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CORE II Block 1 > Lipid biosynthesis and transport > Flashcards

Lipid biosynthesis and transport Flashcards

1
Q

Name the three main type of lipids.

A
  • isoprenoids
  • glycosphingolipids
  • phospholipids
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2
Q

What are the critical roles of lipids in biology?

A
  • energy store
  • cell membranes
  • signaling molecules
  • co-factors
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3
Q

What is the job of acyl-glycerols?

A

Energy storage

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4
Q

In general, what is the mechanism of fatty acid storage?

A

Free fatty acids are esterified to glycerol.

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5
Q

Which intermediate does triacylglycerol and phospholipid biosynthesis have in common?

A

phosphatidic acid

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6
Q

How is the phosphatidic acid intermediate made?

A
  1. glycerol-3-phosphate (sourced from glycolysis) is condensed with a coA-activated fatty acid
  2. product is condensed with second coA-activated fatty acid to form phosphatidic acid
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7
Q

What is the source of glycerol-3-phosphate for the formation of phosphatidic acid?

A

source = dihydroxyacetone phosphate, converted to glycerol-3-phosphate by glycerol-3-phosphate dehydrogenase.(this mecanism is a back-up to the aspartate-malate shuttle)

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8
Q

Starting from the phosphatidic acid intermediate, describe triacylglycerol synthesis.

A
  1. the phosphate on phosphatidic acid is hydrolyzed by phosphatase
  2. A third coA-activated fatty acid condenses with the last remaining hydroxyl group, forming triacylglycerol
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9
Q

Why is triacylglycerol formation an efficient storage method for lipids?

A

The molecule is extremely hydrophobic, so the molecules pack tightly together (they fall out of solution after forming lipid droplets), creating a phase separate from the surroundings.

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10
Q

From what lipid type are glycosphingolipids, eicosanoids, triacylglycerides, and phospholipids derived?

A

Fatty acids

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11
Q

Describe the generic phospholipid structure.

A
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12
Q

From what structure does phosphoiipid synthesis begin?

A

phosphatidic acid

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13
Q

Describe phospholipid synthesis in general.

A

A phosphatidic acid is esterified to alcohol via phosphate group. The phosphate must be activated, or the alcohol must be activated, because a high energy phosphoester bond will be formed.

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14
Q

What are two examples of phosphatidic acid being activated in the synthesis of phospholipid?

A
  • phosphatidyl serine: CTP is condensed onto the phosphate moiety with a loss of pyrophosphate. The alcohol on serine then attacks the first phosphate, causing the CMP to leave as a leaving group (net negative charge)
  • phosphatidyl inositol: same as above, but the alcohol is inositol (net negative charge)
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15
Q

What is an example of phosphatidic acid being activated in the synthesis of phospholipid?

A

-phosphatidyl ethanolamine: starting with diacylglycerol, ethanolamine is activated with CTP to become CDP-ethanolamine. The diacylglycerol alcohol attacks the first phosphate, causing CMP to leave as a leaving group (neutral charge phospholipid)

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16
Q

What is PAF?

A

Platelet-activating factor. It is an ether lipid responsible for platelet aggregation and smooth muscle contraction in the airway.

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17
Q

What is the subclass of ether lipids containing unsaturated bonds?

A

plasmologens

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18
Q

Describe the molecular makeup of waxes.

A

They are long chain fatty acids esterified to very hydrophobic alcohol. This is what sets them apart.

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19
Q

How are sphingolipids different from glycerophospholipids?

A

Sphingolipids do not start from an acylglycerol such as diacylglycerol or phosphatidic acid, but rather from a sphingosine backbone. Only difference is that an alcohol is replaced with an amine.

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20
Q

Describe the modifications that a sphingosine molecule can undergo

A

Because it has three polar positions, it can undergo three modifications within a single molecule.

  • The addition of a fatty acid would make a ceramide
  • addition of UDP-glucose to ceramide makes a cerebroside
  • addition of phosphatidylcholine to ceramide makes a sphingomyelin
21
Q

Desribe a ganglioside.

A

A variety of oligosaccharides can be added to one of the remaining two polar positions on a ceramide, providing the opportunity for a large amount of diversity in a single class of molecules.

22
Q

Which diseases are associated with ganglioside formation?

A
  • tay-sacchs: defective gangliosidase
  • guillain-barre: anti-ganglioside immune response
  • viruses can also use gangliosides as attachment sites
23
Q

What are the sources of lipid diversity?

A
  1. different fatty acyl chains (length, saturation)
  2. different head groups
  3. phospholipids vs ether lipids vs sphingolipids
  4. glycosylation patterns
24
Q

How does lipid diversity differ between species?

