Midterm II: Fatty Acid Synthesis (Ben)

Question 1

In what 3 tissues does fatty acid synthesis occur?

And in what part of the cell?

Answer 1

• Liver
• Adipose Tissue
• Lactating Mammary Gland
• in the cytosol

Question 2

What enzyme produces fatty acids?

What is special about its structure and function?

Answer 2

Fatty Acid Synthase

• one single polypeptide chain with 7 different active sites for different reactions in separate domains

Question 3

What is the “committed step” in fatty acid synthesis?

Reactants?

Enzyme + co-factor?

Products?

Answer 3

The first step… carboxylation…

Reactants: Acetyl-CoA + HCO3- + ATP

Enzyme: Acetyl-CoA Carboxylase + Biotin

Products: Malonyl-CoA

Question 4

How is biotin connected to acetyl-CoA carboxylase?

Answer 4

• covalently bound to a lysine side chain of the enzymes “biotin carrier protein”

Question 5

Acetyl-CoA Carboxylase is a “multienzyme protein” and performs a 2-step reaction.

What is the first step?

Answer 5

Biotin Carboxylase transfers a carboxyl group from bicarbonate to biotin in the presence of ATP.

Question 6

Acetyl-CoA Carboxylase is a “multienzyme protein” and performs a 2-step reaction.

What is the second step?

Answer 6

Carboxyl transferase transfers the carboxyl group from biotin to acetyl-CoA, forming malonyl-CoA

Question 7

How are biotin + its attached carboxyl group moved between the two enzymes within Acetyl-CoA carboxylase?

Answer 7

via biotin carrier protein

Question 8

What is the protein responsible for fatty acid synthesis?

How many enzymatic actions does it have?

Answer 8

Fatty Acid Synthase

• has 6 enzymes and an acyl carrier protein

Question 9

What first happens to fatty acid synthase before it can begin building a fatty acid chain?

Answer 9

Acetyl group of acetyl-CoA is transferred from…

ACP-SH to…

Cys-SH of …

β-ketoacyl-ACP synthase​ part of FA synthase, via…

MAT

Question 10

On what special part of acyl carrier protein do molecules bind to become involved in fatty acid synthesis?

Answer 10

the -SH group on the vitamin panthothenic acid

(in 4’-phosphopantetheine form)

Question 11

On which enzyme in the fatty acid synthase complex is the Cys-SH which binds acetyl-CoA found?

Answer 11

β-ketoacyl-ACP synthase

or “KS” part

Question 12

What happens in fatty acid synthesis after acetyl-CoA binds to Cys-SH?

Answer 12

a Malonyl group is transferred to the ACP-SH by MAT

Question 13

Once both the malonyl and acetyl groups are connected to fatty acid synthase…

What is the first step of fatty acid synthesis?

Reactants?

Enzyme?

Products?

Answer 13

Condensation…

Reactants: Acetyl + Malonyl groups

Enzyme: β-ketoacyl-ACP Synthase

Product: Acetoacetyl-ACP + CO₂

Question 14

After condensation of the malonyl + acetyl groups…

what is the 2nd step in fatty acid synthesis?

Reactants?

Enzyme?

Products?

Answer 14

Reduction of carbonyl group…

Reactants: Acetoacetyl-ACP + NADPH + H⁺

Enzyme: β-Ketoacyl-ACP Reductase (KR)

Products: β-Hydroxybutyryl-ACP + NADP⁺

Question 15

After reduction of the carbonyl group at C-3…

what is the 3rd step of fatty acid synthesis?

Reactants?

Enzyme?

Product?

Answer 15

Dehydration to form a double bond…

Reactants: β-Hydroxybutyryl-ACP

Enzyme: β-Hydroxybutyryl-ACP Dehydratase (DH)

Product: trans-Δ²-Butenoyl-ACP + H₂0

Question 16

After dehydration to form a double bond…

what is the 4th step in fatty acid synthesis?

Reactant?

Enzyme?

Product?

Answer 16

Reduction of the double bond…

Reactant: trans-Δ2-Butenoyl-ACP + NADPH + H⁺

Enzyme: Enoyl-ACP-Reductase

Product: Butyryl-ACP + NADP⁺

Question 17

After reduction of a double bond…

what is the 5th and final step of fatty acid synthesis before the cycle begins again?

Answer 17

Translocaton of Butyryl-ACP to the Cys-SH of β-Ketoacyl-ACP Synthase (KS)

Question 18

After the formation of a butyryl group, how does each subsequent round of the fatty acid synthase cycle begin?

Answer 18

ACP is recharged with another malonyl group by MAT

Question 19

How many fatty acid synthase cycles does it take to produce palmitate?

And how is it finally released from the enzyme?

Answer 19

After 7 cycles a 16C palmitate is produced…

…and released by thioesterase enzyme

Question 20

What is the overall reaction for the seven cycles of fatty acid synthase activity leading to production of one palmitate?

(Including the synthesis of the Malonyl-CoA substrates.)

Answer 20

Question 21

What condition in lipid synthesizing tissues makes them favorable environments for this process?

Answer 21

High [NADPH]/[NADP⁺] ratio

(in the cytosol of adipose + liver cells)

Question 22

Where does most of the NADPH used in FA synthesis come from?

Answer 22

Pentose Phosphate Pathway

and some from Malic Enzyme

Question 23

How does acetyl-CoA get from the mitochondria to the cytosol for FA synthesis?

Answer 23

it is transported out of the mitochondria as citrate after combining with oxaloacetate via citrate synthase

Question 24

What happens to citrate after it has left the mitochondria for FA synthesis?

Enzyme + co-factors?

Product?

Answer 24

it is converted back to acetyl-CoA via citrate lyase

• this requires ATP and yields oxaloacetate, ADP + Pi

Question 25

After oxaloacetate is released from citrate in the cytosol, what happens to it?

Answer 25

It is converted back into _malate_ via _malate dehydrogenase_ using NADH + H⁺

Question 26

What 2 things can happen with malate that has been formed from oxaloacetate in the cytosol? And what byproduct of one of these things contributes directly to fatty acid synthesis?

Answer 26

1. re-enters the mitochondria directly 2. conversion to _pyruvate_ via _malic enzyme_ + NADP⁺ * ^​yields _NADPH_ for FA synthase

Question 27

What are 3 regulatory mechanisms of _acetyl-CoA Carboxylase_?

Answer 27

1. _Negative feedback inhibition_ - via palmitate 2. _Allosteric Stimulation_ - via citrate 3. _Covalent Modification_ - insulin ---\> **_de_**phosphorylation ---\> activation

Question 28

Increased levels of what two citric acid cycle molecules inhibit a key citric acid cycle enzyme? (which then leads to increased FA synthesis)

Answer 28

Increased _acetyl-CoA_ and _ATP_ inhibit _isocitrate dehydrogenase_ - this leads to increased citrate + FA synthesis

Question 29

What is the mechanism for _glucagon_'s effect on fatty acid synthesis?

Answer 29

* **Gs** * **Adenylyl Cyclase** * **[cAMP] increase** * **PKA** * **Phosphorylation/deactivation of ACC**

Question 30

How does the presence of _Palmitoyl-CoA_ affect FA synthesis? (Generally and via what mechanisms?)

Answer 30

It _inhibits_ it, via... * _Ac-CoA Carboxylase_ inhibition * Inhibition of _citrate movement_ to cytosol * _G-6-P Dehydrogenase_ inhibition * key pentose phosphate pathway enzyme * leads to decreased [NADPH]