Part III => METABOLISM and ENERGY. 3.2 Glucose Catabolism 3.2a Glycolysis Pathway 3.2b Glycolysis Regulation 3.2c Fermentation

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1 Part III => METABOLISM and ENERGY 3.2 Glucose Catabolism 3.2a Glycolysis Pathway 3.2b Glycolysis Regulation 3.2c Fermentation

2 Section 3.2a: Glycolysis

3 Synopsis 3.2a - Dietary starch (eg bread, rice and potatoes) is hydrolyzed into glucose by the combined action of enzymes such as amylase (saliva) and maltase (small intestine) - Glycolysis involves the breakdown/oxidation of glucose into pyruvate using a wide array of enzymes and the free energy released in the process is either used to synthesize ATP or captured in the form of NADH - In terms of chemical reactions, the glycolytic enzymes catalyze phosphorylation (transferase), isomerization (isomerase), bond cleavage (lyase), dehydrogenation (oxidoreductase), and hydrolysis (hydrolase) - Of the six major classes/families of enzymes (see 2.5), all but ligase are involved in mediating glycolysis! - The 10-reaction sequence of glycolysis is divided into two stages: Stage I Energy investment/expenditure Stage II Energy recovery/payoff - Overall glycolytic reaction: Glucose + 2NAD + + 2ADP + 2P i <=> 2Pyruvate + 2NADH + 2ATP + 2H 2 O + 2H +

4 Glycolysis Overview G / kj.mol Glycolysis can be divided into two main stages Stage I (Steps 1-5) Stage II (Steps 6-10) Glycolysis is accompanied by a net G of -76 kj per mole of glucose converted to two moles of pyruvate

5 Glycolysis Stage I (Investment): Glucose GAP 1 2 In Stage I (Steps 1-5): 3-1 molecule of glucose is converted to 2 molecules of glyceraldehyde-3- phosphate (GAP) - 2 molecules of ATP are utilized (energy investment) 4 5

6 (1) Glucose Glucose-6-Phosphate G = -34 kj/mol - Transfer of the terminal phosphoryl group of ATP to glucose to generate G6P - Thermodynamically favorable powered by the free energy released due to ATP hydrolysis! - Catalyzed by hexokinase (HK) a non-specific enzyme that not only catalyzes the phosphorylation of glucose but also other hexoses such as mannose and fructose - As is true for kinases in general, hexokinase requires Mg 2+ divalent ions for catalytic activity the Mg 2+ ion is believed to shield the negative charges on - and -phosphate oxygen atoms within ATP, so as to render its -phosphate atom more accessible to nucleophilic attack by the CH 2 OH group of glucose

7 (2) Glucose-6-Phosphate Fructose-6-Phosphate G 0 kj/mol - Isomerization of 6-membered G6P to 5-membered F6P - Thermodynamically neutral operates near equilibrium! - Catalyzed by phosphoglucose isomerase (PGI) - Requires ring opening of G6P followed by isomerization and subsequent ring closure to generate F6P

8 (3) Fructose-6-Phosphate Fructose-1,6-Bisphosphate G = -19 kj/mol - Phosphorylation of F6P to FBP a rate-determining step of glycolysis - Thermodynamically favorable thanks to the free energy provided by ATP hydrolysis! - Catalyzed by phosphofructokinase (PFK) in a manner akin to phosphorylation of glucose to G6P by hexokinase (Step 1) requires Mg 2+ ion as a cofactor! - PFK serves as a key regulatory player in glycolysis its catalytic activity is allosterically enhanced by AMP/ADP and inhibited by ATP (vide infra)! - Note that bisphosphate refers to the attachment of two phosphate groups to separate moieties in lieu of directly to each other which would be diphosphate as in ADP

