- Inner mitochondrial membrane is impermeable to Acetyl-CoA;

Tricarboxylate transporter translocates it out of mitochondria as citrate

- Brings acetyl-CoA from mitochondrial matrix to cytosol as citrate

- Facilitates directional transport

- Citrate synthase catalyzes formation of citrate within the mitochondrial matrix

- ATP-citrate lyase catalyzes reverse rxn in the cytosol to release Acetyl-CoA

USE Acetyl-CoA Carboxylase (ACC) which is multi-domain/multi-functional and has biotin carboxylase & carboxyltransferase activity

Functional domains:

i. biotin carboxylating domain (BC)

ii. biotin binding domain (BB)

iii. Carboxyltransferase domain (CT)

MECHANISM: Carboxylation driven by ATP hydrolysis

ENZYME: Biotin Carxoylase activity of ACC

MECHANISM: transcarboxylation

ENZYME: Transcarboxylase activity of ACC

STEP 1:      STEP 2:

Repeat 2 and 3 for an extra malonyl-CoA and keep adding on malonyl-CoA

START: acetyl-CoA

ENZYME: malonyl/acetyl CoA-ACP transacylase (MAT)

END: acetyl-ACP

ENZYME: beta-ketoacyl-ACP synthase

START: malonyl-CoA

ENZYME: malonyl/acetyl CoA-ACP transacylase (MAT)

END: malonyl-ACP

*Top shows step 1a and 2, bottom shows the loading

ENZYME: KS subunit of FAS

END: beta-ketobutyryl-ACP

ENZYME: Ketoacyl-ACP reductase (KR)

END: beta-hydroxybutyryl-ACP

ENZYME: Hydroacyl-ACP Dehydrase (DH)

END: trans-deltasquared- Butenoyl-ACP

ENZYME: Enoyl-ACP reductase (ER)

END: Butyryl-ACP

Done by KS

i.e. move from the ACP to the KS

Condensation via KS domain

Question: for C16-Palmitic acid, how many rounds of FA synthesis needed?

ANSWER: Since it has 16 carbons in the chain and each round produces 2 carbons, need 8 rounds

NOTE: New carbons always added on the carboxyl side, so the oldest malonyl-CoA added will be at the end of the chain

ENZYME: Thioesterase (TE) which cleaves the ester bond

END: A Fatty Acid chain

In mammalian cells, FA synthesis is catalyzed by the multi-domain, multi-function enzyme FAS that contains SIX enzyme activities (KS, KR, DH, ER, MAT, HS)

ACP is the prosthetic group involved in all steps of the enzymatic reactions

STARTER = acetyl-ACP, EXTENDER = malonyl-ACP (so 1 acetyl ACP : many malonyl-ACPs)

- FA synthase primarily produces palmitate (C16:0)

- In eukaryotic cells, Elongation happens in both the cytoplasm and the ER (endoplasmic reticulum)

- Microsomal FA Elongation that happens in ER involves acyl-CoA (malonyl CoA) derivatives; it's performed by separate enzymes bound to cytoplasmic face of ER membrane

In Humans...

EVOVL1-7: Elongation of very-long-chain FAs (7 genes)

KAR: beta-ketoreductase

HACD1-4: 3 hydroxyacyl-CoA dehydratase (4 genes)

TER: Trans-2,3-enoyl-CoA reductase

Eukaryotic cells produce several ER-bound acyl-CoA desaturases that catalyze production of mono- and polyunsaturated FAs

Diff desaturases exhibit activity at bonds 9, 6 and 5

Two distinct families: 

Typically, the first cis-double-bond is introduced between C9 and 10 (delta 9) and its reaction catalyzed by SCD

----

Additional double bonds are added by 6- and 5-desaturases (FADs)

PROBLEM: Humans unable to introduce double bonds beyond bond 9.

- SOLUTION: Fatty acids must come in from the diet:

Activated by...

- citrate (allosterically)

- Dephosphorylation (Insulin-activated protein phosphatase 2A or PP2A)

INActivated by...

- Palmitoyl-CoA (fatty acid-CoA) via feedback inhibition

- Phosphorylation (AMP kinase, or AMPK; is an Energy sensor)

Regulators of actual FA synthesis: reciprocal regulation of beta-oxidation

- Malonyl-CoA inhib's CPT1 activity so FA synthesis inhib's beta-oxidation

- High FA concentration suppresses FA synthesis by blocking malonyl-CoA formation

diagram for regulation of ACC