5B- Synthesis of FA's and TG's

Question 1

How is citrate made in the mitochrondria?

Answer 1

ii. The pathway begins with glycolysis, which converts glucose –> pyruvate in the cytosol. The pyruvate enters the mitochondria, it’s converted to acetyl CoA by pyruvate dehydrogenase (PDH) or to oxaloacetate by pyruvate carboxylase.
iii. When acetyl CoA levels are high, PDH is inhibited and pyruvate carboxylase is activated, converting pyruvate –> oxaloacetate. When [oxaloacetate] increases, it condenses with acetyl CoA to form citrate.

Question 2

What is the role of citrate being made in the mitochondria and the cytoplasmic acetyl CoA?

Why all this work?

Answer 2

iv. Citrate is made and transported across the mitochrondial membrane back to the cytosol, where citrate lyase converts citrate –> OAA + Acetyl CoA

Why? 1. Because PDH is only in the mitochondria. We can’t make cytosolic Acetyl CoA using PDH!

Question 3

What are the 2 places that NADPH comes from?

Answer 3

i. Pentose Phosphate Pathway- discussed in section 5D
ii. Recycling of oxaloacetate from citrate lyase

Question 4

How is NADPH made in the cytosome from OAA?

Answer 4

i. OAA + NADH –> Malate + NAD+ using the enzyme malate dehydrogenase
ii. Malate + NADP+ –> Pyruvate + NADPH + CO2 using malic enzyme

Question 5

What is the Km of muscle LPL? Why is this significant?

Answer 5

ii. The low Km of muscle LPL allows use of fatty acids from chylomicrons and VLDL even when [lipoproteins]blood is low.

Question 6

What is the Km of adipose tissue? Why is this significant?

Answer 6

iii. The K¬m in adipose is high and is therefore active after a meal when fats are high in the blood.

Question 7

What happens to the free fatty acids and glyceol after they are broken down by adipose tissue LPL?

Answer 7

1. When the free fatty acids enter the cell (after being freed from the triacylglyceride), they basically get put back into triglycerides right away.
2. When they come in, they’re activated to fatty acyl CoA. This reacts with glycerol-3 phosphate to from triacylglycerol.

Question 8

How do we convert Acetyl CoA to Malonyl CoA?

Answer 8

a. To begin to make fats, we first have to get the 2 carbon acetyl CoA into a 3 carbon molecule. This is established by converting acetyl CoA + CO2+ ATP –> malonyl CoA + ADP + Pi using biotin and acetyl CoA carboxylase.

Question 9

What are the activators and inhibitors of Acetyl CoA carboxylase?

Answer 9

i. Activators- citrate, dephosphoryation (in the fed state), insulin
ii. Inhibitors- Palmityl CoA (the end product of fatty acid synthesis), phosphorylation by AMP-activated protein kinase (when energy levels are low).

Question 10

What are the steps of palmitic acid synthesis?

Answer 10

ii. To begin, an acetyl group is added to a -SH of Cysteine on another subunit.

The malonyl CoA attaches to the ACP -SH group and condenses and releases CO2.

2 moles of NADPH is used for reducing the chain, which is shown below.

v. The 4-carbon chain on the ACP is transferred to the Cyteine -SH group next door. Once there, the -SH group on the ACP is free and a cytosolic malonyl CoA can bind to it just like before. the malonyl group on the ACP condenses with the 4-carbon chain on the Cys to make a 6-carbon chain on the ACP.
vi. This whole process repeats until we have 16 carbons off the ACP, which is palmitic acid.

Question 11

What are the 5 things that insulin stimulates?

Answer 11

i. Synthesis and release of LPL
ii. Phosphofructokinase-1
iii. Phosphofructokinase-2
iv. Dephosphorylation of pyruvate dehydrogenase (so pyruvate from glycolysis can go into TCA)
v. Conversion of glucose to fatty acids in adipose cells

Question 12

How does glucagon stimulate lipolysis?

Answer 12

b. When we fast, insulin decreases as glucagon increases. Glucagon causes cAMP levels to rise in adipose cells, stimulating lipolysis. It follows this mechanism:

Question 13

What is the role of carnitine:palmitoyltransferase I (CPT I) in fatty acid synthesis?

Answer 13

What really matters, in terms of regulation and everything, is that CPT I is inhibited by malonyl CoA .

Malonyl CoA levels are increased when acetyl CoA carboxylase is activated (remember back to #4?). So basically, when fatty acid synthesis is taking place, fatty acid oxidation is inhibited.

