Glucose Metabolism. Glycolysis Expectations

Transcription

1 Glucose Metabolism Pratt and Cornely, Chapter 13 Glycolysis Expectations Memorize/learn Figure 13.2 Know overall reaction and stages Explain chemical logic of each step Enzyme mechanisms presented in book 1

2 Glycolysis Ten enzymes that take glucose to pyruvate Cytosol ATP and NADH Reactions and Enzymes of Glycolysis ATP ATP P i + NAD + ADP ADP 2x ADP ADP NADH 2x 2x 2x ATP ATP Hexose and triose phases Energy input and payoff phases 2

3 Energy Input Energy Payoff 3

4 Know... Substrates Co substrates Products Enzyme names 1. Hexokinase Previous concepts: Induced fit, kinase Energy use/production? Chemical logic? 4

5 Problem 3 (Notice miswording) The G o value for hexokinase is 16.7 kj/mol, and the G value under cellular conditions is similar. What is the ratio of G 6 P to glucose under standard conditions at equilibrium if the ratio of ATP:ADP is 10:1? How high would the ratio of G 6 P to glucose have to be to reverse the hexokinase reaction by mass action? 2. Phosphoglucose Isomerase Previous concepts: Isomerization Energy use/production? CNCEPT: Near equilibrium Chemical logic? Stereochemistry reverse does not produce mannose! 5

6 3. PFK 1 Previous concepts: Allosteric inhibition Energy use/production? Chemical logic? First committed step of glycolysis Why? regulation 6

7 Regulation 4. Aldolase Previous concepts: Standard free energy is +23kJ, but it is a near equilibrium reaction Energy use/production? Chemical logic? Beginning of triose stage 7

8 Aldolase Mechanism 5. Triose Phosphate Isomerase Previous concepts: Catalytic perfection Energy use/production? Chemical logic? Most similar to which previous reaction? 8

9 6. Glyceraldehyde 3 P DH Previous concepts: Redox and dehydrogenase Energy use/production? Chemical logic? GAPDH Mechanism 9

10 7. Phosphoglycerate Kinase Previous concepts: High energy bond Energy use/production? Substrate level phosphorylation Chemical logic? Coupled to reaction 6 Coupled Reactions GAPDH = 6.7 kj/mol PG Kinase = 18.8 kj/mol verall: 10

11 8. Phosphoglycerate Mutase Previous concepts: Covalent catalysis Energy use/production? Chemical logic? Mutase isomerization with P transfer Mechanism Not a simple transfer What happens if the bisphosphate escapes? 11

12 9. Enolase Concept: Phosphoryl group transfer potential Energy use/production? Chemical logic? 10. Pyruvate Kinase Energy use/production? Chemical logic? Regulation: F 1,6 BP can act as a feedforward activator to ensure fast glycolysis 12

13 Standard Free energies are up and down Free energies under cellular conditions are downhill Three irreversible verall Energetics Amino acid and nitrogen metabolism Fate of Pyruvate Aerobic Energy Gluconeogenesis Anaerobic in higher organisms Anaerobic in microorganisms 13

14 The Problem of Anaerobic Metabolism With oxygen, the NADH produced in glycolysis is re oxidized back to NAD + NAD + /NADH is a co substrate which means If there is no oxygen, glycolysis will stop because The solution to the problem is to The solution in Yeast Pyruvate is decarboxylated (cofactor?) to acetaldehyde Acetaldehyde transformed to ethanol What type of reaction? What cofactor? NAD + is regenerated to be reused in GAPDH 14

15 Lactate formation The Solution in Us Balanced equation We don t operate anaerobically... Most energy still trapped in lactate Back to pyruvate, then acetyl CoA Citric acid cycle 15

16 ther sugars enter glycolysis High fructose diet puts sugars through glycolysis while avoiding major regulation step Glucose Metabolism verview Keep the main pathway purposes distinct But learn details of chemistry and regulation based on similarities 16

17 Glucose Metabolism verview Energy Production ATP Pentose Phosphate Pathway Gluconeogenesis Glycogen metabolism Pentose Phosphate Pathway Pyruvate DHAP Glycerol (Triacylglycerides) H H DHAP H Ribose, NADPH H H (P) Glycogen Synthesis Glycogen Glycogen Lactate Glycogen Degradation Pyruvate Amino Acids Gluconeogenesis Precursors for Gluconeogenesis Names of compounds? Type of reaction? Type of enzyme? Cofactor(s)? More on lactate processing later H H H NH 2 H P 3 H 17

18 Chemistry of Gluconeogenesis Chemically opposite of glycolysis (mainly) Energetically costly no perpetual motion machine! Points of regulation Glycolysis Step 1: costs 1 ATP Step 3: costs 1 ATP Step 7: makes 2 ATP Step 10: makes 2 ATP Gluconeogenesis Step 10: no change Step 8: no change Step 3: costs 2 ATP Step 1: costs 4 ATP equivalents 18

