8 - Fatty acid synthesis

Question 1

Where does fatty acid synthesis occur?

Answer 1

The cytosol

There are no transporters for acetyl CoA (it must be transported across membranes as citrate by tricarboxylate carrier)

Question 2

Give the broad steps for the synthesis of fatty acids (the longest - 16 carbon palmitate) from acetyl-CoA (LONG)

Answer 2

• Acetyl-CoA is generated in the mitochondrial matrix
• Acetyl-CoA is converted to citrate via citrate synthase
• Citrate is transported into the cytosol via tricarboxylate carrier
• Citrate is converted to acetyl-CoA by citrate lyase (requires ATP)
• Acetyl-CoA is converted to malonyl CoA via acetyl CoA carboxylase (biotin) RLS

Fatty acid synthase (following repeated)

1. Condensation - loss of CO2 and condensation of acetyl group from KS to malonyl group on ACP (acetoacyl ACP)
2. Reduction - carbonyl group of acetoacyl-ACP is reduced to alcohol (β-hydroxybutyryl-ACP)
3. Dehydration - loss of water from C2 and C3 to create a double bond (trans-butenoyl-ACP)
4. Reduction - of double bond to produce butyryl-ACP

Acyl group elongation

• The 4 carbon unit is transferred to the thiol group of β-ketoacyl (ACP synthase)
• A second malonyl-CoA moiety is added to the thiol group of ACP and the condensation is repeated to form a 6-carbon fragment

Acyl group release
- The cycle is repeated until a 16 carbon fatty acyl chain (palmitate) is formed and released from ACP (release catalyzed by thiolase)

Question 3

What is acyl carrier protein (ACP)?

Answer 3

The acyl carrier protein (ACP) is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4’-phosphopantetheine moiety.

Question 4

Fatty acyl groups have limited solubility, therefore they are found as ____?

Answer 4

Thioesters of:

• cysteine
• Acyl carrier protein (ACP)
• Coenzyme A

In addition to improving solubility, these thioesters can serve as reactive forms in biosynthetic reactions.

Phosphate containing groups on ACP and CoA have charges which keep it on one side of the membrane (lipophobic)

Question 5

What is the rate limiting step of fatty acid synthesis?

Describe the important enzyme involved and what happens when it is phosphorylated

Answer 5

Conversion of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase (ACC).

ACC polymerizes (when active) into long filaments (visualizable by EM).

Phosphorylation of ACC blocks polymerization and inactivates the enzyme.

It is a biotin carrier and serves as a biotin carboxylase and transcarboxylase.

Question 6

Describe the mechanism for the generation of malonyl CoA

Answer 6

Activates CO2 by attaching it to a ring nitrogen of biotin on the carrier.

Reaction uses bicarbonate as source of CO2.

Transcarboxylase activity transfers the CO2 from biotin to acetyl-CoA, generating malonyl-CoA. This CO2 is the same one that is lost by condensation by acetoacyl-ACP in fatty acid synthase (allowing carbanion formation and attack on acetyl group - first acyl group).

THe flexible arm of biotin is the key to transfer between different regions of the protein

Question 7

Why does obesity result from consuming too many carbs in respect to fatty acid synthesis?

Answer 7

When lots of carbs are consumed you get lots of glycolysis in the liver and lots of pentose phosphate pathway providing lots of NADPH for fatty acid synthesis (beta-ketoacyl ACP reductase and enoyl ACP reductase)

Extra acetyl-CoA in the mitochondrial also feeds into fatty acid synthesis in the regular way to make palmitate (C16).

Palmitate is esterified with glycerol-3-phosphate to make tripalmitin (a triacylglycerol) which is transported as VLDL to adipose tissue for storage.

In short, obesity results from too much energy and metabolites generated facilitating the highly energy-costly process of fatty acid synthesis!

Question 8

Give the two reactions for fatty acid synthesis

Answer 8

Acetyl-CoA carboxylase
7 acetyl-CoA + 7 ATP + 7 CO2 -> 7 malonyl-CoA + 7 ADP + 7 Pi

Fatty acid synthase
Acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14H+ -> palmitate + 8 CoASH + 6 H2O + 7 CO2 + 14 NADP

Question 9

What influences the polymerization (regulation) of acetyl-CoA carboxylase?

Answer 9

+ citrate (substrate)
+ insulin

• malonyl-CoA (product)
• Glucagon/epinephrine
• palmitoyl-CoA

malonyl-CoA also inhibits beta oxidation (the opposing process)

Question 10

Describe the species differences in fatty acid synthase

Answer 10

Bacteria/plants: seven activities in seven different polypeptides (don’t need a lot of fatty acids)

Yeast: seven activities in two polypeptides (insures you get good substrate channeling and all required proteins present at same time)