Chapter 11: Glycolysis PDF

Chapter 11: Glycolysis Dr. Asmaa Abu Obaid 1 ✓Define Glycolysis ✓List and recognize each of the 10 steps in glycolysis ✓Differentiation the irreversible reactions along with their substrates, products and enzymes ✓Calculate the yield of energy during glycolysis ✓Define carbohydrate a...

Chapter 11: Glycolysis Dr. Asmaa Abu Obaid 1 ✓Define Glycolysis ✓List and recognize each of the 10 steps in glycolysis ✓Differentiation the irreversible reactions along with their substrates, products and enzymes ✓Calculate the yield of energy during glycolysis ✓Define carbohydrate and differentiate between the different types Dr. Asmaa Abu Obaid 2  gluconeogenesis, glycolysis, and the citric acid cycle, all of which are key to carbohydrate metabolism and energy production Gluconeogenesis synthesizes glucose from three- carbon precursors, while glycolysis breaks down glucose into pyruvate, releasing energy Pyruvate can be further converted into acetyl CoA, which enters the citric acid cycle where it is oxidized to CO₂ and water. These pathways also intersect with the metabolism of non-carbohydrate molecules like amino acids and lipids. Dr. Asmaa Abu Obaid 3  The Enzymatic Reactions of Glycolysis Glycolysis, a major energy source in animals Glycolysis is a sequence of ten enzyme-catalyzed reactions by which glucose is converted to pyruvate The conversion of one molecule of glucose to two molecules of pyruvate is accompanied by the net conversion of two molecules of ADP to two molecules of ATP and the reduction of two molecules of NAD+ to two molecules NADH The enzymes of this pathway are found in most living species and are located in the cytosol, In some mammalian cells (such as those in the retina and some brain cells), it is the only ATP- producing pathway Dr. Asmaa Abu Obaid 4  It can be divided into two stages: the hexose stage and the triose stage. Hexose stage: 2 ATP are consumed per glucose Triose stage: 4 ATPare produced per glucose Net: 2 ATP produced per glucose Dr. Asmaa Abu Obaid 5 The Ten Steps of Glycolysis Each chemical reaction prepares a substrate for the next step in the process A hexose is cleaved to two trioses Interconversion of the trioses allows both to be further metabolized via glycolytic enzymes ATP is both consumed and produced in glycolysis Dr. Asmaa Abu Obaid 6 Dr. Asmaa Abu Obaid 7 Two phases of glycolysis Dr. Asmaa Abu Obaid 8 Dr. Asmaa Abu Obaid 9 Dr. Asmaa Abu Obaid 10  Step 1: Phosphorylation by hexokinase Transfers the γ-phosphoryl of ATP to glucoseC- 6 oxygen to generate glucose 6- phosphate(G6P) Mechanism: attack of C-6 hydroxyl oxygen of glucose on the γ-phosphorous of ATP2- Four kinases in glycolysis: steps 1,3,7, and 10 All four kinases require Mg2+and have a similar mechanism Dr. Asmaa Abu Obaid 11 Properties of hexokinases Broad substrate specificity-hexokinases can phosphorylate glucose, mannose and fructose Isozymes-multiple formsof hexokinase occur in mammalian tissues and yeast Hexokinases I, II, III are active at normal glucose concentrations (Kmvalues ~10-6 to 10-4M) Hexokinase IV (Glucokinase, Km~10-2M) is active at higher glucose levels, allows the liver to respond to large increases in blood glucose Dr. Asmaa Abu Obaid 12 Hexokinase vs. glucokinase Tissue-specific isozymes: With high glucose levels, glucokinase is active. Because glucokinase is never saturated with glucose, the liver can respond to large increases in blood glucose by phosphorylating it for entry into glycolysis or the glycogen synthesis pathway. Dr. Asmaa Abu Obaid 13  transporters, there is a chance for them to leave the cell.... That is why the glucose is phosphorylated by ATP to become glucose-6-phosphate, which now bears a charge. This disqualifies it from leaving through glucose transporters Dr. Asmaa Abu Obaid 14  Step 2: Conversion of G6P to F6P; Isomerization aldose ketose The enzyme is also known as phosphoglucose isomerase (PGI) Dr. Asmaa Abu Obaid 15 2. Glucose 6-Phosphate Isomerase Converts glucose 6-phosphate(G6P) (an aldose) to fructose 6- phosphate(F6P) (a ketose) Enzyme preferentially binds the α-anomer of G6P (converts to open chain form in the active site) Enzyme is highly stereospecific for G6P and F6P Isomerase reaction is near-equilibrium in cells Dr. Asmaa Abu Obaid 16 Step 3: phosphorylation by Phosphofructokinase-1 (PFK-1) Dr. Asmaa Abu Obaid 17 Phosphofructokinase-1 (PFK-1) Catalyzes transfer of a phosphoryl group from ATP to the C-1 hydroxyl group of F6Pto form fructose 1,6-bisphosphate(F1,6BP) PFK-1 is metabolically irreversible and a critical regulatory point for glycolysis in most cells (PFK-1 is the first committed step of glycolysis) A second phosphofructokinase (PFK-2) synthesizes fructose 2,6- bisphosphate(F2,6BP) Dr. Asmaa Abu Obaid 18 Step 4: