lecture 23 - beta oxidation and TCA cycle intro Flashcards
How are fatty acids generated?
Fatty acids are generated from lipid droplets in adipose tissue
Triacyl glycerol (TAG) is insoluble – accumulates in specialised lipid droplets.
Fatty acids that are freed up from TAG are bound to the carrier protein albumin in the blood.
How are fatty acids activated?
Two step reaction catalysed by acyl coA synthetase, breaks two phosphoanhydride bonds and therefore consumes 2 ATP equivalents:
1.fatty acid + ATP -> acyl adenylate + PPi (IRREVERSIBLE because of PPi hydrolysis)
2. acyl adenylate + HS-CoA -> acyl CoA + AMP
overall:
fatty acid + CoA + ATP -> acyl-CoA + AMP + PPi
Fatty acids activation is through a high energy C-S bond using CoA
It needs 2 ATP
How is fatty acid CoA transported into mitochondria?
Requires transport shuttle - regulatory step
Fatty acyl CoA is converted to acyl carnitine by carinitine palmitoyl transferase I (CPTI) on outer mitochondrial membrane
Acyl carnitine shuttled across IMM by translocase
Carnitine palmitoyl transferase II (CPTII) on matrix face of inner mitchondrial membrane transfers back acyl group to recreate fatty acid CoA in mitochondrial matrix
What is key evidence for the beta oxidation of fatty acids?
The classical experiments of Franz Knoop: fed benzylated fatty acids of different length to dogs
analysis of derivatives in urine revealed phenylacetic acids when fed even-number C chain fatty acids and benzoic acids when fed odd-number C chain fatty acid
Franz Knoop used the first ever metabolic labelling experiment to show that the b-carbon is oxidised in fatty acid degradation
What is beta oxidation?
b-oxidation of fatty acyl CoA uses a cycle of 4 reactions to ‘dismember’ the fatty acid by 2 carbon units. shortens acyl CoA by two carbons each cycle
Yields 1 acetyl-CoA for each cycle
oxidation
hydration
oxidation
thiolysis
What are the steps of beta oxidation?
- first oxidation of acyl CoA (Cn) –> enoyl CoA by acyl CoA dehydrogenase, reduces FAD to FADH2
- hydration of enoyl CoA –> L-3-hydroxyacyl CoA by enoyl CoA hydratase (stereospecific forming only L-isomer of 3-hydroxyacyl CoA)
- Second oxidation of L-3-hydroxyacyl CoA to 3-ketoacyl CoA by L-3-hydroxyacyl CoA dehydrogenase - reduces NAD+ to NADH
- thiolysis of 3-ketoacyl CoA to acyl CoA (Cn-2) + acetyl CoA by thiolase
How does b-oxidation take place in unsaturated fatty acids?
breakdown is same as saturated FA until reach double bond then
isomerase converts cis enoyl CoA to trans double bond
trans enoyl CoA is then substrate in oxidation pathway
How is pyruvate converted to acetyl CoA?
Pyruvate dehydrogenase (PDH) catalyzes this conversion via three separate reactions:
- Decarboxylation – release of CO2
- Generation of acetyl-CoA: transfer of the acetyl group to CoA
- Oxidation: transfer of electrons from the carbonyl group to NAD+
Describe PDH
PDH is a giant multienzyme complex
8 X E2 in the core
24 X E1 surround the core
12 X E3 surround the core
Next to the permanently modified cofactors NAD+ and CoA, PDH uses three catalytic cofactors:
thiamine pyrophosphate (TPP)
lipoic acid (as a lipoamide coupled to a lysine sidechain)
FAD
What is the TCA cycle?
The TCA cycle converts the two carbons of the acetyl group into CO2. The acetyl carrier coenzyme A is used as a shuttle between the TCA cycle and catabolic pathways.
1st half of the cycle: Removal of two carbons in the form of CO2
2nd half of the cycle: Regeneration of oxaloacetate from succinate
The TCA cycle is a metabolic cycle, its intermediates are only needed in catalytic amounts as carriers of acetate’s two carbons for their conversion to CO2.
The TCA cycle produces reducing intermediates – 3 NADH and 1 FADH2 – that can be used by the oxidative phosphorylation chain
Entry into the cycle is through acetyl-CoA produced by the multi-enzyme complex pyruvate dehydrogenase
PDH generates NADH as well
The acetate entering the cycle is completely ‘burned’ – converted into CO2
