Introduction to glucose metabolism

Overview of glucose metabolism

Objectives critical importance of glucose Recognizing the critical importance of glucose as the main carbohydrate of blood & main fuel of human cells. sources of blood glucose Recalling the sources of blood glucose in feed & fasting states. glucose transport Recognizing glucose transport into cells basic concepts & directions (pathways) of metabolism. Understand the basic concepts & directions (pathways) of metabolism.

General importance of carbohydrates in human body Provide energy 1- Provide energy through metabolism pathways and cycles Store energy 2- Store energy in the form of: starch (in plants) glycogen (in animals and humans) Supply carbon 3- Supply carbon for synthesis of other compounds. Form structural components 4- Form structural components in cells and tissues.

A constant source of GLUCOSE is an absolute requirement for human life as it is: A constant source of GLUCOSE is an absolute requirement for human life as it is: 1- Preferred energy of the brain 2- Required energy source for cells with no or few mitochondria (as RBCs) 3- Essential source of energy for exercising muscles (substrate for anerobic glycolysis) Critical importance of glucose

Metabolic Pathways of Glucose Production and Utilization Glucose Glycogenesis Gluconeogenesis Hexose interconversion Glycogenolysis Glycolysis Krebs cycle HMP/PPP Hexose interconversion Production Utilization

GLUCOSE GLUCOSE Pyruvate Acetyl CoA Citric Acid Cycle NADH & FADH2 Electron transport chain (flow of electrons) ATP Formation of ATP (oxidative phosphorylation) Metabolismof Glucose Glucose HEXOSE MONOPHOSPHATE PATHWAY Ribose-5 Phosphate Glycogen GLYCOGENSYNTHESIS GLYCOLYSIS NADPH Lactate Lactate Oxygen & Mitochondria No Oxygen No Mitochondria OR BOTH

Pathways of glucose metabolism 1- Catabolic pathways: 1- For providing energy (ATP): Glycolysis lactate Anaerobic Glycolysis: end product is lactate pyruvate Aerobic Glycolysis: end product is pyruvate 2- For providing synthetic products: Hexose monophosphate pathway (Produces NADPH & Ribose 5-Phosphate) 2- Synthetic pathways: Glycogen synthesis

Sources of Glucose to human Body Glucose can be obtained from three primary sources: Carbohydrate in Diet: Carbohydrates are sources for glucose of the body after meals. Excess glucose is stored in the form of glycogen in liver & skeletal muscles. Glycogen degradation (Glycogenlysis): Glycogen (synthesized from glucose molecules) is stored in liver & skeletal muscles. liver glycogen In cases of fasting, liver glycogen is degraded to yield glucose for blood. muscle glycogen In cases of muscular exercise, muscle glycogen is degraded to secure glucose for muscles as a source of energy. Gluconeogenesis (Glucose Synthesis): It is the synthesis of glucose from non carbohydrate sources (as some amino acids) It occurs in prolonged fasting

Sources of glucose of carbohydrate diet Free Monosaccharides 1- Free Monosaccharides: glucose & fructose mainly glucose & fructose Fructose is converted into glucose in liver Disaccharides 2- Disaccharides: Sucrose - Sucrose (glucose & fructose) - Lactose - Lactose (glucose & galactose) - Maltose - Maltose (glucose & glucose) They are digested into monosaccharides (glucose, fructose & galactose) in the intestine. Fructose & galactose are converted into glucose in the liver Polysaccharides 3- Polysaccharides : Starch - Starch (plant source e.g. rice, potato, flour) Glycogen - Glycogen (animal source) They are digested into glucose in the GIT

Sources of glucose of carbohydrate diet

Glucose transport into cells 1- Na+-independent facilitated diffusion transport: Transport occurs with concentration gradient No No requirement for ATP It is conducted by a group of at least 14 glucose transporters (GLUT-1 to 14)

Glucose transport into cells

GLUT-1 GLUT-1 is abundant in RBCs & Brain GLUT-2 GLUT-2 is found in liver, kidney & b-cells of the pancreas Function in both directions (from blood to cells & from cells to blood) GLUT-3 GLUT-3 primary glucose transporter in neurons GLUT-4 GLUT-4 is abundant in adipose tissue & skeletal muscles Number is increased by insulin GLUT-5 GLUT-5 is the primary transporter of fructose GLUT-7 GLUT-7 is expressed in gluconeogenic tissue (as the liver) mediates glucose flux across ER membrane Glucose transport into cells