cleavage by Aldolase Dihydroxyacetone phosphate (DHAP) is derived from C-1 to C- 3 of fructose 1,6-bisphosphate, and glyceraldehyde 3- phosphate (GAP) is derived from C-4 to C-6. Dr. Asmaa Abu Obaid 19 4. Aldolase Aldolase cleaves the hexose F1,6BP into two triose phosphates: glyceraldehyde 3-phosphate(G3P) and dihydroxyacetone phosphate(DHAP) Reaction is near-equilibrium, not a control point There are two distinct classes of aldolases. class I enzymes are found in plants and animals; class II enzymes are more common in bacteria, fungi, and protists. Many species have both types of enzyme Dr. Asmaa Abu Obaid 20 Interesting hint about Aldolases The key to understanding the strategy of glycolysis lies in appreciating the significance of the aldolase reaction. It’s best to think of this as a near-equilibrium biosynthesis reaction and not a degradation reaction. Aldolases evolved originally as enzymes that could catalyze the synthesis of fructose 1,6-bisphosphate. This reaction occurred at the end of a biosynthesis pathway leading from pyruvate to glyceraldehyde 3- phosphate and dihydroxyacetone phosphate. During glycolysis, flux in the triose stage is in the opposite direction—toward pyruvate synthesis. The first steps of glycolysis—the hexose stage—are directed toward formation of fructose 1,6-bisphosphate so that it can serve as substrate for the reversal of the pathway leading to its synthesis. Keep in mind that the glucose biosynthesis pathway (gluconeogenesis) evolved first. It was only after glucose became readily available that pathways for its degradation evolved. Dr. Asmaa Abu Obaid 21 Step 5: Isomerization through Triose Phosphate Isomerase (TPI) the two molecules produced by the splitting of fructose 1,6-bisphosphate, only glyceraldehyde 3- phosphate is a substrate for the next reaction in the glycolytic pathway Dr. Asmaa Abu Obaid 22 5. Triose Phosphate Isomerase (TPI) Conversion of DHAP into glyceraldehyde 3-phosphate(G3P) Reaction is very fast (diffusion controlled), and only the D-isomer of G3P is formed Radioisotopic tracer studies show: One G3P molecule: C1,2,3 from Glucose C4,5,6 Second G3P: C1,2,3 from Glucose C3,2,1 Dr. Asmaa Abu Obaid 23 Keeping Track of Carbons Dr. Asmaa Abu Obaid 24  Second phase of glycolysis, where ATP and NADH are formed Dr. Asmaa Abu Obaid 25  Step 6: oxidation with Glyceraldehyde 3- Phosphate Dehydrogenase (GAPDH) Conversion of G3P to 1,3- bisphosphoglycerate(1,3BPG) Molecule of NAD+ is reduced to NADH Oxidation of the aldehyde group of GAP proceeds with large negative free- energy change This is an oxidation–reduction reaction; the oxidation of glyceraldehyde 3- phosphate is coupled to the reduction of NAD to NADH Dr. Asmaa Abu Obaid 26  Step 7: phosphorylation by Phosphoglycerate Kinase (PGK) In the next step of glycolysis, the C-1 phosphoryl group of 1,3 bisphosphoglycerate is transferred to ADP to form ATP. The remaining energy is conserved in the form of reducing equivalents (NADH). The NADH formed in the glyceraldehyde 3- phosphate dehydrogenase reaction is reoxidized, either by the membrane- associated electron transport chain Dr. Asmaa Abu Obaid 27 Step 6 and 7 are coupled Transfer of phosphoryl group from the energy-rich mixed anhydride 1,3BPG to ADP yields ATP and 3-phosphoglycerate(3PG) Substrate- level phosphorylation-Steps 6 and 7 couple oxidation of an aldehyde to a carboxylic acid with the phosphorylation of ADP to ATP This reaction is the first ATP-generating step of glycolysis. Dr. Asmaa Abu Obaid 28  Step 8: shift of the phosphoryl group by Phosphoglycerate Mutase Catalyzes transfer of a phosphoryl group from one part of a substrate molecule to another Reaction occurs without input of ATP energy Mechanism requires 2 phosphoryl-group transfer steps Dr. Asmaa Abu Obaid 29  Step 9: dehydration by Enolase: 2PG to PEP 2-Phosphoglycerate(2PG) is dehydrated to phosphoenolpyruvate(PEP) Elimination of water from C-2 and C-3 yields the enol-phosphate PEP PEP has a very high phosphoryl group transfer potential because it exists in its unstable enol form Enolase requires Mg for activity. Two magnesium ions participate in this reaction: a “conformational” ion binds to the carboxylate group of the substrate, and a “catalytic” ion participates in the dehydration reaction. Dr. Asmaa Abu Obaid 30  Step 10: substrate level phosphorylation by Pyruvate Kinase (PK) ✓Catalyzes a substrate-level phosphorylation ✓Metabolically irreversible reaction ✓Pyruvate kinase gene can be regulated by various hormones and nutrients ✓Because pyruvate kinase is regulated, the concentration of phosphoenolpyruvate is maintained at a high enough level to drive ATP formation during glycolysis. Dr. Asmaa Abu Obaid 31 Summary Dr. Asmaa Abu Obaid 32 Dr. Asmaa Abu Obaid 33 Dr. Asmaa Abu Obaid 34