Title: Glucose Catabolism Ch. 14
1Glucose Catabolism Ch. 14
- Overview
- Reactions
- 1-5 Energy invested, 6-10 energy recouped
- Fermentation
- Anaerobic end products Ethanol or Lactate
- Control - Substrate cycle PFK vs FBPase
- Other Hexoses - Fructose, galactose, mannose
- Pentose Phosphate pathway
- Produces NADPH and nucleotide sugars
2 Overview
- Glucose 2NAD 2ADP 2Pi -gt 2 Pyruvate
2NADH 2ATP 2 H2O 4H - The C6 sugar is phosphoryated twice (- 2 ATP)
- Cleaved to give two 3 carbon units
- Oxidized 2(NAD -gt NADH)
- 2 phosphoryl groups from each of 2 C3 units
transferred to ADP (4 ATP)
3Glycolysis Reactions 1-5
- 1. Hexokinase
- Glucose ATP -gt Glucose-6-phosphate ADP Pi
- 2. Phospho Glucose Isomerase
- G6P -gt Fructose-6-phosphate
- 3. Phospho Fructokinase
- F6P ATP -gt Fructose 1,6-bisphosphate ADP Pi
- 4. Aldolase
- FBP -gt Glyceraldehyde-3-phosphate
Dihydroxyacetone phosphate - 5. Triose Phosphate Isomerase
- DHAP ltgt GAP
4Hexokinase
- Glucose ATP -gt Glucose-6-phosphate ADP Pi
- ATP supplied as Mg2 ATP
- Active site cleft closes over substrates water
excluded
5Phospho Glucose Isomerase (PGI)
CH2OPO32-
CH2OH
O
O
H
H
OH
H
OH
OH
OH
H
H
- Glucose-6-phosphate -gt Fructose-6-phosphate
- General Acid protonates O - ring opens
- Enediolate (C1 - C2) intermediate
- Enol -gt Keto at C2
- Ring closure by attack of O on C2 CO
6Phospho Fructokinase (PFK)
CH2OPO32-
CH2OPO32-
O
OH
H
H
- F6P ATP -gt Fructose 1,6-bisphosphate ADP Pi
- Second input of ATP
- Mechanism similar to hexokinase
- Rate determining step in glycolysis
- Allosterically regulated by AMP, ATP and citrate
7Aldolase
CH2OPO32- C O HO C H H C OH H C OH
CH2OPO32-
CH2OPO32- C O HO C H H C OH H C OH
CH2OPO32-
DHAP
GAP
- FBP -gt Dihydroxyacetone phosphate
Glyceraldehyde-3-phosphate - DHAP GAP numbered from center
- C3 bears the phosphoryl group
- Schiff base or metal ion catalysed reaction
8Triose Phosphate Isomerase (TIM)
- DHAP ltgt GAP Keq 0.0473
- Only GAP utilized in subsequent reactions
- TIM is an a/b barrel, highly efficient
- Enediolate intermediate mechanism
9Glycolysis Reactions 6-10
- 6. Glyceraldehyde-3-Phosphate Dehydrogenase
- GAP NAD Pi -gt 1,3-Bisphosphoglycerate
NADH H - 7. Phosphoglycerate kinase
- 1,3-BPG ADP -gt 3-Phosphoglycerate ATP
- 8. Phosphoglycerate Mutase
- 3PG -gt 2-phosphoglycerate
- 9. Enolase
- 2PG -gt Phosphenolpyruvate H2O
- 10. Pyruvate Kinase
- PEP ADP -gt Pyruvate ATP
10Glyceraldehyde-3-Phosphate Dehydrogenase(GAPDH)
- GAP NAD Pi -gt 1,3-Bisphosphoglycerate NADH
H - Inactivated by iodoacetate - Cysteine in active
site - 3H at C1 is transferred to NADH - direct hydride
transfer - Thio ester acyl enzyme intermediate is displaced
by Pi - Acyl phosphate is a high energy bond
- Overall the reaction is endergonic but removal of
1,3BPG by PGKdrives the reaction forward
11Phosphoglycerate kinase (PGK)
O C O- H C OH CH2OPO32-
- 1,3-BPG ADP -gt 3-Phosphoglycerate ATP
- First ATP synthesis step
- ADP and ATP in complex with Mg
- ?Gis -12 kJ M-1
12Phosphoglycerate Mutase (PGM)
O C O- H C OPO32- CH2OH
O C O- H C OH CH2OPO32-
- 3PG -gt 2-phosphoglycerate
- PGM contains phospho-histidine
- Determined by 32P transfer from 3PG
- Enzyme first transfers PO3 to make 2,3BPG
- 2,3BPG intermediate occasionally dissociates
- 2,3BPG is an allosteric effector of hemoglobin
- Side reactions in erythrocytes convert 1,3 2,3
BPG - 2PG ?G hydrolysis -17.6 kJ /mol
13Enolase
O C O- H C OPO32- CH2OH
