptns affecting activity and regulation: Glucose-6-phosphate++ synthase (+) glycogenesis (excess substrate).- - phosphorylase (-) glycogenolysis & (+) glycogenesis. ATP+ + synthase (+) glycogenesis- - phosphorylase (-) glycogenolysis. Ca+2++ phosphorylase kinase (+) glycogen phosphorylase glycogenolysis-Muscle contraction ---> Ca+2 release (+) phosphorylase glycogenolysis(+) glucose ATP generation for ensuing cycles of muscle contraction.
Insulin:++ phosphodiesterase - cAMP - protein kinase++ protein phosphataseA. stimulates glycogenesis:b- a-form of glycogen synthase (activation)activation of glycogenesis in both liver and muscle.B. inhibits glycogenolysis:a- b-form of glycogen phosphorylase (inactivation)This leads to inactivation of glycogen phosphorylase(conversion of active to the inactive form)decrease glycogenolysis in both liver and muscle.
B. Glucagon (in liver) andepinephrine (in liver and muscles):Both hormones produce activation of adenyl cyclase thusincreasing cAMPThis produces activation protein kinase.This converts:1. Active a- to inactive b-form of glycogen synthase(phosphorylated), thus inhibiting synthase.Accordingly glucagon & epinephrine --- glycogenesis.2.Inactive b- to active a-form of glycogen phosphorylase,thus activating glycogen phosphorylase.Accordingly glucagon & epinephrine +++glycogenolysis.
Regulation according tonutritional status:A. In the well fed state:Glycogen synthase is allosterically (+) by G6P (which is present in high concentrations).Glycogen phosphorylase is (-) by G6P & ATP, i.e.(-)glycogenolysis & (+)glycogenesis stores bl glucoseB. During starvation:There are decreased levels of G6P & ATP, thus(-)glycogenesis & (+)glycogenolysis to supply blood glucose.
Muscle glycogen andblood glucoseMuscle glycogen can be converted toBl glucose via indirect pathways:Coris cycle:during muscle exerciseGlucose- alanine cycle:during starvation
Glycogen storagediseases:Inherited deficiencies of specific enzymes ofglycogen metabolism.Von Gierkes disease (most common)Cause: deficiency of G-6-phosphatase.It is characterized by:-enlargement of liver and kidneys-hypoglycemia-hyperlipemia-hypercholestorelemia.
GluconeogenesisSynthesis of glucose from non- carbohydrateprecursors.These precursors are metabolic intermediates.Importance:Supply blood glucose in case of CHO deficiency>18 hrs. (fasting, starvation and low CHO diet).Site:Cytosol of liver cells
Steps:By reversal of glycolysis.3 glycolytic key enzymes are reversed by4 key enzymes of gluconeogenesisas follows:
Glucogenic Precursors:They give directly or indirectly pyruvate, oxaloacetate or any intermediates of glycolysis or Krebs cycle. They include:1. Lactate:It is released by R.B.Cs. and by skeletal muscles during exercise, then transferred to the liver to form pyruvate then glucose.2. Glycerol:It is produced from digestion of fats and from lipolysis.
3. Glucogenic amino acids:Ptns are the main sources of blood glucose especially after 18 hrsdue to depletion of liver glycogen.-Some amino acids by deamination directly form pyruvic acid or oxaloacetic.-Others may give intermediates of Krebs cycle which go through the cycle eventually yielding oxaloacetic acid.4. Propionyl CoA:Many amino acids may give propionyl CoA through their catabolism. Also the last 3 carbons of odd chain fatty acids form propionyl CoA and thus give glucose. This is uncommon in humans.
Regulation ofgluconeogenesis:Gluconeogenic regulatory key enzymes are those whichreverse the glycolytic key enzymes.Glycolysis and gluconeogenesis are reciprocally controlled:Insulin:(secreted after carbohydrate meal)--- gluconeogenic key enzymes (at the same time it acts as inducer of glycolytic key enzymes) decrease bl. Glucose.Anti-insulin hormones (glucagon, epinephrine,glucocorticoids & growth hormone):(secreted during fasting, stress or severe muscular exercise)+++ gluconeogenic key enzymes, thus increasing glucose. increased blood gluconeogenesis
Blood GlucoseConcentration of bloog glucose:fasting blood glucose (8-12 hrs. after the last meal) is 70-110 mg/dL.It increases after meals but returns to fasting level within 2 hrs.Sources of blood glucose:Dietary carbohydrates.Glycogenolysis (during fasting for less than 18 hrs.).Gluconeogenesis (during fasting for more than 18 hrs.).
