Glycolysis & TCA Cycle Flashcards
(36 cards)
1
Q
Metabolism
A
chemical conversions in biological systems
- series of enzyme catalysed reactions with net thermodynamic favorability
2
Q
Anabolism
A
Synthesis of macromolecules from precursor molecules
3
Q
Catabolism
A
Breakdown of nutrients into ‘waste’
4
Q
Main types of Metabolic Reactions
A
- hydrolysis/dehydration
- oxidation/reduction
- isomerisation
- C-C cleavage
- group transfer
5
Q
Glycolysis
A
energy producing pathway also providing synthesis precursors
- first step in oxidation of glucose to carbon dioxide
- can occur with or without oxygen
6
Q
Net Glycolysis Reaction
A
glucose + 2Pi + 2ADP + 2NAD+ –> 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H20
7
Q
TCA Cycle
A
final common pathway for oxidation of all fuel molecules / can also give biosynthesis intermediates
8
Q
2 Phases of Glycolysis
A
- Preparatory phase
- phosphorylation of glucose using 2 ATP
- cleavage to 2x3C sugars - Payoff phase
- oxidation of 3C sugars to product ATP
- each reaction happens twice per glucose molecule
9
Q
Step 1 of Glycolysis
A
Phosphorylation of Glucose - C6
- Glucose 6 Phosphate formed
- hexokinase catalyses this
- activates glucose; some energy from ATP hydrolysis conserved in the molecule
- keeps it in the cell (no transporter available for it)
10
Q
Step 2 of Glycolysis
A
Isomeration
- fructose 6 phosphate formed
- aldose to ketose sugar
- carbonyl group moved from C1 to C2
11
Q
Step 3 of Glycolysis
A
Phosphorylation
- fructose 1,6 bisphosphate
- group transfer reaction of phosphate onto C1: both C phosphorylated ensures that both 3C sugars made will have phosphate groups
- phosphofructokinase catalyses this
12
Q
Step 4 of Glycolysis
A
Carbon - Carbon cleavage
- C2 carbonyl facilitates C-C bond cleavage at correct position
- dihydroxyacetone phosphate and glyceraldehyde 3-phosphate formed
- dihydroxyacetone phosphate isomerised to glyceraldehyde 3-phosphate via intramolecular redox reaction (H transfer from C1 to C2), this is simply an isomerisation
13
Q
Step 5 of Glycolysis
A
Oxidation by NAD+ and phosphorylation
- 1,3 bisphosphoglycerate
- uses inorganic phosphate
- phosphorylation coupled to glyceraldehyde 3-phosphate by a thioester intermediate
- energy from this oxidation trapped in 1,3 BPG to later power ATP production
14
Q
Step 6 of Glycolysis
A
ATP Production
- net yield of ATP here is 0
- energy rich / high phosphoryl transfer power
- 3-phosphoglycerate formed
- phosphate from position 1 transferred to ADP
- group transfer reaction
15
Q
Step 7 of Glycolysis
A
Phosphate Moved from 3 position to 2 position
- needed for final steps
- 2-phosphoglycerate formed
- phosphoglycerate mutase catalyses this
16
Q
Step 8 of Glycolysis
A
Dehydration
- water removed to give phosphoenolpyruvate
- activating phosphate group for transfer to ADP
17
Q
Step 9 of Glycolysis
A
ATP Production
- pyruvate formed
- unstable enol form stabilised
- high phosphoryl transfer potential arises from enol-ketone conversion driving force
- net yield of 2 ATP
- ATP made due to substrate level phosphorylation
18
Q
Substrate Level Phosphorylation
A
Steps 6,9 of glycolysis
- mechanism of ATP synthesis in glycolysis
- both 1,3 BPG and phosphoenol pyruvate have higher phosphoryl transfer power than ATP
- they’re also unstable so it is favorable to transfer a phosphate to give a stable molecule
19
Q
Aerobic Conditions
A
- glucose fully oxidised to carbon dioxide using coenzyme A and the TCA cycle
20
Q
Anaerobic Conditions
A
- NAD+ not regenerated via oxidative phosphorylation so additional reactions are needed to regenerate them to continue glycolysis
- oxidised to ethanol or lactate
- buildup of these can be toxic however so limit glycolysis rate
21
Q
Allosteric Regulation of Glycolysis
A
- done at key/irreversible points in the pathway
- hexokinase
- pyruvate kinase
- phosphofructokinase: key control point as this is the first commited step of glycolysis
- downregulated by ATP increases
22
Q
Glycolysis in the Muscle vs. Liver (PFK 1 regulation)
A
Different isozymes of PFK
