Fatty acid synthesis & Amino acid metabolism

Lecture 6 *INCOMPLETE*

Why is excess glucose converted into fat & not glycogen?

easier to store fat as less water is needed to store it

fat is more energy dense

Why would the [NADPH]/[NADP] ratio be kept high?

Favours reductions & fatty acid synthesis

Why would the [NAD]/[NADH] ratio be kept high?

Favours glycolysis

How is carbon exported from the mitochondria into the cytosol?

citrate is synthesised in mitochondria:

oxaloacetate + acetyl-CoA —→ citrate + CoASH

ATP-citrate-lyase in cytoplasm:

ATP + citrate + CoA —→ oxaloacetate + acetyl-CoA + ADP + P_i

What is the first stage of fatty acid synthesis?

Biotinyl-enzyme + HCO₃^- + ATP + ADP + P_i —→ carboxybiotinyl-enzyme

carboxybiotinyl-enzyme + acetyl-CoA —→ malonyl-CoA

Reaction occurs in 2 steps: a CO₂ activation & a carboxylation

*NB - this is the committed & the rate limiting step of fatty acid synthesis

What is the second stage of fatty acid synthesis?

Acetyl CoA binds to phosphopantotheine on acyl-carrier protein on fatty acid synthase. Acetyl group then shifted to the cysteine residue also on fatty acid synthase. Then malonyl CoA binds to phosphopantotheine.

So at the end of this stage: Acyl group bound to the cysteine residue & malonyl group bound to phosphopantotheine.

What is the third stage of fatty acid synthesis?

Acetyl group from acetyl-CoA reacts with bound malonyl-CoA to form butyryl-CoA and CO₂.

What is the fourth stage of fatty acid sythesis?

2 carbons from malonyl-CoA are added in a condensation reaction to butyryl-CoA to form a 6 carbon fatty acid over the course of several reactions.

These reactions are…

1. Initial condensation

2. reduction with NAPDH

3. dehydration

4. Reduction with NADPH

6 carbon molecule is formed.

What is the 5th stage of fatty acid synthesis?

6 carbon molecule keeps reacting with malonyl-CoA & goes through the cycle 7 times total, extending the chain by 2 carbons each rotation until it reaches C16 and forms palmitate.

Effectively, what does the malonyl-CoA act as during fatty acid synthesis?

a 2 carbon donor to acetyl-CoA

*NB - 3rd carbon released as CO₂ in the formation of butyryl-CoA

What molecules inhibit acetyl-CoA carboxylase (ACC)?

glucagon

adrenaline

palmitoyl-CoA

AMPK through phosphorylation

What molecules activate ACC?

insulin

citrate

ChREBP - transcription factor that upregulates enzyme that is itself regulated by nutrients

How can malonyl-CoA be recycled?

malonyl-CoA decarboxylase can convert malonyl-CoA to acetyl-CoA & CO₂

Describe the structure of fatty acid synthase.

Very large molecule that has an acyl-carrier protein (ACP) bound to a phosphopantotheine residue.

Also bound to a cysteine residue too.

What are the 4 ways fatty acid synthesis & oxidation can be controlled?

control of malonyl-CoA synthesis

control of phosphorylation of ACC

control of glucose uptake by insulin

control of TAG synthesis

How can malonyl-CoA synthesis be controlled?

Insulin activates acetyl-CoA carboxylase by stimulating its dephosphorylation —→ increases malonyl-CoA

Glucagon increases ACC phosphorylation —→ decreases malonyl-CoA

How is the control of ACC phosphorylation achieved?

Direct phosphorylation - 5’-AMP increases phosphorylation via AMPK

Allosteric effectors - citrate, palmitoyl-CoA

How does insulin control TAG synthesis?

Stimulates the enzymes that add acyl groups to glycerol

Stimulates glucose metabolism to provide glycerol backbone for TAG synthesis.

How is nitrogen excreted in mammals?

as urea

ideal because water soluble, neutral pH & non-toxic

What prosthetic group is used by aminotransferases?

pyridoxal phosphate

transfers from a large number of amino acids to glutamate, alanine & aspartate.

What is the role of alanine?

carrier of ammonia & pyruvate from skeletal muscle to the liver.

*NB - links amino acid metabolism with the Cori cycle.

Describe stage 1 of the glucose-alanine cycle.

excess ammonia + glutamate —→ glutamine.

catalyst: glutamine synthetase

requires energy from ATP hydrolysis

Describe stage 2 of the glucose-alanine cycle.

Glutamine enters the liver and is acted on by glutaminase. NH₄⁺ is liberated in the mitochondria.

*NB - liver also releases N₂ from glutamate as NH₃ using glutamate dehydrogenase in oxidative deamination.