Wk3 Metabolism pt2 Flashcards
What happens when ATP is low?
Phosphofructokinase and glycolysis are switched on to generate ATP
What is pyruvate converted to when oxygen levels are low?
Lactate. Produces 2 ATP rapidly but much more is produced by TCA cycle (anaerobic)
what is the citric acid cycle?
- Also known as tricarboxylic acid cycle (TCA)
- accounts for 2/3 of total oxidation of carbon in most cells
- takes place in the mitochondrial matrix
- Oxygen is required for the downstream electron transport chain as oxygen is a final electronic sceptre for NADH to lose electrons and NAD plus returns to the cycle
Acetyl CoA
CoA + 2CO2
3NAD+
3NADH
FAD
FADH2
GDP+Pi
GTP
What is NAD+?
- Derived from vitamin niacin (B3)
- Acts as a coenzyme in several redox reactions
- Oxidation in respiratory chain generates 2.5 molecules of ATP
What is FAD?
- Derived from vitamin riboflavin (B2)
- Attaches covalently to irs enzyme (prosthetic group)
- Succinate dehydrogenase contains FAD and is bound to the inner membrane of the mitochondria and is an integral part of the respiratory chain
- FADs oxidation in succinate dehydrogenase generates 1.5 molecules of ATP
What are anaplerotic reactions?
- reactions which fill in missing metabolites for important with metabolic pathways
- Direct conversion of pyruvate to oxalaxetate
- oxalacetate/aspirate conversion
- glutamate/a-ketoglutarate conversion
- malate to pyruvate conversion (malic enzyme)
Last step of electron transport chain
2e- + 2H+ + 1/2 O2 = H2O
Process in electron transport chain
- when electrons leave NADH they are high in energy
- every time they are passed on to one of the complex is they lose energy
- this energy is then used to pump protons from the mitochondrial membrane into the inter membrane space
- The proton gradient is then used to produce ATP
- The transporter two electrons to complex one and three will extra food for H + each into the into membrane space
- during the proton pumping there are no counter ions pumped over the membrane
- this results in a charge separation and a possible concentration gradient so the electrochemical potential is 150 to 250 MV
- this potential difference provides energy for ATP synthesis
- 3H plus ions are needed to make one ATP plus 1H plus to translocate ATP to the cytosol
- cytochrome c oxidase (complex IV) which catalysed the transfer of the electrons to molecular oxygen can be inhibited by cyanide carbon monoxide and azide
What is substrate level phosphorylation
transfer of phosphate from a substrate to ATP
What is oxidative phosphorylation
formation of ATP coupled to oxidation of NADH or FADH2 by oxygen
What does ATP synthase do
- protons in the into membrane space cannot be allowed to flow back into the matrix and be lost as heat
- The electrochemical gradient is used to drive synthesis of ATP via confirmational change in ATP synthase
Oxidation of cytosolic NADH
❖ NADH molecules formed during glycolysis are located in the cytosol
❖ However, NADH can only be oxidised inside the mitochondria and NADH are unable to cross the mitochondrial membrane
❖ 2 mechanisms exist which enable ‘reducing equivalents’ to be transferred from the cytosol into the mitochondrion
❖ the glycerol phosphate shuttle (important in insects) uses cytosolic NADH to reduce DHAP to form glycerol-3-phosphate which then diffuses into the mitochondria and is oxidised by the mitochondrial glycerol-3- phosphate dehydrogenase to form DHAP and FADH2
Oxidation of cytosolic NADH: the malate/aspartame and glycerol phosphate shuttle
❖ The malate/aspartate shuttle starts with cytosolic oxaloacetate
❖ malate dehydrogenase reduces OAA to form malate, which is then transported into the mitochondria
❖ inside the mitochondria the reaction is simply reversed by the mitochondrial malate dehydrogenase
❖ however as OAA is unable to cross the inner mitochondrial membrane it has to be transaminated to aspartate which then can be transported into the cytosol, where it is converted back into OAA by the cytosolic aspartate aminotransferase
❖ while usually 10 H+ per NADH can be pumped across the membrane to form ATP, in this case the glutamate aspartate carrier (to maintain glutamate and aspartate concentrations) uses 1 H+. Hence the ATP production is only 2.25 molecules of ATP per cytosolic NADH
How many ATP molecules should you get from 1 molecule of glucose?
- oxidation of NADH = extrusion of 10H+ ions = 2.5 molecules ATP
- FADH2 = extrusion of 6H+ = 1.5 ATP
- in eukaryotes, cytosolic NADH loses another H+ due to malate/aspartame shuttle
- using historic P/O = 38 molecules
- insects = 36 ATP
- modern non-integer P/O = 31 ATP using malate/aspartate, 29.5 using glycerol phosphate shuttle
- no set number?
What is brown adipose tissue?
❖ Rare compared with white fat
❖ Mitochondria largely uncoupled, so energy released as heat rather than captured
as ATP
❖ Important for maintaining body temp, especially in neonates (hibernation)
❖ Uncoupling proteins (UCP) provide proton channel
❖ Dinitrophenol is also an uncoupler
What else can be used to produce ATP?
❖ Fatty acids can be oxidised, although it is a slow process
❖ Proteins can be broken down to release amino acids which then can be broken down further to produce ATP
❖ Lactate! (Certain cells in brain use this)
How to control metabolism
❖ Primary control - the level of ATP
❖ Levels of intermediates affect local rates
❖ 3 major control strategies:
❖ Enzyme levels
❖ Enzyme activities
❖ Substrate availability
❖ Many points of control, mostly at steps unique to the pathway
GLUT1
All mammalian tissues
1mM
Basal glucose uptake
GLUT2
Liver cells
15-20mM
Removes excess glucose from blood
Pancreatic beta cells
15-20 mM
Plays role in regulation of insulin
GLUT3
All mammalian tissues
1mM
Basal glucose uptake
GLUT4
Muscle cells
5mM
Amount in plasma membrane increases with endurance training
Muscle and fat cells
5mM
What are control points in TCA cycle?
Pyruvate dehydrogenase
Isocitrate dehydrogenase
A-ketoglutarate dehydrogenase
Inhibited by ATP, NADH and acetyl CoA
What are different organs metabolic profiles?
❖ Brain - consumes ~60% of body glucose at rest. Can use ketone bodies in starvation
❖ Muscle - resting muscle uses fatty acids. Anaerobic muscle draws on glycogen stores (75% of glycogen stored in muscle)
❖ Kidneys - 0.5% of body mass, use 10% of body glucose (Na+- K+ ATPase)
❖ Liver - major site of conversion (Cori cycle)
❖ Differential uptake and specific isoenzymes account for differences
What diseases are associated with defects in carbohydrate metabolism?
❖ A range of diseases resulting from mitochondrial defects (neuro/visual symptoms) - eg Leber hereditary optic neuropathy (complex I)
❖ Beriberi (VitB1, Pyr DeH, a-ketoglut DeH), mercury & arsenic poisoning (Pyr DeH and GAP -> 3PGA conversion)
❖ Diabetes, glucosuria
❖ Glycogen storage disease (eg von Gierke’s - absence of glucose 6- phosphatase; McArdle’s - myophosphorylase deficiency; Tarui - PFK; Cori; others)
❖ Cancer (Metabolomics; PKM2)
What is diabetes mellitus?
Imbalance between insulin and glucagon, high blood glucose, excessive ketone body production
Excess ketone body production = acidosis, coma, death