9 (4) Fructose-1,6-Bisphosphate GAP + DHAP G 0 kj/mol + - Cleavage of a single 6-C compound (FBP) into two 3-C compounds (GAP and DHAP) - Thermodynamically neutral operates near equilibrium! - Catalyzed by aldolase (ALD) into two interconvertible 3-C compounds aldolase belongs to the lyase family that catalyze breaking/elimination of bonds by means other than hydrolysis! - Aldolase is a portmanteau of aldol lyase an aldol harbors both an hydroxyl and carbonyl group (ie aldol is an umbrella term for both aldoses and ketoses!) - Note that the atom nomenclature changes upon cleavage of FBP atoms 4/5/6 in FBP become atoms 1/2/3 in GAP, while atoms 1/2/3 in FBP become atoms 3/2/1 in DHAP

10 (5) Dihydroxyacetone Phosphate Glyceraldehyde-3-Phosphate triose phosphate isomerase [DHAP] G 0 kj/mol [GAP] - Interconversion of DHAP to GAP via an enediol intermediate - Thermodynamically neutral operates near equilibrium! - Catalyzed by triose phosphate isomerase (TIM) a perfect enzyme in that it operates near the diffusion-controlled limit with k cat /K M 10 9 M -1 s -1 (see 2.6) - DHAP and GAP are ketose-aldose isomers - Only GAP continues along the glycolytic pathway as GAP is siphoned off, more DHAP is converted to GAP due to equilibrium shift!

11 Glycolysis Stage II (Recovery): GAP Pyruvate 6 7 In Stage II (Steps 6-10): - 2 molecules of GAP are converted to 2 molecules of pyruvate with concomitant generation of high-energy compounds (ATP and NADH) - 4 molecules of ATP via substrate-level phosphorylation molecules of NADH via reduction of NAD +

12 (6) Glyceraldehyde-3-phosphate 1,3-Bisphosphoglycerate HPO 4 2- G 0 kj/mol - Oxidation and phosphorylation of GAP to high-energy 1,3-BPG with concomitant release of NADH cf the high-energy nature of mixed anhydride bond in 1,3-BPG vs the terminal phosphoanhydride bond in ATP! - Hydrogen phosphate (HPO 4 2- ) used as a phosphate donor - NAD + used as an oxidizing agent to oxidize GAP - Thermodynamically neutral operates near equilibrium! - Catalyzed by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) all dehydrogenases require a redox cofactor such as NAD + (H)/FAD(H 2 )

13 (7) 1,3-Bisphosphoglycerate 3-Phosphoglycerate G 0 kj/mol - Dephosphorylation of 1,3-BPG to 3PG - High-energy 1,3-BPG used as a phosphate donor to drive the synthesis of ATP from ADP via substrate-level phosphorylation (first ATP generation) - Thermodynamically neutral in spite of the free energy provided by the hydrolysis of high-energy 1,3-BPG, the reaction operates near equilibrium! - Catalyzed by phosphoglycerate kinase (PGK) requires Mg 2+ ion as a cofactor!

14 (8) 3-Phosphoglycerate 2-Phosphoglycerate G 0 kj/mol - Isomerization of 3PG to 2PG in an intramolecular transfer of phosphate group - Thermodynamically neutral operates near equilibrium! - Catalyzed by phosphoglycerate mutase (PGM) - What do you call an isomerase that catalyzes the intramolecular transfer of a functional group from one position to another? Mutase!

15 (9) 2-Phosphoglycerate Phosphoenolpyruvate G 0 kj/mol - Dehydration of 2-phosphoglycerate to generate high-energy phosphoenolpyruvate (or 2-phosphoenolpyruvate) cf the high-energy nature of phosphoester bond in PEP (the highest-energy phosphate bond in nature) vs the terminal phosphoanhydride bond in ATP! - Thermodynamically neutral operates near equilibrium! - Catalyzed by enolase (enol lyase) an enzyme that belongs to the lyase family of enzymes that catalyze breaking/elimination of bonds by means other than hydrolysis!

