Glycolysis سرنوشت گلوکز

Oxidation of glucose is known as Glycolysis. Either Aerobic  Pyruvate Anaerobic  Lactic Acid Occurs in the Cytosol

120 grams of glucose / day = 480 calories

OrganGlucose transporterHK couplerClassification Brain GLUT1HK-IGlucose dependent Erythrocyte GLUT1HK-IGlucose dependent Adipocyte GLUT4HK-IIInsulin dependent Muscle GLUT4HK-IIInsulin dependent Liver GLUT2HK-IVLGlucose sensor GK  - cell GLUT2HK-IVB ( glucokinase ) Glucose sensor Gut GLUT3-symporter----Sodium dependent Kidney GLUT3-symporter----Sodium dependent Glucose transporters

Glycolyis in Detail

Outline of Glycolysis An Enzyme- and Coenzyme- mediated catabolic pathway Glucose Pyruvate NAD + ATP NADH ATP Pyruvate NAD + NADH 6-Carbon Compounds 3-Carbon Compounds Sugar-SplittingGlyco-lysis!

Synopsis of Glycolysis 1.C 6 (a.k.a., glucose) + ATP  C 6 -P + ADP 2.C 6 -P + ATP  P-C 6 -P + ADP 3.P-C 6 -P  2C 3 -P (this is the sugar-splitting step) (note: the stoichiometry of all of the following are 2 for every one glucose) 4.C 3 -P + NAD + + Pi  P-C 3 -P + NADH + H + 5.P-C 3 -P + ADP  C 3 -P + ATP 6.C 3 -P + ADP  C 3 (a.k.a., pyruvate) + ATP

Substrate-Level Phosphorylation

Mitochondrial Reactions

Pyruvate Pyruvate Aerobic Glycolysis Aerobic Glycolysis Alcohol Lactic Acid Fermentation Fermentation Fate of Pyruvate

Pyruvate - Pyruvate can be further processed: a) aerobically to CO 2 and H 2 O via the citric acid cycle. This yields far more energy in the form of ATP lactic acid (lactate) b) anaerobically to lactic acid (lactate) in muscle and in certain micro-organisms ethanol c) anaerobically to ethanol (fermentation) or, d) anaerobically to amino acids (alanine)

Lactate Fermentation 1. Lactate Fermentation Enzyme = Lactate Dehydrogenase COO - C=O + NADH + H +  H-C-OH + NAD + CH 3 pyruvate lactate - Note: uses up all the NADH (reducing equivalents) produced in glycolysis.

Glycogenesis بیوسنتز گلیکوژن

Digestion Pre-stomach – Salivary amylase :  1-4 endoglycosidase G G G G G G G G  1-4 link G G G G  1-6 link G G G GG G G G G G G G G G G maltose G G G isomaltose amylase maltotriose G G G G  Limit dextrins

Glycogen Metabolism

Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis As glucose residues are added to glycogen, UDP-glucose is the substrate and UDP is released as a reaction product. Glycogen synthesis

UDP-glucose is formed from glucose-1-phosphate:  glucose-1-phosphate + UTP  UDP-glucose + PP i  PP i + H 2 O  2 P i Overall:  glucose-1-phosphate + UTP  UDP-glucose + 2 P i Spontaneous hydrolysis of the ~P bond in PP i (P~P) drives the overall reaction Cleavage of PP i is the only energy cost for glycogen synthesis (one ~P bond per glucose residue). Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.

A glycosidic bond is formed between the anomeric C1 of the glucose moiety derived from UDP-glucose and the hydroxyl oxygen of a tyrosine side-chain of Glycogenin. UDP is released as a product.

Glycosylation at C4 of the O-linked glucose product yields an O-linked disaccharide with  (1  4) glycosidic linkage. UDP-glucose is again the glucose donor This is repeated until a short linear glucose polymer with  (1  4) glycosidic linkages is built up on Glycogenin

Glycogen Synthase catalyzes transfer of the glucose moiety of UDP-glucose to the hydroxyl at C4 of the terminal residue of a glycogen chain to form an  (1  4) glycosidic linkage: glycogen (n residues) + UDP-glucose  glycogen (n +1 residues) + UDP A separate branching enzyme transfers a segment from the end of a glycogen chain to the C6 hydroxyl of a glucose residue of glycogen to yield a branch with an  (1  6) linkage.

Both synthesis & breakdown of glycogen are spontaneous If both pathways were active simultaneously in a cell, there would be a "futile cycle" with cleavage of one ~P bond per cycle (in forming UDP-glucose) To prevent such a futile cycle, Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, by allosteric effectors and by phosphorylation.

Glycogen Synthase is allosterically activated by glucose-6-P (opposite of effect on Phosphorylase) Thus Glycogen Synthase is active when high blood glucose leads to elevated intracellular glucose-6-P It is useful to a cell to store glucose as glycogen when the input to Glycolysis (glucose-6-P), and the main product of Glycolysis (ATP), are adequate.

