Fatty Acid Synthesis

Question 1

Anabolism of fatty acids requires

Answer 1

acetyl coA and malonyl coA (3C intermediate)
NADPH electron donor
to be in the cytosol in animals

Question 2

Catabolism of FA takes place in

Answer 2

mitochondria

Question 3

Phase 1 Biosynthesis of Fatty Acids

Answer 3

Acetyl-coA –> Malonyl coA
Enzyme: acetyl coA carboxylase (ACC)
Cofactors: biotin, HCO3- + ATP transferred to biotin
Location: cytosol

Question 4

acetyl coA carboxylase catalytic units

Answer 4

1) biotin carboxylase: adds carboxyl from HCO3- to biotin using ATP

2) biotin carrier protein: carries in biotin and rotates 180°

3) transcarboxylase transfer carboxyl from biotin to acetyl coA forming malonyl coA

Question 5

Types of fatty acid synthase
Structure of fatty acid synthase

Answer 5

Types I: in vertebrates and fungi, Type II: in bacteria and plants
Subunit Structure: A Killer Executive Does Much Kicking (of) Teeth
ACP (acyl carrier protein)
KR - K reductase
ER - E reductase
DH - dehydratase
MAT - malonyl-acetyl ACP transferase
KS - K synthase
TE (thioesterase)

Question 6

Phase 2 Biosynthesis of Fatty Acids

Answer 6

Enzyme: fatty acid synthase

1) Condensation of acetyl and malonyl: acetyl added to KS, malonyl added to ACP (coA dissociate), then acetyl added to malonyl producing a CO2
2) Reduction by KR of carbonyl producing NADP+
3) Dehydration of C2/C3 by DH producing water and forming double bond
4) Reduction of double bond by ER producing NADP+
5) Translocation of fatty acid chain back to KS

Repeat until full fatty acid palmitate chain has been formed

Question 7

thioesterase does what

Answer 7

frees palmitate from ACP through hydrolytic activity, 1 water used

Question 8

role of malonyl

Answer 8

acts as the donor of 2 carbons to extend fatty acid chain in phase 2 fatty acid synthesis
every time bound to ACP and added onto growing chain bound to KS
Malonyl-coA is the inhibitor of acyl-carnitine transferase I in FA catabolism pathway

Question 9

Overall equation of palmitate synthesis

Answer 9

8 acetyl coA + 7 ATP + 14 NADPH +14 H+ –>
Palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP+ + 6H2O

Question 10

role of palmitate in FA synthesis

Answer 10

Palmitate is principle FA of FA synthesis
Acts as precursor for long chain FA synthesis

Question 11

stearate can be desaturated to form
Palmitoleate is formed by

Answer 11

longer chain fatty acids in the smooth ER and mitochondria

desaturation of palmitate at Δ9 carbon by fatty acyl-coA desaturase enzyme

Question 12

Synthesis of alpha-linolenate

Answer 12

Last 2 desaturations only in plants, must be consumed in humans (essential nutrient)

Palmitate (16:0) –> Stearate (18:0) –> Oleate (18:1Δ9) –> Linoleate (18:2 Δ9,12) –> alpha-linolenate (18:3 Δ9,12,15)

Enzymes: elongation and fatty acyl-coA desaturase

Question 13

How is arachidonate formed?
Structure nomenclature

Answer 13

From linoleate via different (omega-6) desaturation pathway
Arachidonate (20:4 Δ5,8,11,14)

Question 14

Sources of NADPH regeneration
Malic enzyme pathways and significance

Answer 14

Half from pentose phosphate pathway and half from malate/malic enzyme pathway

Malic enzyme pathway:
1) malate in cytosol converted to pyruvate by malic enzyme and reduces NADP+ –> NADPH
2) Pyruvate crosses back to mitochondrial matrix via pyruvate transporter
3) Pyruvate converted to oxaloacetate by pyruvate carboxylase and ATP

OR
1) malate crosses from cytosol to mitochondrial matrix via malate-alpha ketoglutarate transporter
2) Malate is converted to oxaloacetate by malate dehydrogenase reducing NADH in process

Significance: acetyl coA cannot cross mitochondrial membrane

Question 15

Sources of acetyl-coA

Answer 15

pyruvate decarboxylation and amino acid catabolism in mitochondria

Question 16

Role of citrate in fatty acid metabolism regulation

Answer 16

acts as activator of fatty acid synthesis in mitochondria

Acetyl-coA + oxaloacetate –> citrate via citrate synthase in mitochondria
Citrate –> acetyl-coA + oxaloacetate via citrate lyase in cytosol

Regulation of intermediates to feed into different pathways and regeneration of NADPH and NADH

Question 17

What is the rate limiting enzyme of FA biosynthesis?

Answer 17

acetyl-coA carboxylase
citrate stimulates, palmitoyl-CoA inhibits
epinephrine and glucagon trigger inactivation phosphorylation

Question 18

Insulin regulation of fatty acid synthesis

Answer 18

Promotes phosphatase activation of acetyl-coA carboxylase (ACC), leading to acetyl-coA –> malonyl-coA

Malonyl-coA negative feedback on carnitine acyl transferase-1 in FA catabolism pathway for transport into mitochondria

Question 19

Glucagon regulation of fatty acid synthesis

Answer 19

Inactivation phosphorylation of ACC via PKA AMPK (AMP-activated protein kinase)
No malonyl-coA formed, fatty acid mobilization continues

Question 20

Biosynthesis of TAGs from glycerol-3-phosphate

Answer 20

Insulin promoted

1) Dihydroxyacetone phosphate (source glycolysis/glycerol) –> glycerol-3-phosphate
Enzyme: glycerol-3-phosphate dehydrogenase
OR
glycerol –> glycerol-3-phosphate
Enzyme: glycerol kinase

2) Formation of 2 acyl coA and addition to glycerol 3-phosphate –> phosphatidic acid
Enzyme: acyl-coA synthase + ATP and acyl transferase x2

3) formation of glycerophospholipid OR formation of TAG
Enzyme: phosphatidic acid phosphatase + acyl transferase

Question 21

Biosynthesis pathway of cholesterol

Answer 21

4 stages: condensation, phosphorylation, polymerization, and cyclization

1) Condensation: Acetate –> HMG-CoA –> Mevalonate
Enzyme: HMG-coA reductase and HMG-coA synthase
2) Phosphorylation: Mevalonate –> isoprene
3) Polymerization: Isoprene –> squalene
4) Cyclization: Squalene –> cholesterol

Question 22

HMG-coA stands for

Answer 22

β-hydroxy-β-methylglutaryl-CoA

Question 23

Inhibition of cholesterol synthesis (4)

Answer 23

High [AMP] leads to AMPK phosphorylation INactivation of HMG-coA reductase (decreased activity and cholesterol prod)

Glucagon and epinephrine also lead to AMPK inactivation pathway

Insig (insulin-induced gene protein) senses high cholesterol and triggers ubiquitination of HMG-coA reductase

Statins inhibit HMG-coA reductase to lower cholesterol prod

Question 24

Stimulation of cholesterol synthesis (2)

Answer 24

Insulin leads to DEphosphorylation of HMG-coA reductase –> increased activity and cholesterol production

SREBPs (Sterol Regulatory Element Binding Proteins) activate HMG-coA Reductase