Topic 15: Lipogenesis

Question 1

What do we need for synthesis of Triglycerides?

Answer 1

NADPH comes from: HMPS, Isocitrate Cycle, Malic Enzyme

Acetyl-CoA comes from: Glucose, Amino Acids, and in ruminants Acetate or Beta-Hydroxybutyrate

Glycerol-3-Phosphate or DHAP for glycerol backbone

Question 2

Where does Fatty Acid Synthesis take place?

Answer 2

CYTOPLASM of Liver, Lactating Mammary Glands, and Adipose

so NOT inside mitochondria

Question 3

What 2 components make up the Fatty Acid Synthase System?

Answer 3

Acyl Carrier Protein (ACP)
Condensing Enzyme (CE)

Question 4

What is Step 1 of Fatty Acid Synthesis?

Answer 4

2 Carbon Acetyl CoA is combined with HCO3 to make 3 carbon Malonyl CoA via Acetyl-CoA Carboxylase

So, Acetyl CoA –> Malonyl CoA via Acetyl-CoA Carboxylase

Requires an ATP and Bicarb

Question 5

What vitamin is required for Acetyl-CoA Carboxylase and what is this enzyme’s function?

Answer 5

Need Vitamin B7/Biotin. It is the first step in Fatty Acid Synthesis.

Question 6

What activates Acetyl-CoA Carboxylase?

Answer 6

Citrate/Isocitrate

Question 7

What inhibits Acetyl-CoA Carboxylase?

Answer 7

AMP Kinase – phosphorylates it to turn it off

Question 8

What is Step 2 of Fatty Acid Synthesis?

Answer 8

Malonyl CoA to ACP part of complex
Acetyl CoA to CE part of complex

CONDENSATION - knock of CO2, move Acetyl to Malonyl so everything’s on ACP part – 4 carbons now

Review Slide 7 Lipogenesis

Question 9

What is Step 3 of Fatty Acid Synthesis?

Answer 9

REDUCTION – take H+ from NADPH (generates NADP+)

Review Slide 7 Lipogenesis

Question 10

What is Step 4 of Fatty Acid Synthesis?

Answer 10

DEHYDRATION – take off H20

Question 11

What is Step 5 of Fatty Acid Synthesis?

Answer 11

REDUCTION – take H+ from NADPH (generates NADP+)

Now have Butyrate!

Question 12

What is Step 6 of Fatty Acid Synthesis?

Answer 12

TRANSFER – move everything to CE part

Question 13

What is Step 7 of Fatty Acid Synthesis?

Answer 13

RELOAD – put another Malonyl on ACP and repeat the entire process Step 2-6 until you get PALMITIC ACID/PALMITATE (16 C)

Question 14

What is the only Step that requires ATP in fatty acid synthesis?

Answer 14

Step 1: creating Malonyl CoA from Acetyl CoA

ATP powers Acetyl-CoA Carboxylase

Question 15

What side of fatty acid chain do you add each set of 2 carbons? Carboxy or Methyl?

Answer 15

Carboxy. So the “oldest” carbons are at the methyl end

Question 16

Where can mammals desaturate at in fatty acid chains?

Answer 16

from carboxy: 9 then 4, 5, 6, 8

Question 17

What Desaturase are CATS deficient or completely lacking in?

Answer 17

6-Desaturase - so they can’t Desaturate Linoleic Acid in the pathway to form Arachidonic acid

Question 18

Where can mammals NOT desaturate past?

Answer 18

9

Question 19

What is an essential fatty acid for mammals?

Answer 19

Linoleic Acid

alpha-Linolenic Acid

Question 20

How do we make Arachidonic acid from Linoleic Acid?

Answer 20

Desaturation at 6, Elongation, Desaturation at 5 => Arachidonic acid 20:4 (5,8,11,14)

Question 21

How is Fatty Acid Synthesis regulated?

Answer 21

Acetyl CoA Carboxylase - forms Malonyl CoA from Acetyl-CoA (Requires B7/Biotin)

STIMULATED by: Citrate or Isocitrate; Protein Phosphatase (via insulin)

INHIBITED by: Palmitoyl CoA (palmitate/palmitic acid) - end product of Fatty Acid Synthesis; Protein Kinase A (via Glucagon)

Question 22

How do we get Acetyl CoA into the cytoplasm from Mitochondria in NON-ruminants?

Answer 22

Citrate!

Acetyl-CoA + OAA –> Citrate out via transporter –> Citrate via CITRATE LYASE uses an ATP –> Acetyl CoA goes on and OAA has another fate

CoenzymeA DOES NOT MOVE!

Question 23

How do we replenish OAA in the mitochondria after removing it via citrate for FAS?

Answer 23

OAA via Malate DH (NADH back to NAD+) goes to Malate –> Malate via MALIC ENZYME (NADP+ to NADPH) goes to Pyruvate –> Pyruvate goes on

NAD+ is generated (can now be used for Glycolysis)
NADPH is generated (can now be used for FAS)

Question 24

How do we get Acetyl CoA into the cytoplasm from Mitochondria in Ruminants?

Answer 24

Acetate absorbed from gut is directly converted to Acetyl-CoA

Simple and boring.

Question 25

What are the sources of NADPH for FAS?

Answer 25

1. HMPS all animals
2. Malic Enzyme (Malate -> Pyruvate)
   [not so much in Ruminants]
3. Isocitrate Cycle (Isocitrate -> a-KG via Isocitrate-DH)
   [high in ruminants]
   [in mito: NAD+ => NADH]
   [in cyto: NADP+ => NADPH]

Question 26

How does Glycerol-3-P and DHAP contribute to Triglyceride synthesis? (now talking about TAG not just fatty acids)

Answer 26

DHAP –> G-3-P via acyltransferase has fatty acid attached to C1 (lysophosphatidic acid) then C2 (PHOSPHATIDIC ACID)

Phosphatidic Acid is Precursor for TAG

CoA is released upon each reaction

Question 27

Breakdown Non-Ruminant TAG Synthesis.

Answer 27

Aectyl CoA – from Glucose
G-3-P/DHAP – from Glucose
NADPH – HPMS, Isocitrate Cycle, Malic Enzyme
Tissues – many, especially LIVER and ADIPOSE

Question 28

Breakdown Ruminant TAG Synthesis.

Answer 28

Acetyl CoA – from Acetate/BHBA
G-3-P/DHAP – from Glucose (gluconeogenesis)
NADPH – HMPS, Isocitrate Cycle
Tissues – many, especially ADIPOSE (liver is TOO BUSY doing gluconeogenesis!!)

Question 29

Does it cost more to build up or breakdown fatty acids?

Answer 29

Always costs MORE to BUILD UP

Question 30

How much ATP does it cost for FAS?

Answer 30

FAS builds 16 C Palmitic Acid:
7 C-C Cycles
8 ATP used = 8 ATP (to make Malonyl CoA)
14 NADPH = 35 ATP
8 Citrates or Acetates = 8 ATP (takes an ATP to get to Acetyl CoA)

Total = 51 ATP

Question 31

How much ATP did gluconeogenesis vs glycolysis use/yield?

Answer 31

Gluconeogenesis uses 6 ATP

Glycolysis yields 2 ATP