Glycogen Breakdown

Storage Tissues Liver: Glucose for bloodstream Muscle: Glucose for anaerobic ATP synthesis (Glycolysis)

Glycogen Phosphorylase [  (1 —> 4) Linkages]

Phosphoglucomutase

Glycogen Debranching Enzyme [  (1 —> 6) Linkages]

Reactions of Glycogen Breakdown

Glycogen Phosphorylase

Glycogen Debranching Enzyme

Maintenance of Blood Glucose Levels Insulin (peptide from the pancreas) – Produced in response to high glucose – Insulin-dependent glucose transporter (GLUT4) – cAMP decreases Glucagon (peptide from the pancreas) – Produced in response to low glucose – Glucagon receptors (liver) - activation of adenylate cyclase – Glycogen breakdown to glucose-6-P – Glucose-6-phosphatase – Glucose enters bloodstream

Glycogen Storage Diseases are genetic enzyme deficiencies associated with excessive glycogen accumulation within cells Some enzymes whose deficiency leads to glycogen accumulation are part of the inter-connected pathways shown here

Control of enzyme activity Rate limiting step

 When an enzyme defect affects mainly glycogen storage in liver, a common symptom is hypoglycemia, relating to impaired mobilization of glucose for release to the blood during fasting.  When the defect is in muscle tissue, weakness & difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.  Additional symptoms depend on the particular enzyme that is deficient.

The product glucose-6-phosphate may enter Glycolysis or (in liver) be dephosphorylated for release to the blood Liver Glucose-6-phosphatase catalyzes the following, essential to the liver's role in maintaining blood glucose: glucose-6-phosphate + H 2 O  glucose + P i Most other tissues lack this enzyme…why??

Hexose Metabolism متابولیسم فروکتوز و گالاکتوز

Common Hexoses

Sources of Sugars Glucose: lactose (dairy products) and sucrose (table sugar) Fructose: fruits and sucrose Galactose: lactose Mannose: polysaccharides and glycoproteins

Fructose Metabolism

Sucrose (Table Sugar) O-  -D-Glucopyranosyl-(1—> 2)-  -D-Fructofuranoside

All Tissues

Cleavage of Sucrose (  -glucosidase or invertase)

Mutarotation of Fructose

Muscle Metabolism of Fructose (Anaerobic Glycolysis) Large Amounts of Hexokinase

Liver Metabolism of Fructose I (Little Hexokinase)

Liver Metabolism of Fructose II

Liver Metabolism of Fructose III

Complexity of Liver Metabolism Breakdown of Many Metabolites

Fructose Intolerance Too Much Fructose – Fructose-1-P Aldolase is rate-limiting – Depletion of P i – Reduction in [ATP] – Increase in glycolysis – Accumulation of lactate (acid) in blood Fructose-1-P Aldolase Deficiency (Genetic Disease)

Galactose Metabolism

Lactose Metabolism (Dairy Products)

Mutarotation of  -D-Galactose

Glycolytic Enzymes are specific and do not recognize galactose!

Need Epimerization

Phosphorylation of Galactose

Activation of Galactose

Epimerization of UDP-Galactose

Why UDP-Galactose? Glycoproteins Glycolipids (Require UDP-Galactose)

Formation of Glucose-1-P

Formation of Glucose-6-P Glucose-6-P ——> Glycolysis

Galactosemia (Mental Retardation and Death) Treatment Galactose-free diet (reversal of all symptoms except mental retardation)

Cataracts

Pentose Phosphate Pathway Hexose Monophosphate Shunt Phosphogluconate Pathway Generation of NADPH and Pentoses

Overview Function – NADPH production Reducing power carrier – Synthetic pathways Role as cellular antioxidants – Ribose synthesis Nucleic acids and nucleotides

Characteristics: Tissue Distribution Demand for NADPH – Biosynthetic pathways FA synthesis (liver, adipose, mammary) Cholesterol synthesis (liver) Steroid hormone synthesis (adrenal, ovaries, testis) – Detoxification (Cytochrome P-450 System) – liver – Reduced glutathione as an antioxidant (RBC) – Generation of superoxide (neutrophils)

Characteristics: Oxidative and Non-oxidative Phases Oxidative phases – Reactions producing NADPH – Irreversible Non-oxidative phases – Produces ribose-5-P – Reversible reactions feed to glycolysis

NADPH producing reactions Glucose-6-P dehydrogenase 6-P-gluconate dehydrogenase

The Pentose Phosphate Pathway: Non-oxidative phases

Regulation Glucose-6-P dehydrogenase – First step – Rate limiting Allosteric Regulation – Feedback inhibited by NADPH Inducible enzyme – Induced by insulin

Role of NADPH in the RBC Production of superoxide – Hb-Fe 2+ -O 2 -> Hb-Fe 3+ + O 2 -. Spontaneous rxn, 1% per hour O H 2 O -> 2H 2 O 2 Both O 2 -. & H 2 O 2 can produce reactive free radical species, damage cell membranes, and cause hemolysis

Detoxification of Superoxide Anion and Hydrogen Peroxide Antioxidant enzymes – Superoxide dismutase – Glutathione peroxidase – Glutathione reductase

