Carbohydrate Metabolism Major pathway for Glucose oxidation Glycolysis

Carbohydrate Metabolism • Major pathway for Glucose oxidation: – Glycolysis – Citric Acid Cycle • Minor Pathways for Glucose oxidation: – HMP shunt – Uronic Acid pathway • Glycogenesis • Glycogenolysis • Gluconeogenesis

Glycolysis Definition: • Glycolysis is the sequence of 10 reactions that break down one molecule of glucose to: – two molecules of pyruvate (3 C) (in presence of O 2) or to – 2 molecules of lactic acid (in absence of O 2). – with net generation of 2 molecules of ATP and NADH Location: – All cells of the body. Site: – In the cytoplasm of every cell,

Importance Of Glycolysis 1) Supply energy – 2 ATP in absence of O 2 and – 8 ATP in the presence of O 2 2) Supply 2 molecules of Pyruvic acid which enter the mitochondria and converted into: – Acetyl Co. A (active acetate) that are oxidized by Citric Acid Cycle – Oxaloacetic acid, which is the initiator of citric acid cycle. 3) Supply Dihydroxy Acetone Phosphate which is converted to α Glycerophosphate, which can be converted to Triglyceride (Lipogenesis)

4)Synthesis of amino acids: from intermediates of glycolysis e. g. – 3 phosphoglycerate Serine – Pyruvic acid alanine 5)Importance of Glycolysis in RBCs – Energy production: • It is the only pathway that supplies the red cells with 2 ATP. – Reduction of met-hemoglobin: • Glycolysis provides NADH for reduction of met Hb in red cells by the NADH Cyt b 5 methemoglobin reductase system

The Individual Reactions of Glycolysis The pathway consisting of 2 separate phases. • The first require energy: – (2 molecules of ATP are used to convert glucose to fructose 1, 6 -bisphosphate (F 1, 6 BP) • The second is the energy-producing phase: – F 1, 6 BP is degraded to 2 molecules of pyruvate, with the production of 4 molecules of ATP and 2 molecules of NADH]

The Hexokinase (glucokinase) Reaction • This reaction consumes 1 molecule of ATP, and is catalyzed by enzyme hexokinases/glucokinase. • It is irreversible reaction. • Glucokinase is an isoform of hexokinase enzyme present in liver. • The importance of this reaction: – Formation of glucose 6 - phosphate which cannot pass outside the cell because it has no transporter (glucose is trapped inside the cell). – Conversion of glucose into an active form capable of being further metabolized.

Hexokinase Glucokinase Site Extrahepatic tissues Liver Km Low 2 mg% High 200 mg% Velocity of reaction low high Inhibition By G 6 -P No inhibition Induction by insulin Not inducible Importance of the difference between glucokinase and hexokinase: The difference in Km and Vmax allows: The liver glucokinase to act when intracellular glucose level is high (after carbohydrate meals) to remove glucose rapidly from portal blood and prevent large amount of it from reaching systemic circulation. Tissue hexokinase to use glucose when blood glucose level decrease. Allosteric inhibition of hexokinase by its product (G 6 P) allows: Peripheral tissue to use glucose only when it is required to supply energy. Liver glucokinase which is not inhibited by its product continue to accumulate glucose and store it as glycogen

2 - Phosphohexose Isomerase reaction: • Glucose 6 - Phosphate isomerase Fructose 6 - Phosphate 3 - Phosphofructokinase-1, (PFK-1) • Fructose 6 - Phosphate Phosphofructokinase-1 fructose 1, 6 -bisphosphate ATP ADP • This reaction is irreversible. • This is the most important step in regulation of glycolysis. 4 - Aldolase • fructose 1, 6 -bisphosphate Aldolase dihydroxyacetone phosphate + glyceraldehyde 3 P 5 - Triose Phosphate Isomerase: • dihydroxyacetone phosphate Isomerase glyceraldehyde 3 -P

Second phase of glycolysis 6 - Glyceraldehyde-3 -Phosphate Dehydrogenase 1, 3 - bisphoglycerate Pi NAD+ NADH + H+ • This reaction is reversible. • Fate of NADH produced: – In aerobic glycolysis, NADH is oxidized in respiratory chain resulting in synthesis of 3 ATP molecules – In anaerobic glycolysis, NADH is oxidized in reaction catalyzed by lactate dehydrogenase.

7 - Phosphoglycerate Kinase 1, 3 -bisphoglycerate Kinase ADP 3 -phosphoglycerate ATP • The high-energy phosphate of 1, 3 -BPG is used to form ATP (substrate level phosphorylation) • This reaction is reversible. N. B. • In erythrocytes, there is an associated pathway in which 1, 3 biphosphoglycerate is converted to 2, 3 BPG • 2, 3 BPG is an important regulator of hemoglobin's affinity for oxygen. Note that 2, 3 -bisphoglycerate phosphatase degrades 2, 3 BPG to 3 -phosphoglycerate, a normal intermediate of glycolysis without production of ATP. 1, 3 -bisphoglycerate mutase 2, 3 -bisphosphoglycerate phosphatase 3 - phosphoglycerate

8. Phosphoglycerate Mutase: 3 -phosphoglycerate mutase – This reaction is reversible 2 - phosphoglycerate 9. Enolase 2 - phosphoglycerate Enolase phosphoenolpyruvate 10 - Pyruvate Kinase • phosphoenoylpyruvate kinase pyruvic acid ADP ATP • In this reaction, ATP is formed (substrate level phosphorylation) • This reaction is irreversible

Lactate dehydrogenase • This reaction occurs only in anaerobic glycolysis (in RBCs and exercising muscles) • It oxidizes NADH produced by glyceraldehydes 3 P dehydrogenase enzyme by reducing pyruvate to lactate.

The Energy derived from glycolysis • Glycolysis requires two molecules of ATP in the reactions catalyzed by Hexokinase, Phosphofructokinase • Then four molecules of ATP are formed by substrate level phosphorylation in the reactions catalyzed by: – phosphoglycerate kinase, – pyruvate kinase. • Thus anaerobic glycolysis produce 2 molecules of ATP. • Aerobic glycolysis produce, in addition, two molecules of NADH which are transported to mitochondria to produce 6 ATP molecules by oxidation via respiratory chain. The net result is 8 ATP.

Aerobic glycolysis Anaerobic glycolysis Site - Under aerobic conditions e. g. in most tissues decrease oxygen supplies (exercising muscles) or absence of mitochondria (RBCs) End Product - Pyruvic a. - Lactic a. Fate of NADH Oxidized in respiratory chain - Oxidized by enzyme lactate dehydrogenase Energy yield 2 ATP + 2 NADH (6 ATP) = 8 ATP - 2 ATP

Inhibitors of glycolysis • Glyceraldehyde 3 P dehydrogenase enzyme is inhibited by arsenic which form 1 -arseno 3 -phosphoglycerate, thus prevent formation of ATP by phosphoglycerate kinase enzyme. • Enolase enzyme is inhibited by floride. Thus floride is used during collection of blood samples for estimation of glucose, to inhibit glycolysis to keep blood glucose