O C O- C OPO32- CH2
- 2PG -gt Phosphenolpyruvate H2O
- Mg binds first
- Two-step dehydrogenation
- 1. Abstraction of proton from C2
- 2. Elimination of C3 -OH
14Pyruvate Kinase
O C O- C O CH3
O C O- C OPO32- CH2
- PEP ADP -gt Pyruvate ATP
- 2nd ATP generating step
- Beta phosphoryl of ADP attacks P
- Enol pyruvate -gt keto form
15Fermentation
- Anaerobic processes to recycle NAD
- For glycolysis to continue NADH (from GAPDH
reaction) -gt NAD - In muscle, lactate dehydrogenase converts
Pyruvate NADH -gt Lactate NAD - In yeast, pyruvate decarboxylase and alcohol
deydrogenase convert pyruvate NADH to CO2
Ethanol NAD
16Lactate Dehydrogenase (LDH)
O C O- C O CH3
O C O- HO C H CH3
- Pyruvate NADH -gt Lactate NAD
- Hydride transfer to C2 of pyruvate
- Lactate (lactic acid) build-up lowers pH
- Low pH causes muscle fatigue
17Pyruvate Decarboxylase
O C O- C O CH3
H C O CH3
- Pyruvate -gt Acetaldehyde CO2
- Thiamine pyrophosphate (TPP) cofactor
- Attack on Carbonyl carbon release of CO2
- Mechanism Fig 14-20
18Alcohol Dehydrogenase (ADH)
H C O CH3
- Acetaldehyde NADH -gt Ethanol NAD
- Similar to LDH
19Control
- Flux forward rate - reverse rate Jvf - vr
- At equilibrium Flux 0 vf vr
- For rate determining steps vf gtgt vr J vf
- J can be varied by
- Substrate concentration
- Allosteric control
- Covalent modification - eg. Phosphorylation
- Substrate cycles
- independent forward and reverse reactions
- Change in enzyme levels - Genetic control
20Control of Glycolysis in Muscle
- Hexokinase, PFK and Pyruvate Kinase
- the only reactions in glycolysis with large -?G
- PFK is the major regulatory point
- ATP is a substrate and allosteric inhibitor
- Binds to inhibitor site when PFK is in the T
state - AMP binds to R state
21Substrate cycling
- Fructose 1-6 bisphosphatase
- 1,6-Fructose Bisphosphate -gtFructose6P Pi
- Provides a pathway for reverse reaction of PFK
- Opportunity for greater regulatory range
- Net effect of the two reactions is ATP hydrolysis
- exothermic reaction generates heat - Thermogenesis important for insect flight and
Obesity
22Other Hexoses
- Fructose -gt F1P
- Fructokinase makes Fructose-1-P
- Galactose -gt UDP-Glu -gt Glu-1P -gt Glu-6P
- Galactokinase makes Galactose-1-P
- Gal-1P Uridylyl tranferase makes UDP-Gal
- UDP Gal-4- epimerase makes UDP-Glu
- Mannose
- Hexokinase, makes Man-6-P
- phophomannose isomerase Man-6-P -gtF6P
23Pentose Phosphate pathway
- NADPH - Reductant distinct from NADH
- NAD/NADH 1000
- NADP/NADPH 0.01
- 3G6P 6NADP 3H2O gt 6NADPH 6H 3CO2
2F6P GAP - 3 stages
- Oxidations to give NADPH and Ru5P
- Isomerizations to give R5P and Xu5P
- Bond breaking and making to give F6P and GAP
24NADPH production
- G6P dehydrogenase
- G6P NADP gt 6-phosphoglucono-d-lactone NADPH
H - Phosphogluconolactonase
- 6-phosphoglucono-d-lactonegt 6-phosphogluconate
- Phosphogluconate dehydrogenase
- 6-phosphogluconate NADP gt Ru5P NADPH CO2
25Ribulose-5-phosphate
- Ribulose-5-phosphate isomerase
- Ru5P gt Ribose 5P
- R5P is a precursor for nucleotide synthesis
- Ribulose-5-phosphate epimerase
- Ru5P gt Xylulose 5P
26CC bond cleavage
- Transaldolase
- R5P Xu5P gt S7P GAP 55 gt 73
- E4P Xu5P gt F6P GAP 45 gt 63
- Transketolase
- S7P GAP gt E4P F6P 73 gt 64
27Control
- G6PDH is regulated by substrate
- NADP
- G6P dehydrogenase deficiency
- Most alleles have low protein stability
- Low NADPH leads to oxidation damage to
erythrocytes - Linked to malaria resistance