Wk3 Metabolism pt2

Question 1

What happens when ATP is low?

Answer 1

Phosphofructokinase and glycolysis are switched on to generate ATP

Question 2

What is pyruvate converted to when oxygen levels are low?

Answer 2

Lactate. Produces 2 ATP rapidly but much more is produced by TCA cycle (anaerobic)

Question 3

what is the citric acid cycle?

Answer 3

• 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

Question 4

Acetyl CoA

Answer 4

CoA + 2CO2

Question 5

3NAD+

Answer 5

3NADH

Question 6

FAD

Answer 6

FADH2

Question 7

GDP+Pi

Answer 7

GTP

Question 8

What is NAD+?

Answer 8

• Derived from vitamin niacin (B3)
• Acts as a coenzyme in several redox reactions
• Oxidation in respiratory chain generates 2.5 molecules of ATP

Question 9

What is FAD?

Answer 9

• 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

Question 10

What are anaplerotic reactions?

Answer 10

• 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)

Question 11

Last step of electron transport chain

Answer 11

2e- + 2H+ + 1/2 O2 = H2O

Question 12

Process in electron transport chain

Answer 12

• 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

Question 13

What is substrate level phosphorylation

Answer 13

transfer of phosphate from a substrate to ATP

Question 14

What is oxidative phosphorylation

Answer 14

formation of ATP coupled to oxidation of NADH or FADH2 by oxygen

Question 15

What does ATP synthase do

Answer 15

• 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

Question 16

Oxidation of cytosolic NADH

Answer 16

❖ 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

Question 17

Oxidation of cytosolic NADH: the malate/aspartame and glycerol phosphate shuttle

Answer 17

❖ 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

Question 18

How many ATP molecules should you get from 1 molecule of glucose?

Answer 18

• 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?

Question 19

What is brown adipose tissue?

Answer 19

❖ 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

Question 20

What else can be used to produce ATP?

Answer 20

❖ 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)

Question 21

How to control metabolism

Answer 21

❖ 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

Question 22

GLUT1

Answer 22

All mammalian tissues
1mM
Basal glucose uptake

Question 23

GLUT2

Answer 23

Liver cells
15-20mM
Removes excess glucose from blood

Pancreatic beta cells
15-20 mM
Plays role in regulation of insulin

Question 24

GLUT3

Answer 24

All mammalian tissues
1mM
Basal glucose uptake

Question 25

GLUT4

Answer 25

Muscle cells
5mM
Amount in plasma membrane increases with endurance training

Muscle and fat cells
5mM

Question 26

What are control points in TCA cycle?

Answer 26

Pyruvate dehydrogenase
Isocitrate dehydrogenase
A-ketoglutarate dehydrogenase

Inhibited by ATP, NADH and acetyl CoA

Question 27

What are different organs metabolic profiles?

Answer 27

❖ 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

Question 28

What diseases are associated with defects in carbohydrate metabolism?

Answer 28

❖ 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)

Question 29

What is diabetes mellitus?

Answer 29

Imbalance between insulin and glucagon, high blood glucose, excessive ketone body production

Excess ketone body production = acidosis, coma, death