2- Na + -monosaccharide cotransporter system Energy-requiring process that transports glucose against a concentration gradient from low glucose concentrations outside the cell to higher concentrations within the cell It is a carrier-mediated process in which the movement of glucose is coupled to the concentration gradient of Na+, which is transported into the cell at the same time This type of transport occurs in the epithelial cells of the intestine & renal tubules Glucose transport into cells

GLUCOSE TRANSPORT & INSULIN

Glycolysis

Glycolysis Glucose (6C) 2 Pyruvate (3C) 2 ATP 2 ADP 4 ADP 4 ATP 2 NAD 2 NADH+ H +

Glycolysis Glycolysis, is the major pathway for glucose metabolism Glycolysis is the breakdown of glucose to: 1- Provide energy (in the form of ATP) 2- Provide intermediates for other metabolic pathways. It occurs in cytosols of all tissues All sugars can be converted to glucose & thus can be metabolized by glycolysis.

End products of glycolysis 1- In cells with mitochondria & an adequate supply of oxygen Aerobic glycolysis (Aerobic glycolysis) Pyruvate - Pyruvate: enters the mitochondria & is converted into acetyl CoA. Acetyl CoA enters citric acid cycle (Krebs cycle) to yield energy in the form of ATP NADH: - NADH: utilizes mitochondria & oxygen to yield energy In cells with no mitochondria or adequate oxygen (or Both) 2- In cells with no mitochondria or adequate oxygen (or Both) Anaerobic glycolysis (Anaerobic glycolysis) Lactate Lactate: formed from pyruvate (by utilizing NADH)

Pyruvate is the end product is the end product of aerobic glycolysis Lactate is the end product is the end product of anaerobic glycolysis End products of glycolysis NADH is an end product of aerobic glycolysis

Overall reactions of glycolysis

Steps catalyzed By key enzymes

Glycolysis Glucokinase or hexokinase

Hexokinase & glucokinase HEXOKINASE GLUCOKINASE LOCALIZATIONMost tissuesHepatocytes & Pancreas SpecificityBroad specificity for all hexoses Same Kinetics Km Low Km High Affinity Permits efficient phosph. of glucose even when tissue concentration of glucose is low High Km Low Affinity Requires high concentration of glucose for 1/2 saturation So It permits metabolism of glucose when I.C. concentration of glucose in liver cells are increased Vmax Low Vmax Cannot trap glucose more than cell need High Vmax Allow liver to remove flux of glucose from blood (after absorption) To Reduce hyperglycemia after diet & absorption. Effect of insulin (regulation by insulin) Synthesis not affected by insulinSynthesis is increased by insulin

Glycolysis

Glycolysis

Key enzymes in glycolysis 1- Hexokinase & Glucokinase Glucose Glucose 6-phosphate 2- Phosphofructokinase (PFK) Fructose 6-phosphate Fructose 1,6 bisphosphate 3- Pyruvate Kinase (PK) Phosphoenol pyruvate Pyruvate Phosphoenol pyruvate Pyruvate

Regulation of Key Enzyme The regulation of the activity of enzyme is essential for coordinating the metabolic processes. Types of regulation: 1- General: (occurs in all types of enzymes in the body) increasing substrate concentration will lead to increase activity of the enzyme 2-Special regulatory mechanisms: (not all enzymes of the body) i- Allosteric effectors ii- Covalent modification iii. Increase or decrease rate of enzyme synthesis( long –term regulation)

Pyruvate Kinase Covalent Modification

Long-Term Regulation of Glycolysis Insulin: Induction Glucagon: Repression

Energy yield from glycolysis Energy yield from glycolysis 1- Anerobic glycolysis 2 molecule of ATP for each one molecule of glucose converted to 2 molecules of lactate It is a valuable source of energy under the following conditions Oxygen supply is limited 1- Oxygen supply is limited as in muscles during intensive exercise Tissues with no mitochondria 2- Tissues with no mitochondria Kidney medulla RBCs Leukocytes Lens & cornea cells Testes 2-Aerobic glycolysis 2 moles of ATP for each one mol of glucose converted to 2 moles of pyruvate 2 molecules of NADH for each molecule of glucose 2 or 3 ATPs for each NADH entering electric transport chain (ETC) in mitochondria.