19 Step 1a Pyruvate Carboxylase Biotin Costs ATP to make driving force for next reaction First step in biosynthesis of glucose and many other molecules Related to which amino acid? Mixed anhydride Coupled through biotin coenzyme Mechanism 19

20 PEP carboxykinase Step 1b ATP cost to restore PEP C 2 loss drives rxn Step 8 Fructose 1,6 bisphosphatase No additional energy input Phosphate ester hydrolysis is spontaneous 20

21 Step 10 Glucose 6 phosphatase Liver (and others) Not in muscle Problem 34 A liver biopsy of a four year old boy indicated that the F 1,6 Bpase enzyme activity was 20% normal. The patient s blood glucose levels were normal at the beginning of a fast, but then decreased suddenly. Pyruvate and alanine concentrations were also elevated, as was the glyceraldehyde/dhap ratio. Explain the reason for these symptoms. 21

22 Key Regulation At the committed step in glucogenic cells Principle of Reciprocal regulation Local regulation vs Hormone regulation Local regulation Key Regulation AMP/ATP (energy charge) Citrate (feedback) Hormone regulation Fructose 2,6 bisphosphate Gluconeogenesis is inhibited Glycolysis is stimulated 22

23 Problem 39 Brazilin, a compound found in aqueous extracts of sappan wood, has been used to treat diabetics in Korea. It increases the activity of the enzyme that products F 2,6 BP and stimulates the activity of pyruvate kinase. What is the effect of adding brazilin to liver cells in culture? Why would brazilin be an effective treatment for diabetes? Glucose Metabolism verview Energy Production ATP Pentose Phosphate Pathway Gluconeogenesis Glycogen metabolism Pentose Phosphate Pathway Pyruvate DHAP Glycerol (Triacylglycerides) H H DHAP H Ribose, NADPH H H (P) Glycogen Synthesis Glycogen Glycogen Lactate Glycogen Degradation Pyruvate Amino Acids Gluconeogenesis 23

24 Storage molecule Primer necessary Very large! Multiple ends allow for quick synthesis and degradation Glycogen Step 1 Chemistry of Synthesis Near equilibrium The link to glucose 6 phophate, our central molecule 24

25 Chemistry of Synthesis Step 2 Count high energy bonds Pyrophosphatase Common motiff UDP glucose: activated for incorporation Step 3 Glycogen synthase Growing end is non reducing UDP released Chemistry of Synthesis 25

26 Energetics of Synthesis Total cost: one ATP equivalent from G 6 p Chemistry of Degradation Glycogen phosphorylase Key Regulation site Inorganic phosphate as a nucleophile Remake G 1 P with no ATP cost 26

27 27 verall Energetics H H H H P H H H H P-P-Uridine H H H H H H H Glucose-6-P UDP UTP 2Pi Pi Key Enzymes H H H H P H H H H P-P-Uridine H H H H H H H Glucose-6-P UDP UTP 2Pi Pi Glycogen Synthase Glycogen Phosphorylase

28 Glycogen Storage Diseases Many disrupt glycogen breakdown in muscle and/or liver (hypoglycemia, enlarged liver, muscle cramps...) Glucose Metabolism verview Energy Production ATP Pentose Phosphate Pathway Gluconeogenesis Glycogen metabolism Pentose Phosphate Pathway Pyruvate DHAP Glycerol (Triacylglycerides) H H DHAP H Ribose, NADPH H H (P) Glycogen Synthesis Glycogen Glycogen Lactate Glycogen Degradation Pyruvate Amino Acids Gluconeogenesis 28

29 Pentose Phosphate Pathway Dual Purpose Synthesis of reducing potential Synthesis of 5 carbon sugars At cost of one carbon worth of carbohydrate Net reaction: Complex, 2 Stage Process xidative Stage Generates reducing power and ribose Non oxidative stage Regenerates 3 and 6 carbon sugars from 5 carbon sugars 29

30 xidative Stage Step 1: G 6 P DH Lactone formation xidative Stage Step 2: Also a spontaneous hydrolysis Practice mechanism, carbohydrate orientation 30

31 xidative Stage Step 3: xidative decarboxylation We will see this process again Biosynthesis of Ribose 31

32 Non oxidative Stage To understand purpose, realize that we generally need to make much more NADPH than ribose Problem: stuck with C5, but need C6 and C3 Solution: Shunt C5 back to C6 through near equilibrium reactions PPP Reactions Epimerase Isomerase Transketolase Transaldolase 32

33 Transketolase Use cofactor (B 1 ) to overcome chemical problem Mechanism 33

34 Different Modes for Different Purposes Problem 58 A given metabolite may follow more than one metabolic pathway. List all possible fates of glucose 6 P in (a) a liver cell and (b) a muscle cell. 34

35 Summary of glucose metabolism 35