Fatty acids and lipids

Question 1

Where does fatty acid biosynthesis occur?

Answer 1

cytoplasm

Question 2

Prior to the fatty acid synthesis, what are the 4 steps to prepare acetyl CoA?

Answer 2

1) acetyl group shuttle transports acetyl CoA from mitochondrion to cytosol:
- citrate synthase converts acetyl CoA to citrate, which passes through membrane into cytosol
- citrate lyase in cytosol condenses citrate with cytoplasmic CoA to reform acetyl CoA

2) the 2 carbon acetyl CoA is converted to a 3 carbon malonyl CoA by Acetyl CoA carboxylase at expense of 1 ATP
3) Acetyl transferase replaces the CoA with ACP (acyl carrier protein) to produce Acetyl ACP
4) Malonyl transferase replaces CoA with ACP to produce Malonyl ACP

Question 3

What is the carbon donor?

Answer 3

Malonyl ACP

Question 4

What are the 4 steps of fatty acid biosynthesis? what is the enzyme at each step?

Answer 4

1) Condensation - 2 Carbon from Malonyl ACP (decarboxylation of malonyl ACP) added to carboxyl end of Acetyl ACP to produce 4 carbon beta-ketoacyl ACP
- enzyme = beta-ketoacyl synthase

2) Reduction - NADPH used to reduce beta-ketoacyl ACP to D-3-Hydroxybutyryl ACP and produce NADP+
- enzyme = beta-ketoacyl ACP reductase

3) Dehydration - D-3-hydroxybutyryl ACP dehydrated to Crotonyl ACP
- enzyme = dehydratase

4) Reduction - NADPH used to Crotonyl ACP to Butyryl ACP and produce NADP+
- enzyme = enoyl reductase

• repeated - malonyl ACP adds 2 carbons to butyryl ACP and so on ….

Question 5

what is the name of the 16 carbon fatty acid?

Answer 5

palmitate

Question 6

Once the acyl-ACP molecule is 16 carbons long, what happens?

Answer 6

thioesterase cleaves off the ACP molecule to leave the 16C palmitate fatty acid

Question 7

Where are the enzymes located for the synthesis of long chain fatty acids (longer than 16C)?

Answer 7

on cytosolic face of ER

Question 8

What additional enzyme is involved in the synthesis of unsaturated fatty acids?

Answer 8

fatty acid desaturases - insert cis double bond

Question 9

What are the names of the two essential fatty acids and why can mammals not synthesise these?

Answer 9

linolete and linolenate

• mammals cannot synthesise these as they have double bonds beyond carbon 9 and mammals have desaturases that can inset double bonds up to carbon 9 only.

Question 10

What is the key regulatory step in fatty acid biosynthesis?

Answer 10

acetyl Co-A carboxylase (converts acetyl CoA to malonyl CoA)

Question 11

How is acetyl CoA carboxylase regulated globally and locally? (brief - just the names of the type of regulation)

Answer 11

Global Regulation
- Reversible phosphorylation:
AMP activates AMP-activated kinases = inhibit
ATP reduces activation of AMP-activated kinases = stimulates
- Hormonal control:
Insulin = stimulates
Glucagon and Adrenaline = inhibit

Local Regulation:
- Allosteric regulation:
early substrates (citrate = stimulates, palmitate = inhibits)

Question 12

How is Acetyl CoA carboxylase globally regulated?

Answer 12

Reversible phosphorylation:

• low energy = increase AMP = activates AMP-activated protein kinase = phosphorylates acetyl CoA carboxylase = inhibit fatty acid synthesis (preserve resources as fatty acid synthesis is consumptive)
• high energy = increase ATP = inhibits AMP-activated protein kinase = less phosphorylation of acetyl CoA carboxylase = more active carboxylase = stimulate fatty acid synthesis (cell has sufficient energy to do so)

Hormonal control:

• Insulin stimulates protein phosphatase 2A = dephosphorylates acetyl CoA carboxylase = fatty acid synthesis occurs
• Insulin inhibits AMP-activated protein kinase = reduce inactivation of acetyl CoA carboxylase = allow fatty acid synthesis (insulin means high glucose = high energy)
• Glucagon and Adrenaline activate protein kinase A = inhibit protein phosphatase 2A = phosphorylation of acetyl CoA carboxylase = inhibit fatty acid synthesis (glucagon means low glucose = low energy)

Question 13

How is Acetyl CoA carboxylase locally regulated?

Answer 13

Allosteric regulation:

• citrate activates acetyl CoA carboxylase
• binding of citrate to a phosphorylated acetyl CoA carboxylase can partially activate it = overrides the global regulation*
• Malonyl-CoA
• Palmitate inhibits acetyl CoA carboxylase

Question 14

How are ingested triacylglycerols broken down and absorbed? (4 steps)

Answer 14

1) triacylglycerols from into micelles with aid from bile salts
2) pancreatic lipases break down the triacylglycerols into 2 fatty acids and a monoacylglycerol, which diffuse across membrane and reform into triacylglycerols
3) triacylglycerols combine with other lipids and proteins to form chylomicrons
4) chylomicrons pass into lymphatics, then into blood and are moved directly to muscle and used as energy source or stored in adipocytes

Question 15

How are triacylglycerols broken down in adipocytes to glycerol and three fatty acids? (3 steps)

Answer 15

1) Adrenaline activates beta-adrenergic GPCR which results in a conformational change and the G protein alpha subunit exchanges ADP for ATP. ATP-activated G protein dissociates from trimer and activates adenylate cyclase to produce cAMP second messenger from ATP

2) cAMP activates protein kinase A that phosphorylates:
- Perilipin (which inhibits adipose triacylglycerol lipase) = adipose triacylglycerol lipase now active and removes one fatty acid to produce diacylglycerol and a fatty acid
- Hormone sensitive lipase = once phosphorylated it is activated and removes the final fatty acid producing monoacylglycerol and another fatty acid.