16 (10) Phosphoenolpyruvate Pyruvate G = -23 kj/mol - Dephosphorylation of phosphoenolpyruvate (or 2- phosphoenolpyruvate) to pyruvate or systematically, -ketopropionate - High-energy PEP used as a phosphate donor to drive the synthesis of ATP from ADP via substratelevel phosphorylation (second ATP generation) - Thermodynamically favorable thanks to the free energy provided by the hydrolysis of high-energy PEP! - Catalyzed by pyruvate kinase (PK) requires Mg 2+ (or Mn 2+ ) ion as a cofactor! Formate Acetate Propionate

17 Exercise 3.2a - What happens during the two phases of glycolysis? - How many ATP molecules are invested and how many are recovered from each molecule of glucose that follows the glycolytic pathway? - Write the reactions of glycolysis, showing the structural formulas of the intermediates and the names of the enzymes that catalyze the reactions. Distinguish between thermodynamically neutral and favorable steps. - Summarize the types of catalytic mechanisms involved. Do any glycolytic enzymes require cofactors? - What high-energy compounds are synthesized during glycolysis?

18 Section 3.2b: Glycolysis Regulation

19 Synopsis 3.2b - Enzymes that function with a large negative G are candidates for flux-control points - Phosphofructokinase (PFK), the major regulatory point for glycolysis in muscle, is allosterically inhibited by ATP and activated by AMP/ADP - Substrate cycling allows the rate of glycolysis to respond rapidly to changing needs

20 Thermodynamics of Glycolysis (in erythrocytes) Step Enzyme Name Enzyme Family G / kj.mol -1 G / kj.mol -1 1 Hexokinase (HK) Transferase Phosphoglucose isomerase (PGI) Isomerase Phosphofrucktokinase (PFK) Transferase Aldolase (ALD) Lyase Triose Phosphate Isomerase (TIM) Isomerase Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) Oxidoreductase Phosphoglycerate kinase (PGK) Transferase Phosphoglycerate mutase (PGM) Isomerase Enolase (ENO) Lyase Pyruvate kinase (PK) Transferase Garrett R & Grisham CM (2005). Biochemistry (3rd ed). Belmont, CA: Thomson Brooks/Cole. pp Recall that G = G + RT lnk eq (see 1.1) where G is the actual free energy change under non-equilibrium (steady-state) conditions, and G is the standard free energy equilibrium! - Since cellular processes operate under steady-state rather than equilibrium setting, the free energy changes associated with various glycolytic steps are largely concerned with G! - Of the 10 steps of glycolysis, only three (Steps 1/3/10) operate far from equilibrium ( G << 0) implying that they COULD be largely responsible for flux control!

21 G Profile for Glycolysis Only Steps 1, 3 and 10 are associated with large negative G!

22 PFK Is the Major Flux-Controlling Enzyme - Actual free energy changes ( G) associated with various glycolytic steps suggest that the major candidates for flux control are: - Hexokinase (HK) => Step 1 - Phosphofructokinase (PFK) => Step 3 - Pyruvate kinase (PK) => Step 10 - Of these three enzymes, only PFK plays a central role in controlling the rate of flow of metabolites (or flux) through glycolysis why?! - HK is not absolutely critical for glycolysis (much less serve as a regulatory point!) since glycolysis in skeletal muscle often does not require HK (due to the breakdown of glycogen into glucose-6-phosphate via glycogenolysis) see On the other hand, PK catalyzes the final step of glycolysis thus its ability to control flux through glycolysis becomes somewhat moot - Simply put, the conversion of F6P to FBP in Step 3 by PFK is the major ratedetermining or rate-limiting (or the slowest) step of glycolysis - How is PFK regulated? In PFK, ATP not only binds to the active site (to serve as a substrate) but also to an allosteric site (to serve as an allosteric inhibitor)