muscle:
- low ph = inhibition
- lactate production slows glycolysis down to prevent damage
- high ATP decreases affinity for substrate (energy charge ratio regulates enzyme)
liver: enzyme also controlled by concentrations of biosynthetic intermediates
- PFK inhibited by citrate
- fructose 2,6 BP activates PFK
23
Q
Reversible Phosphorylation
A
- liver: pyruvate kinase
- hormone triggered phosphorylation (glucagon)
- phosphorylation by cyclic AMP dependent protein kinase makes enzyme less active
- low glucose levels = phosphorylation = glycolysis slowed to conserve glucose
- this prevents the liver from using up all the glucose
24
Q
TCA Cycle
A
- complete oxidation of glucose to produce reduced electron carriers that feed into the ETC
- final common pathway of all fuel molecule oxidation
25
Pyruvate Dehydrogenase
- links glycolysis to TCA cycle
- oxidative decarboxylation of pyruvate to acetyl CoA in mitochondrial membrane
- large enzyme complex made of 3 enzymes and 5 cofactors
26
Reaction Mechanism of PDH
1. decarboxylation
2. oxidation
3. transfer to CoA
27
Coenzyme A
- activated carrier of acyl groups
- contains reactive thiol group that reacts with carboxylic acid to form a thioester
- derives from vitamin B5
- thioester hydrolysis has large negative free energy so acetyl is readily transferred to other molecules
- CoA addition activates acetyl group
28
Net Reaction of TCA Cycle
acetyl CoA + 3NAD + FAD + ADP/GDP + Pi + 2H2O = 2CO2 + 3NADH + FADH2 + ATP/GTP + CoA + 2H+
- essentially an oxidation of acetyl CoA to carbon dioxide and reduction of electron carriers
- ATP synthesis is 2.5 ATP per NADH and 1.5 ATP per FADH2
29
Steps in TCA Cycle
1. 2 carbon acetyl-CoA condensation (aldol condenstoin with hydrolysis) with oxaloacetate to form citrate (6C)
2. isomerisation to isocitrate (reposition hydroxyl group to set up decarboxylation)
3. oxidative decarboxylation to form a-ketoglutarate (5C)
4. oxidative decarboxylation and CoA addition to form succinyl-CoA (4C)
5. substrate level phosphorylation to succinate (forms GTP and uses thioester bond hydrolysis to drive synthesis)
6. oxidation to form fumarate (FADH reduced because energy change of reaction not high enough to reduce NAD)
7. hydration to for malate (water added across double bond)
8. oxidation to form oxaloacetate (oxidation of hydroxyl to carbonyl) ; ie. regeneration of starting compound
30
ATP Generation from TCA Cycle
1. substrate level phosphorylation of succinyl CoA to succinate
2. oxidative phosphorylation using reduced electron carriers in ETC
31
NAD (nicotinamide adenine dinucleotide)
- nicotinamide ring
- synthesised from vit. B3
1. addition of 2H and 2e
2. removal of one proton
- forms NADH
32
FAD (flavin adenine dinucleotide)
- flavin ring
- synthesised from vit. B2
1. addition of 2H and 2e at one time
33
Biosynthesis and Anabolism in TCA Cycle
- also produces building blocks for synthesis of biomolecules
- citrate gives fatty acids/sterols
- a-keto glutarate gives amino acids
- succinyl-CoA gives porphyrins
- oxaloacetate gives purines/pyrimidines and amino acids
34
Production of Oxaloacetate to replenish cycle
- carboxylation of pyruvate
- anaplerotic reaction
- enzyme: pyruvate carboxylase (biotin ring)
- biotin attached via amide linkage to lysine
- biotin used to attach bicarbonate that is then transferred to pyruvate
- uses ATP for energy
35
Allosteric Regulation of TCA cycle
- response to ATP levels in cell (energy charge)
1. pyruvate dehydrogenase (inhibited by ATP/NADH/acetyl CoA)
2. isocitrate dehydrogenase (inhibited by ATP/NADH)
3. a-keto glutarate dehydrogenase (inhibited by ATP/NADH/succinyl CoA)
36
Regulation of Pyruvate Dehydrogenase
- allosteric regulation and reversible phosphorylation
- phosphorylation of serine by PDH kinase on E1 subunit inactivates
- kinase is activated by ATP, acetyl CoA, NADH
- dephosphorylation of serine by PDH phosphatase on E1 subunit activated
eg. muscle phosphatase is activated by calcium ions. this signals that ATP production is needed and stimulates the TCA cycle