Regulation of BloodGlucose:Four factors are important for regulating blood glucose level:I. Gastrointestinal tract.II. LiverIII. Kidney.IV.Hormones.
I. Gastrointestinaltract:1. It controls the rate of glucose absorption. The maximum rate of glucose absorption is1 gm/kg body weight/ hour.An average person weighing 70 gm will absorb 70 gm glucose/ hour.2. Glucose given orally stimulates more insulin than intravenous glucose. This may be due to secretion of glucagon- like substance by intestines. This stimulates B-cells of pancreas to secrete more insulin. This is called anticipatory action.
II. Liver: The liver is the main blood glucostatMaintains blood glucose level within normal as follows:A. If blood glucose level increases, the liver controls this elevation and decreases it through:1. Oxidation of glucose via major and minor pathways.2. Glycogenesis.3. Lipogenesis.B. If blood glucose level decreases, the liver controls this drop and increases it through:1. Glycogenolysis.2. Gluconeogenesis.
III. Kidney:All glucose in blood is filtered through the kidneys, it then completely returns to the blood by tubular reabsorption.If blood glucose exceeds a certain limit (called renal threshold), it will pass in urine causing glucosuria.Renal threshold: it is the maximum rate of reabsorption of glucose by the renal tubules. Normally the renal threshold for glucose is 180 mg/100mL.
IV. Hormones:A. Insulin (the only hypoglycemic hormone):Action of insulin:Insulin decreases bl glucose level by:1. +++ oxidation of glucose2. +++ glycogenesis3. --- glycogenolysis4. --- glyconeogenesis5. +++ lipogenesis
B. Anti-Insulin Hormones:(hyperglycemichormones):1. Growth Hormone:It elevates the blood glucose level by stimulating gluconeogenesis.2. Thyroxine:It elevates the blood glucose level by:Increasing the rate of absorption of glucose from intestines.Stimulating gluconeogenesis and glycogenolysis.Inhibiting glycogenesis.3. Epinephrine (adrenaline):It increases the blood glucose level by increasing glycogenolysis in both liver and muscles.4. Glucagon:It increases the blood glucose level by increasing glycogenolysis in liver only.
Mechanism of BloodGlucose Regulation(Glucose Homeostasis)The blood glucose level is regulated by the balance between the action of insulin and anti-insulin hormones (hyperglycemic hormones).After a carbohydrate meal:Bl glucose increases, stimulating the secretion of insulin which tends to decrease the blood glucose level by its various actions.During fasting:Bl glucose is low; this stimulates the secretion of the anti- insulin hormones (hyperglycemic hormones) which by their various mechanisms lead to increasing the blood glucose level.The net result is a condition of glucose equilibrium, or what we call the homeostatic mechanism.
Abnormalities of BloodGlucose LevelThese may be in the form of: Hyperglycemia HypoglycemiaHyperglycemia: (Diabetes Mellitus):It is due to:decreased insulin secretion and/orhypersecretion of anti-insulin
Hypoglycemia:-It is the decrease in blood glucose level below the fasting level.At a level of 50mg/100 mL convulsions occurAt a level of 30 mg/100 mL coma and death result.-Hypoglycemia is more dangerous than hyperglycemia because glucose is the only fuel to the brain.Causes:i. Excess insulin: a) Overdose of insulin. b) Tumor of B-cells of pancreas (insulinoma).ii. Hyposecretion of anti-insulin hormones: (hypo-functions of the pituitary gland, adrenals & thyroid gland). insulin acts unopposed causing lowering of blood glucoseiii. Liver disease: hypoglycemia is due to decreased glycogen stores and
GlucosuriaPresence of detectable amounts of glucose in urine (>30 mg/dL).Causes:A. Hyperglycemic glocusuria:Bl glucose exceeds the renal threshold (180mg/dL). It is caused by:1. Diabetes mellitus.2. Emotional or stress glucosuria (epinephrine glucosuria)3. Alimentary glucosuria;It is due to increased rate of glucose absorption as in cases of gastrectomy or gastrojejunostomy.B. Normoglycemic or renal glucosuria:1. Congenital renal glucosuria (diabetes innocens):due to congenital defect in renal tubular reabsorption of glucose.2. Acquired renal disease (e.g. nephritis).