Oxidative phosphorylation: The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP coupled to the transfer of electrons from reduced coenzymes to molecular oxygen via the electron transport chain (ETC); it occurs in the mitochondria. Substrate-level phosphorylation: The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP (or GDP to GTP) coupled to cleavage of a high-energy metabolic intermediate (substrate). It may occur in cytosol or mitochondria Substrate-level phosphorylation & Oxidative phosphorylation

Energy yield from glycolysis Energy yield from glycolysis In anaerobic glycolysis: 2 ATP for one glucose molecule In aerobic glycolysis Glycolysis: 2 ATP 2 NADH: 2 X 3 = 6 ATP NADH Pyruvate Acetyl CoA 2 Pyruvate produce 2 Acetyl CoA (& 2 NADH): 2 X 3 = 6 ATP 2 Acetyl CoA in citric acid cycle: 2 X 12 = 24 ATP

Energy yield of aerobic glycolysis 2 Lactate 2 Lactate Oxygen & Mitochondria No Oxygen No Mitochondria OR BOTH GLUCOSE 2 PYRUVATE 2NAD+ 2 NADH 6 ATP = 2 X 3 = 6 ATP 2 ACETYL CoA 2 ACETYL CoA CITRIC ACID CYCLE 24 ATP = 2 X 12 = 24 ATP 2NAD+ 2 NADH 6 ATP = 2 X 3 = 6 ATP Energy yield of anaerobic glycolysis 2 ATP Net = 2 ATP/ glucose molecule Net = 38 ATP / glucose molecule

Genetic defects of glycolytic enzymes Pyruvate kinase deficiency (95% of cases) PK deficiency leads to a reduced rate of glycolysis with decreased ATP production. PK deficiency effect is restricted RBCs. As RBCs has no mitochondria & so get ATP only from glycolysis. RBCs needs ATP mainly for maintaining the bio- concave flexible shape of the cell. PK deficiency leads to severe deficiency of ATP for RBCs. So, RBCs fail to maintain bi-concave shape ending in liability to be lysed (hemolysis). chronic hemolytic anemia Excessive lysis of RBCs leads to chronic hemolytic anemia.

Fate of pyruvate Lactate 1- Lactate: in anaerobic glycolysis (in cytosol) Acetyl CoA 2- Acetyl CoA: in aerobic glycolysis (in mitochondria & available oxygen) Oxalacetate 3- Oxalacetate: required for: 1- Citric acid cycle (condenses with acetyl CoA): to yield energy (ATP) OR 2-Gluconeogenesis (to synthesize glucose)

LACTATE LACTATE Oxygen & Mitochondria No Oxygen No Mitochondria OR BOTH Glucose PYRUVATE ACETYL CoA ACETYL CoA CITRIC ACID CYCLE OXALACETATE Fate of Pyruvate glycolysis Gluconeogenesis

Formation of acetyl CoA from pyruvate Pyruvate (end product of aerobic glycolysis) is transported into the mitochondria. mitochondrial matrixacetyl CoA In the mitochondrial matrix, pyruvate is converted to acetyl CoA by pyruvate dehydrogenase complex pyruvate dehydrogenase complex (multienzyme complex) This reaction is irreversible Pyruvate dehydrogenase complex Pyruvate dehydrogenase complex is composed of three enzymes & five coenzymes Coenzymes of the complex Coenzymes of the complex are derived from water soluble vitamins: Thiamine pyruphosphate, TPP 1- Thiamine pyruphosphate, TPP (derived from thiamine, vitamin B1) NAD+ 2- NAD+ (derived from niacin) FAD 3- FAD (derived from riboflavin) Lipoic acid 4- Lipoic acid Coenzyme A 5- Coenzyme A (derived from pantothenic acid)

Pyruvate dehydrogenase complex PYRUVATE ACETYL CoA

PDH Complex: Covalent Regulation Pyruvate dehydrogenase complex (active) Pyruvate dehydrogenase complex (inactive) P ATP AD P Protein Kinase Glucagon + H2OH2O PiPi Insulin - Protein Phosphatase e + + Glucagon Insulin PDH

Citric acid cycle (Krebs cycle) Citric acid cycle or, (Krebs cycle)

