Lecture 1 Glycolysis.pptx. .

Biochemistry research

1. LECTURE 1 GLYCOLYSIS Likando Chababa , B.Sc , M.Sc (Biochemistry) BIOCHEMISTRY II

2. GLYCOLYSIS   The glycolytic pathway is a pathway used by all tissues for the breakdown of glucose. Glucose is metabolized to pyruvate by the pathway of glycolysis, which can occur aerobically. When it occurs anaerobically, the end product is lactate . Aerobic tissues metabolize pyruvate to acetyl-CoA, which can enter the citric acid cycle for complete oxidation to CO 2 and H 2 O, linked to the formation of ATP in the process of oxidative phosphorylation. Glucose is the major fuel of most tissues

3. Reactions of Glycolytic Pathway All reactions occur in the cytoplasm . The conversion of glucose to pyruvate occur in two stages : The first being the priming stage and The second being energy generation stage . The priming stage corresponds to energy investment in the form of ATP where glucose and other hexose sugars are converted to the common product called glyceraldehyde-3-phosphate . In the energy generation state , a net of two ATP molecules are formed per glucose molecule metabolised. In addition to ATP, two molecules of NADH + H + are formed if the end product is pyruvate . If the end product is lactate , then NADH + H + are converted to NAD + .

4. Reactions of Glycolytic Pathway Phosphorylation of glucose : In this reaction, glucose is first phosphorylated at the hydroxyl group on C-6 and thus glucose 6-phosphate thus formed in the presence of ATP. The reaction is catalyzed by the specific enzyme glucokinase in liver cells and by non-specific Hexokinase in liver and extrahepatic tissues. Glucokinase activity in hepatocytes is also increased by insulin . As blood glucose levels rise following a meal, the cells of the pancreas are stimulated to release insulin into the portal circulation. Glucose + ATP Glu 6-P + ADP

5. Reactions of Glycolytic Pathway Isomerization of glucose 6-phosphate: The isomerization of glucose 6-phosphate to fructose 6-phosphate is catalyzed by phosphohexose isomerase . The reaction is readily reversible and is not a rate-limiting or regulated step. Glu 6-P Fru 6-P

6. Reactions of Glycolytic Pathway Phosphorylation of fructose 6-phosphate : The irreversible phosphorylation reaction catalyzed by phosphofructokinase 1(PFK-1) is the most important control point and the rate-limiting step of glycolysis. PFK-1 is controlled by the available concentrations of the substrates ATP and fructose 6-phosphate and fructose 2,6-bisphosphate activates PFK-1 of glycolysis. Fructose 6-phosphate is phosphorylated to Fructose 1,6-bisphosphate through an ATP input . Fru 6-P + ATP Fru 1,6-bisp + ADP

7. Reactions of Glycolytic Pathway Cleavage of Fructose 1,6-bisphosphate: Fructose 1, 6-bisphosphate is split by the enzyme aldolase into two molecules of triose phosphates : An aldotriose , glyceraldehyde 3-phosphate (GAP) and A Ketotriose , Dihydroxyacetone phosphate (DHAP). Fru 1,6-bisp GAP + DHAP

8. Reactions of Glycolytic Pathway Isomerization of dihydroxyacetone phosphate: Triose phosphate isomerase interconverts dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Dihydroxyacetone phosphate must be isomerized to glyceraldehyde3-phosphate for further metabolism by the glycolytic pathway. This isomerization results in the net production of two molecules of glyceraldehyde 3-phosphate from the cleavage products of fructose 1,6-bisphosphate. DHAP GAP

9. Reactions of Glycolytic Pathway Oxidation of glyceraldehyde 3-phosphate: The conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate (1,3-BPG) by glyceraldehyde 3-phosphate dehydrogenase is the first oxidation-reduction reaction of glycolysis. 2GAP + 2NAD + 2P i 2 1,3-BPG + 2NADH + 2H +

10. Reactions of Glycolytic Pathway Synthesis of 3-phosphoglycerate producing ATP: 1,3-BPG is converted to 3-phosphoglycerate (3-PG), the high-energy phosphate group of 1,3-BPG is used to synthesize ATP from ADP . This reaction is catalyzed by phosphoglycerate kinase , which, unlike most other kinases, is physiologically reversible. Two molecules of 1,3-BPG are formed from each glucose molecule, this kinase reaction replaces the two ATP molecules consumed by the earlier formation of glucose 6-phosphate and fructose 6-Phosphate. 2 1,3-BPG + 2ADP 2 3-PG + 2ATP

11. Reactions of Glycolytic Pathway Shift of the phosphate group from carbon 3 to carbon 2: The shift of the phosphate group from carbon 3 to carbon 2 of phosphoglycerate by phosphoglycerate mutase is freely reversible 2 3-PG 2 2-PG

12. Reactions of Glycolytic Pathway Dehydration of 2-PG: The dehydration of 2-phosphoglycerate by enolase redistributes the energy within the 2-phosphoglycerate molecule, resulting in the formation of phosphoenolpyruvate (PEP), which contains a high-energy phosphate plus water . The reaction is reversible despite the high-energy nature of the product. 2 2-PG 2 PEP + 2H 2 O

13. Reactions of Glycolytic Pathway Formation of pyruvate producing ATP: The conversion of PEP to pyruvate is catalyzed by pyruvate kinase , the third irreversible reaction of glycolysis. The equilibrium of the pyruvate kinase reaction favours the formation of ATP. This is the third irreversible reaction of the glycolytic pathway. 2 PEP + 2 ADP 2 Pyruvate + 2 ATP

14. Reactions of Glycolytic Pathway Under anaerobic conditions, pyruvate is converted to lactate through a redox reaction. The reaction helps to yield NAD+ from reaction 6. The enzyme Lactate dehydrogenase catalyses this reversible reaction. Pyruvate + NADH + H + Lactate + NAD +

15. Energy Yield of Glycolysis Anaerobic: Glu + 2ATP + 4ADP + 2NAD + + 2P i + 2NADH + 2H + 2Lactate + 4ATP + 2ADP + 2NAD + + 2 NADH + 2H + + 2H 2 O Net Equation: Glu + 2ADP + 2P i 2Lactate + 2ATP + 2H 2 O Aerobic: Glu + 2ATP + 4ADP + 2NAD + + 2P i 2Pyruvate + 4ATP + 2ADP + 2NAD + + 2 NADH + 2H + + 2H 2 O Net Eq : Glu + 2ADP + 2P i + 2NAD + 2Pyruvate + 2ATP + 2H 2 O + 2NADH + 2H +

16. Energy Yield of Glycolysis On-going aerobic glycolysis requires the oxidation of most of this NADH by the electron transport chain , producing approximately three ATP for each NADH molecule entering the chain. Therefore aerobic glycolysis results in the production of a total of 8 ATP molecules . Despite the production of some ATP during glycolysis, the end products, pyruvate or lactate, still contain most of the energy originally contained in glucose. The Tricarboxylic Acid (TCA) cycle is required to release that energy completely.