2.2 - Cell Metabolism 2 Flashcards
1
Q
The Krebs / TCA cycle overview
A
- each turn produces 3 NADH, 2 CO2, 1 GTP, 1 FADH2
- the Krebs cycle enzymes (with one exception) are soluble proteins located in the mitochondrial matrix space
- the bulk of ATP is generated when the reduced coenzymes are re-oxidised with the help of oxygen during oxidative phosphorylation, therefore the TCA cycle only operates under aerobic conditions
- NADH –> 3 ATP
- FADH2 –> 2 ATP
- net 38 ATP per glucose
- glycolysis and the TCA cycle provide building blocks for further biosynthesis reactions
2
Q
TCA cycle stages
A
1: 4C oxaloacetate + 2C acetyl CoA –> 6C citrate
- condensation reaction
2: 6C citrate –> 6C isocitrate
- isomerisation
3: 6C isocitrate –> 5C a-ketoglutarate
- oxidative decarboxylation
- produces NADH + H+ + CO2
4: 5C a-ketoglutarate –> 4C succinyl CoA
- oxidative decarboxylation
- produces NADH + H+ + CO2
5: 4C succinyl CoA –> 4C succinate
- group transfer (of CoA)
- produces GTP
6: 4C succinate –> 4C fumerate
- hydration
- produces FADH2
7: 4C fumerate –> 4C malate
- reduction
8: 4C malate –> 4C oxalocatetate
- redox
- produces NADH + H+
3
Q
How do amino acids enter the TCA cycle?
A
- proteins –> AA, which feed into different points of the TCA cycle depending on the AA
- the general strategy of AA degradation is to remove the amino group (eventually excreted as urea) whilst the carbon skeleton is funnelled into the production of glucose (glucogenic) / fed into TCA cycle to synthesise fatty acids and ketone bodies (ketogenic)
- protein metabolism involves transamination reactions - amine group is transferred from one amino acid to a keto acid forming a new pair of amino and keto acids (group transfer)
4
Q
Alanine metabolism as an exemplar
A
alanine + a-ketoglutarate –> pyruvate + glutamate
- alanine aminotransferase
- pyruvate (keto acid) enters Krebs cycle as acetyl CoA is formed on decarboxylation
- glutamate is re-converted to a-ketoglutarate by glutamate dehydrogenase which generates NH4+, which is ultimately converted to urea
5
Q
NADH transport
A
- inner mitochondrial membrane is impermeable to NADH
- NADH from glycolysis –> mitochondria to regenerate NAD+ (finite) so glycolysis can continue
- high energy electrons from cytosol to mitochondria through shuttles
6
Q
Glycerol phosphate shuttle
A
- skeletal muscle, brain
- e- from NADH (rather than NADH itself) are carried across the mitochondrial membrane via a shuttle
1. Cytosolic glycerol-3-phosphate dehydrogenase transfers e- from NADH to DHAP to generate G3P (NAD+ regenerated)
2. A membrane bound form of the same enzyme transfers the e- to FAD, which then passes it to coenzyme Q (part of ETC)
7
Q
Malate-aspartate shuttle
A
- liver, kidney, heart
- aspartate –(aspartate transaminase)–> oxaloacetate –(malate dehydrogenase)–> malate in CYTOSOL
- (aspartate to OAA is transamination, OAA to malate regenerates NAD+ from NADH)
- malate-a-ketoglutarate antiporter transports malate in, a-ketoglutarate out of mitochondria
- same enzymes catalyse the reverse reaction in the mitochondria, forming aspartate
- electrons in NADH now in the mitochondria as NAD+ –> NADH
- glutamate-aspartate antiporter = aspartate out, glutamate into mitochondria
- redox reactions occurring
8
Q
TCA cycle defects in cancer
A
- mutations in some TCA genes have been shown to decrease TCA activity and enhance aerobic glycolysis, where lactate is produced despite there being ample O2 (Warburg effect)
- if we can force the cells to utilise oxidative phosphorylation instead, can we turn them into non-malignant
- defected enzymes: isocitrate dehydrogenase, succinate dehydrogenase, fumerase
9
Q
Fatty acid metabolism - B-oxidation
A
- in mitochondria
- produces acetyl CoA eventually, to enter TCA cycle
- firstly, fatty acids are converted into an acyl CoA species
- fatty acid + ATP + acetyl CoA –> acyl CoA + AMP + PPi (2 mol inorganic phosphate, 2 bonds broken)
- acyl CoA synthetase
10
Q
The carnitine shuttle
A
- generation of the acyl CoA species occurs on the outer mitochondrial membrane
- to transport into matrix it is coupled to the molecule carnitine to form acyl carnitine
- carnitine and acyl carnitine are moved to and from the matrix by a translocase
- cytoplasmic side: carnitine –> acyl carnitine by carnitine acyltransferase I, regenerating CoA from acyl CoA
- in the mitochondria, carnitine acyltransferase II catalyses the reverse reaction producing carnitine (and acyl CoA in the matrix as required)
- acyl carnitine in + carnitine out of mitochondria by translocase
11
Q
Primary carnitine deficiency
A
- autosomal recessive disorder
- symptoms appear during infancy/early childhood and include encephalopathies, cardiomyopathies, muscle weakness, hypoglycaemia
- mutations in SLC22A5 gene which encodes a carnitine transporter results in reduced ability of cells to take up carnitine, needed for B-oxidation of fatty acids
12
Q
Beta oxidation cycle
A
- acyl CoA undergoes a sequence of oxidation, hydration, oxidation and thiolysis reactions
- results in production of one molecule of acetyl CoA + an acyl CoA species which is 2C shorter than original
- each cycle produces 1 molecule of NADH + FADH2
- cycle repeats until 4C acyl CoA ends up as 2 acetyl CoA
- B-oxidation of 16C palmitoyl CoA produces 8mol acetyl CoA in 7 B-oxidation cycles
- palmitoyl CoA + 7 FAD + 7 NAD+ + 7 H2O + 7 CoA –> 8 acetyl CoA + 7 FADH2 + 7 NADH
- produces net 129 ATP
13
Q
Ketone body formation
A
- acetyl CoA from B-oxidation only enters TCA cycle if B-oxidation = carbohydrate metabolism (OAA needed for entry)
- if fat breakdown dominates, acetyl CoA –> acetoacetate, D-3-hydroxybutyrate, acetone (ketone bodies)
- catalysed in the liver
14
Q
Fatty acid biosynthesis / lipogenesis
A
- involves two enzymes: acetyl CoA carboxylase + fatty acid synthase
- fatty acids are formed sequentially by decarboxylative condensation reactions involving the molecules acetyl-CoA and malonyl-CoA
- following each round of elongation, the fatty acid undergoes reduction and dehydration by the sequential action of ketoreductase (KR), dehydratase (DH) and enol reductase (ER)
- the growing fatty acyl group is linked to an acyl carrier protein (ACP)
- overall reaction: acetyl CoA (2C) + 7 malonyl CoA (3C) + 14 NADPH + 14 H+ –> palmitate (16C) + 7 CO2 + 6 H2O + 8 CoA-SH + 14 NADP+
- elongation longer than 16C occurs separately from palmitate synthesis in the mitochondria and endoplasmic reticulum
- desaturation of fatty acids requires the action of fatty acyl-CoA desaturases
15
Q
B-oxidation vs lipogenesis
A
- carriers: CoA vs ACP
- reducing power: FAD/NAD+ vs NADPH
- locations: mitochondrial matrix vs cytoplasm
Similarities - same reactions but in reverse