Case Study 21 yo male medical student with malaria Treated with primaquine Four days later: – Black colored urine – Low RBC count – Elevated reticulocyte count – RBC with Heinz bodies – Low hemoglobin – Elevated serum bilirubin Pt recovered in a few days

G6PDH Deficiency and Hemolytic Anemia Most common genetic enzymopathy – 400 hundred variants of G6PDH deficiency – Mediterranean, Asian, African descent 400 million people affected worldwide 50% of Kurdish men 10-14% of African-American men with G6PD deficiency

Worldwide distribution of G6PD deficiency: 1995

G6PD Deficiency Distribution of G6PD deficiency coincides prevalence of malaria G6PD deficiency may impart some degree of malaria resistance – Also sickle cell anemia

G6PD Deficiency Exposure to anti-malarial drugs (Primaquine) results in increased cellular production of superoxide and hydrogen peroxide (Primaquine sensitivity) Other chemicals known to increase oxidant stress – Sulfonamides (antibiotic) – Asprin and NSAIDs – Quinadine and quinine – Napthlane (mothballs) – Fava beans (vicine & isouramil)

Fava Beans Grown worldwide – Important in Middle East – High in protein – Frost resistant perennial Genetically modified fava bean being developed – Low in vicine and isouramil Favism

Case Study 21 yo male medical student with malaria Treated with primaquine Four days later: – Black colored urine – Low RBC count – Elevated reticulocyte count – RBC with Heinz bodies – Low hemoglobin – Elevated serum bilirubin Pt recovered in a few days

Symptoms Black colored urine – Hemolysis may result in urinary excretion of hemoglobin Low RBC count & low hemoglobin – Result of high rate of hemolysis Elevated bilirubin – Catabolism of heme

RBCs with Heinz Bodies Precipitation of hemoglobin due to disulfide bond formation between Hb molecues Upper photo shows distorted RBCs with large Heinz bodies Bottom photo shows RBC stained with methylene blue

Elevated Reticulocytes A RBC containing granules or filaments representing an immature stage in cell development Normally constitutes 1% of circulating RBCs Reticulocytosis – Elevation of reticulocytes – Indicative of active erythropoiesis in red bone marrow

Defective G6PDH Results in enzyme with unstable structure – Patient with 10% of normal activity – Enough to generate NADPH under normal condition Newly made RBCs have normal 6PDH activity – Patients recover quickly (8 days)

The Fate of Pyruvate سرنوشت اسید پیروویک

Pyruvate Oxidation That’s a fairly well oxidized carbon……and there it goes …and there go its electrons…

Acetyl CoA acetyl Coenzyme A

The PDH complex requires 5 different coenzymes: CoA, NAD+, FAD+, lipoic acid and thiamine pyrophosphate (TPP). Three of the coenzymes of the complex are tightly bound to enzymes of the complex (TPP, lipoic acid and FAD+) and two are employed as carriers of the products of PDH complex activity (CoA and NAD+). p yruvate + CoA + NAD+  CO2 + acetyl-CoA + NADH + H+

TCA Cycle Kreb’s Cycle Citric acid Cycle

Oxaloacetate  Citrate What’s this? a.k.a., Citric Acid a.k.a., Tricarboxylic Acid

Krebs Citric Acid Cycle

Note that these are per Acetyl-CoA That means two turns of Krebs cycle per Glucose

Krebs Citric Acid Cycle oxaloacetate citrate citric acid

Electron Transport Chain

ATP Bookkeeping glycolysis pyruvate oxidation Krebs cycle electron transport electron transport

ATP Bookkeeping One glucose yields: 2 ATP in glycolysis 2 NADH in glycolysis 2 NADH as pyruvate enters citric acid cycle 2 ATP in citric acid cycle 6 NADH in citric acid cycle 2 FADH 2 in citric acid cycle

G l u c o n e o genesis

Substrates for Gluconeogenesis: Lactate, pyruvate, glycerol, propionny-CoA and certain Amino Acids but never FAT!!! The Cori cycle involves the utilization of lactate, produced by glycolysis in non-hepatic tissues, (such as muscle and erythrocytes) as a carbon source for hepatic gluconeogenesis. In this way the liver can convert the anaerobic byproduct of glycolysis, lactate, back into more glucose for reuse by non-hepatic tissues. Note that the gluconeogenic leg of the cycle (on its own) is a net consumer of energy, costing the body 4 moles of ATP more than are produced during glycolysis. Therefore, the cycle cannot be sustained indefinitely.

The glucose-alanine cycle is used primarily as a mechanism for skeletal muscle to eliminate nitrogen while replenishing its energy supply. Glucose oxidation produces pyruvate which can undergo transamination to alanine. This reaction is catalyzed by glutamate-pyruvate transaminase, GPT (also called alanine transaminase, ALT in Figure).

Stage 1 – postprandial All tissues utilize glucose Stage 2 – postabsorptive KEY – Maintain blood glucose Glycogenolysis Glucogneogenesis Lactate Pyruvate Glycerol AA Propionate Spare glucose by metabolizing fat Stage 3- Early starvation Gluconeogenesis Stave 4 – Intermediate starvation gluconeogenesis Ketone bodies Stage 5 – Starvation