Citric acid cycle is the final pathway where the oxidative metabolism of Carbohydratesproteinslipids Carbohydrates (as glucose), proteins (amino acids) & lipids (fatty acids) ATP to yield energy (ATP) Acetyl CoA Acetyl CoA is the end product for oxidation of carbohydrates, lipids & proteins Acetyl CoA oxalacetate Acetyl CoA condenses with oxalacetate to form citrate (first reaction of the cycle) 9 ATP (by oxidative phosphorylation) 3 NADH are produced = 3 X 3 = 9 ATP (by oxidative phosphorylation) 2 ATP (by oxidative phosphorylation) One FADH2 is produced = 1 X 2 = 2 ATP (by oxidative phosphorylation) One ATP by substrate level phosphorylation One ATP is produced (by substrate level phosphorylation) Net = 12 ATP / one acetyl CoA

TCA CYCLE - SUMMARY Acetyl CoA 3 NAD 3 NADH + H 1 FAD 1 FADH 2 1 ADP 1 ATP 2 CO 2

Citric acid cycle (Krebs cycle) Citric acid cycle or,(Krebs cycle)

GLUCONEOGENESIS GLUCONEOGENESIS Gluconeogenesis is the synthesis of glucose from glucogenic precursors which are not of carbohydrate origin (gluconeogenic precursors) It occurs during prolonged fasting to synthesize glucose for tissues requiring continuous supply of glucose as a source of energy: Brain, RBCs, Kidney medulla, Lens, Cornea, Testes, sk.ms Gluconeogenesis occurs ONLY in the liver & kidneys

Gluconeogenic precursors glycolysis 1- Intermediates of glycolysis by reverse of steps of glycolysis (except 4 steps that need 4 different enzymes) citric acid cycle 2- Intermediates of citric acid cycle are converted to oxalacetate then to glucose Lactate 3- Lactate Lactic acid formed of anaerobic glycolysis in cells as RBCs & skeletal muscles are transported in blood to liver to be converted to pyruvate then to glucose (Cori cycle) Glycerol 4- Glycerol Glycerol is derived from the lipid triacylglycerol in adipose tissue. Glycerol is convered into dihydroxyactone phosphate (intermediate of glycolysis) then to glucose. amino acids 5- Glucogenic amino acids of proteins Glucogenic amino acids are deaminated to form a-ketoacids a-keto acids are converted to pyruvate or intermediates of citric acid cycle then to glucose

Lactate Lactate GLUCOSE Pyruvate Pyruvate Intermediate of CITRIC ACID CYCLE Oxalacetate Precursors of ofGluconeogenesis Glucogenic amino acids in proteins as sk. ms. Glycerol Triacylglycerol in adipose tissue Gluconeogenesis Fatty acids

Glycerol as a gluconeogenic Substrate GlycerolGlycerol 3-phosphate GK Dihydroxyacetone phosphate Glycerol 3-phosphate dehydrogenase NAD+ NADH Glucose GK: Glycerol kinase only in liver & kidneys ATPADP

Glucogenic Amino Acids Glutamate Glutamine MethionineValine PhenylalaninTyrosine Amino acids AspartateAspargine Pyruvate Glycine, Alanine Amino acids

Cori Cycle

Gluconeogenic pathway

Pruvate Carboxylase and PEP-CK Vs Pyruvate kinase

Fructose 1,6-bisphosphatase Vs PFK-1 Fructose 1,6-bisphosphatase Vs PFK-1

Glucose 6-Phosphatase Vs. Glucokinase

Energy Consumed During Gluconeogenesis

GLUCOSE Unique enzymes of gluconeogenesis Reactions 1, 2, 3 & 4 are catalyzed by enzymes NOT used in glycolysis 1- Pyruvate to oxalacatate pyruvate carboxylase by pyruvate carboxylase 2- oxalacetate to phosphoenol pyruvate PEP carboxykinase by PEP carboxykinase 3- Fructose 1,6 bisphosphate to fructose 6 phosphate fructose 1,6 bisphosphatase by fructose 1,6 bisphosphatase 4- Glucose 6-phosphate to glucose glucose 6-phosphatase by glucose 6-phosphatase GLUCONEOGENESIS Other reactions of gluconeogenesis are catalyzed by same enzymes of glycolysis in the reverse direction