23 Allosteric Regulation of PFK PFK Activity In respiring muscle: [ATP]/[ADP] ~ 10:1 F6P / mm - PFK is allsoterically inhibited by ATP a built-in control to ensure that when ATP is in excess (there is little demand for energy production), glycolysis is shut off! - On the other hand, AMP/ADP serve as allosteric activators of PFK rising cellular concentrations of AMP/ADP are indicative of energy shortage and thus serve as signals for the production of ATP - Acting in concert, AMP/ADP and ATP allsoterically modulate the enzymatic activity of PFK thereby enabling PFK to play a key role in the control of flux through glycolysis

24 Substrate Cycling Fine-Tunes Flux control 3 3 In resting muscle both PFKase and FBPase are active => low flux In active muscle PFKase is active but FBPase inhibited => high flux - In addition to its allosteric modulation, the Step 3 of glycolysis is further attuned in many mammalian tissues via substrate cycling: - F6P is cycled forth to FBP by phosphofrucktokinase (PFKase) - FBP is cycled back to F6P by fructose-1,6-bisphosphatase (FBPase) a hydrolase! - Substrate cycling can thus potentially decrease the flux through glycolysis by essentially placing a metabolic intermediate (F6P) into a holding pattern until the demand rises!

25 Exercise 3.2b - Which glycolytic enzymes are potential control points? - Describe the mechanisms that control phosphofructokinase activity - What is the metabolic advantage of a substrate cycle?

26 Section 3.2c: Fermentation

27 Synopsis 3.2c - Under aerobic conditions, the glycolytic end-product pyruvate is completely oxidized to CO 2 and H 2 O via the citric acid cycle (see 3.5) - Under anaerobic conditions, the glycolytic end-product pyruvate is converted (or reduced) to either lactate or ethanol in a metabolic process referred to as fermentation - Fermentation can be classified into two major groups: (1) Homolactic fermentation (in muscle) (2) Alcoholic fermentation (in yeast) - In each case, NADH is reoxidized to NAD + to ensure glycolytic continuity thus fermentation plays a key role in the regeneration of energy under anaerobic conditions!

28 O 2 Metabolic Fate of Pyruvate

29 Homolactic Fermentation (in muscle) - During strenuous activity when oxygen is in short supply, ATP is largely synthesized via anaerobic glycolysis in muscles - The pyruvate end-product of glycolysis is reduced to lactate in order to regenerate NAD + (from NADH) which is required for the continuity of anaerobic glycolysis in a process known as homolactic fermentation - Homolactic fermentation is catalyzed by lactate dehydrogenase (LDH) - Under resting conditions when oxygen is no longer limiting, lactate is converted back to pyruvate via equilibrium shift to allow its complete oxidation via the citric acid cycle - When transported to the liver via the bloodstream, lactate can also be used to synthesize glucose via gluconeogenesis generation of glucose from noncarbohydrate sources (including amino acids) - In anaerobic glycolysis, glycolysis and homolactic fermentation are essentially coupled in that glucose is oxidized to lactate in a seamless fashion a virtue of erythrocytes (see 3.1)

30 Alcoholic Fermentation (in yeast) - Under anaerobic conditions, the major source of ATP in yeast is also obtained via anaerobic glycolysis - The pyruvate end-product of glycolysis is ultimately converted via a two-step process called alcoholic fermentation to ethanol: (1) Pyruvate decarboxylase (PDC) decarboxylates pyruvate to acetaldehyde using thiamine pyrophosphate (TPP) as a cofactor with concomitant release of CO 2 which serves as a leavening agent in bread (2) Alcohol dehydrogenase (ADH) subsequently reduces acetaldehyde to ethanol (the active ingredient in wine and beer) with concomitant regeneration of NAD + (from NADH) which is required for the continuity of anaerobic glycolysis - Due to the release of CO 2 as a by-product, alcoholic fermentation is commercially exploited in the production of alcoholic beverages as well as in the rising of bread (leavened bread)

31 Exercise 3.2c - Describe the three possible fates of pyruvate - Compare homolactic and alcoholic fermentation in terms of the products and the cofactors required