3) monoacylglycerol lipase removes the final fatty acid

Question 16

How are fatty acids activated? name the enzyme

Answer 16

link to cytosolic CoA to produce acyl-CoA at expense of ATP

- enzyme = acyl CoA synthetase

Question 17

How does adaptive control regulation fatty acid biosynthesis?

Answer 17

long-term control is mediated by changes in the rates of synthesis and degradation of participating enzymes

Question 18

Define lipid

Answer 18

A diverse group of naturally occurring molecules that are soluble in non-polar organic solvents (such as alcohol and chloroform) and insoluble/partially soluble in water.

Question 19

What is produced by the final thiolysis of a fatty acid with an uneven number of carbons?

Answer 19

produces propionyl CoA and acetyl CoA (instead of two acetyl CoA molecules that can go into the CA cycle)

Question 20

What is the fate of propionyl CoA produced by the final thiolysis of a fatty acid with an uneven number of carbons?

Answer 20

converted into succinyl CoA and fed into the CA cycle.

Question 21

what are the three groups of lipids

Answer 21

neutral lipids (triacylglycerols)
polar lipids (phospholipids and glycolipids)
Steroids (cholesterol)

Question 22

What are the two types of phospholipid?

Answer 22

1) phosphoglycerides (fatty acid ester bonded to positions 1 and 2 of glycerol, position 3 bonded to phosphate)
2) Sphingophospholipid (sphingosine long hydrocarbon tail replaces one fatty acid, fatty acid bound via amide bond to sphingosine, phosphate bonded too)

Question 23

What are the two types of glycolipid?

Answer 23

1) glycosylglycerides (same as phosphoglyceride but phosphate group replaced with 1 or 2 galactose moieties)
2) sphingoglycolipid (same as sphingophospholipid but the phosphate group is replaced with up to 7 sugar residues in chains or branches)

Question 24

name the six most important phosphoglycerides

Answer 24

phosphatidylethanolamine
phosphatidylcholine
phosphatidylserine
phosphatidylinositol
phosphatidylglycerol
cardiolipin

Question 25

What is the major intermediate in the synthesis of the key phosphoglycerides?

Answer 25

phosphatidate

Question 26

What lipids have the most contribution to the percentage lipid content of biological membranes?

Answer 26

phosphoglycerides

Question 27

How are lipids amphipathic?

Answer 27

hydrophobic head = polar

hydrophobic hydrocarbon tails = non-polar

Question 28

Why do lipids form micelle or bilayer in aqueous environment?

Answer 28

thermodynamically favourable structures.

Question 29

How do phospholipases break down a phosphoglyceride?

Answer 29

Phospholipase A1 hydrolyses the ester bond between the fatty acid and the C1 of glycerol

Phospholipase A2 hydrolyses the ester bond between the fatty acid and the C2 of glycerol

Phospholipase C hydrolyses the bond between the phosphate and the C3 of glycerol

Phospholipase D hydrolyses the bond between the phosphate and the X group

Question 30

What are the four methods of fatty acid oxidation control?

Answer 30

1) reciprocal control of hormone sensitive lipase by adrenaline and insulin to control the availability of fatty acids for downstream reactions.
2) malonyl CoA of fatty acid synthesis inhibits carnitine acyl transferase 1 to prevent a futile cycle
3) NADH inhibits L-3-hydroxyacyl CoA dehydrogenase in the second oxidation reaction
4) Acetyl CoA inhibits beta-ketothiolase in the thiolysis reaction

Question 31

Following activation of fatty acids, how are they transferred from the cytoplasm to the mitochondria? name the enzyme at each step (3 steps in total)

Answer 31

1) Acyl CoA reacts with carnitine, releasing cytosolic CoA to produce acyl carnitine
- enzyme = carnitine acyl transferase 1.

2) acyl carnitine is translocated to mitochondrion
- enzyme = a translocase

3) Acyl group reacts with mitochondrial CoA, releasing carnitine, which is shuttled back to cytoplasm
- enzyme = carnitine acyl transferase 2

Question 32

What are the four steps of fatty acid oxidation? name the enzyme at each step

Answer 32

1) Oxidation - acyl CoA oxidised to trans-delta2-Enoyl CoA and FADH2
- enzyme = acyl CoA dehydrogenase

2) Hydration = hydrated between carbon 2 and carbon 3 to produce L-3-hydroxyacyl CoA
- enzyme = enoyl CoA hydratase

3) Oxidation of the double bond between C2 and C3 to a keto group producing 3-ketoacyl CoA and NADH
- enzyme = L3-hydroxyacyl CoA

4) Thiolysis - removal of the 2 carbon acetyl CoA using CoA to produce Acetyl CoA (goes to CA cycle) and another Acyl CoA that is 2 carbons shorter and goes back though the cycle again.
- enzyme = beta-ketothiolase

Question 33

What is produced by one cycle of fatty acid oxidation?

Answer 33

1 FADH2, 1 NADH, Acetyl CoA

• produce ATP via ETC and CA cycle

Question 34

How many acetyl CoA molecules are produced by the final oxidation of the 4 carbon acyl CoA?

Answer 34

2 (if have even number)