Question 14

What are the effects of carnitine deficiency?

Answer 14

i. Classical CPTII deficiencies is characterized by recurrent episodes of acute myoglobinuria precipitated by prolonged exercise or fasting (lol didn’t we just have a test question on this?)
ii. Lipid deposits are found in the skeletal muscles
iii. CPK and long-chain acyl carnitine levels are increased in the blood.
iv. All episodes seen in patients occur during fasting, infections or vomiting (basically, when you need fats for energy) but you can’t utilize fats because they can’t be transported into the cell!

Question 15

What are the effects of biotin deficiency?

Answer 15

acetyl CoA carboxylase requires biotin to function. This enzyme is required for the synthesis of fats.

pyruvate is converted into OAA using pyruvate carboxylase. This enzyme requires biotin to function. Again, synthesis of fats would be affected.

iii. Symptoms are basically from the physiology: you can’t make fatty acids out of free acetyl CoA’s, therefore:
1. Fasting- low blood glucose due to no β-oxidation, and relying soley on gluconeogenesis, glycogenolysis, glycolysis and ketone bodies.
2. Fed- you take in fats but you can’t store them. Increased [fats]plasma.

Question 16

What are the effects of pantothenic acid (B5) deficiency?

Answer 16

i. Aka vitamin B5, panothenate is essential for the synthesis of CoA. Without it, you cannot make acetyl CoA and therefore you can’t make fatty acids.
ii. Furthermore, you need CoA for the formation of ACP on the FAS.
iii. Deficiencies are pretty much like Biotin (B7). You can’t make fatty acids.

Question 17

What substrates are used in fatty acid desaturation?

Answer 17

O2, NADH and cytochrome b5 combine to oxidize both the fatty acid and NADH

Question 18

Why is fatty acid desaturation important?

Answer 18

i. We need to have polyunsaturated fatty acids with double bonds 3 and 6 carbons from the methyl end to make eicosanoids.

Question 19

Why are linoleic and linolynic acid important for our diets?

Answer 19

b. Linoleic (18:2 Δ9,12) and linolenic (18:3 Δ9,12,15) acids are considered essential fatty acids because they contain the ω6 bond (a double bond 6 carbons from the terminal end of the fatty acid). We have to get these from our diet. They are converted into arachidonic acid, which is essential for the synthesis of eicosanoids!

Question 20

How are triglycerides synthesized in the liver?

Answer 20

a. In the liver and in adipose tissue, triacyl glycerols are produced by the reaction G3P + 2 Fatty acids –> phosphatidic acid. This then releases Pi to form diacylgycerol, which then combines with a third fatty acid and form triacylglycerol.

Question 21

What is the structure of Glycerophospholipids?

Answer 21

1. They are normal triglycerides except 1 of the fatty acids is replaced with a phosphate bound to a head group. The nonpolar tail and the polar head make the molecule amphipathic, which is important in making cell membranes.

Question 22

What is the function of glycerophospholipids?

Answer 22

1. They are components of cell membranes, blood lipoproteins, bile and lung surfactant. The source of polyunsaturated fatty acids (arachidonic acid) and therefore serve as precursors to eicosanoids.

Question 23

What is the structure of sphingolipids?

Answer 23

1. They have a backbone of sphingosine, which is an 18-carbon amino alcohol. They have a phosphatebound head group and a fatty acid chain as well.

Question 24

What is the function of sphingolipids?

Answer 24

1. Important in forming the myelin sheath around nerves. Serve as the intracellular communication and as the antigenic derminates in ABO blood types.

Question 25

What is the structure of glycolipids?

Answer 25

1. Basically the same structure as the sphingolipid except the phosphate head group is repleaced with a carbohydrate molecule.

Question 26

What is the function of glycolipids?

Answer 26

1. Their main function is to serve as markers for cellular recognition.

Question 27

What is phosphatidylcholine and how is it made from diacylglycerol?

Answer 27

a. Phosphatidylcholine is a type of glycerophospholipid, in which the head group is a choline molecule. Essentially, diacylglycerol reacts with CDP-choline to synthesize phosphtidylcholine.

Question 28

How is phosphtidylcholine important in lung surfactant?

Answer 28

c. There are 4 major components to lung surfactant: dipalmitoylphosphtidylcholine, phosphtidylglycerol, apoproteins and cholesterol.
i. These reduce the surface tension within the alveoli, preventing their collapse. Without the components of the surfactant, breathing is near impossible.