A
  • immune recognition may differ: humans have systems to recognize bacterial lipids.
  • anti-infectives: we have anti-fungals against fungal-specific lipid pathways.
25
Q

Describe the role of pancreatic lipase.

A

Pancreatic lipase breaks down dietary triacylglycerols into fatty acids and monoacylglycerol.

26
Q

Describe the role of hormone-sensitive lipase.

A

It mobilizes fat stores from adipose.

27
Q

Describe the role of lipoprotein lipase.

A

It breaks down triacylglycerides into fatty acids (TAs made by endogenously) for reuptake by adipoe (after reassembly within the adipocyte)

28
Q

At which positions on phospholipid can lipases act?

A

At all positions… fatty acids can be cleaved off, or the phosphate-alcohol ester can be cleaved off.

29
Q

In what overal physiological process are eicosanoids involved?

A

They are local hormones involved in paracrine or autocrine signaling

30
Q

From what molecule are eicosanoids derived?

A

arachidonic acid cleaved from phospholipids. a phospholipase cleaves off a fatty acid moiety.

31
Q

What are the three families of eicosanoids?

A
  1. leuokotrienes
  2. prostaglandin
  3. EETs
32
Q

Why do high levels of eicosanoids never build up in the blood?

A

Because they are local hormones, they are inherently unstable with short half lives, so they don’t accumulate.

33
Q

Plasma concentrations of eicosanoids are less than 10^-9 M. Eicosanoid receptors require concentrations >>10^-9 M to elicit a response. How is this feasable?

A

This works becasue they are local signals that never even enter the plasma. It is feasable for a cell type to produce that much eicosanoid molecule to elicit a response and to have the signal quickly degraded.

34
Q

What does it mean for eicosanoids to be pleiotropic?

A

It means that one signal molecule has many (and sometimes contradictory) phenotypic effects. This is possible based on the ifferent responses elicited by an eicosanoid from different cell types.

35
Q

What are some of the responses elicited by eicosanoids?

A
  • inflammation
  • fever
  • smooth muscle contraction
  • gastric mucosa maintenance
  • renal absorption
  • platelet aggregation
  • childbirth
36
Q

What is difficult about treating pain caused by prostaglandins? (made from arachidonic acid by PGHS-1 and PGHS-2 enzymes)

A

There are two different isoforms of these enzymes, also called cox1 and cox2. Cox1 is a housekeeping isoform (homeostasis) and is always turned on. Cox2 is only turned on if there is a problem, causing pain and inflammation. NSAIDS, which inhibit these enzymes involved in prostaglandin synthesis, targets both isoforms, so people who use NSAIDS long term end up with ulcers due to inhibition of the housekeeping isoform.

37
Q

How might drug developers overcome the issue with long-term NSAID usage?

A

Because the drug does not differentiate between isoforms, drug developers might make a drug based on the isoform active site, making an NSAID which fits into cox2 but not cox1. Pharm companies have tried this, but they are associated with cardiac problems, because cox2 also produces prostacyclin which expands blood vessels.

38
Q

Where in the prostaglandin/thromboxane pathway is cox1 and cox2 located?

A

the first step after arachidonic acid.

39
Q

How do leukotrienes achieve functional diversity?

A

They are modifed differently depending on which enzymes act upon them.

40
Q

What pathologies are leukotrienes involved in?

A
  • asthma
  • anaphylaxis
  • inflammation

all of these are caused by local hormones, so leukotrienes are local hormones

41
Q

What are EETs?

A

They are a family of eicosanoids that are epoxified at different positions, lending to decreased stability. Because they are unstable, it is hard to determine what they do. They may be involved in cardioprotection or tumor production.

42
Q

Describe transport of dietary lipids.

A
  1. Dietary triacylglycerols are solubilized by bile salts, which are detergents made from cholesterol
  2. pancreatic lipase releases fatty acids and monoacylglycerol
  3. these components are taken up by mucosal cells and reassembled into TAs
  4. TAs associate with apolipoproteins within mucosal cells to form chylomicrons
  5. chylomicrons pass through lymph to blood and drop off TAs to muscle and adipose tissues, where lipoprotein lipase releases TAs into fatty acids and monoacylglycerol before uptake by the cells.
  6. once inside adipose cells, the TAs are reassembled for storage. Muscle cells will oxidize the FAs for energy.
43
Q

What is the overarching problem with lipid transport?

A

Blood is aquous but lipids are hydrophobic.

44
Q

Describe the structure of chylomicrons.

A

On the very inside (nonpolar) are TAs and esterified cholesterol,. On the outside are phospholipids due to polar headgroups, allowing the chylomicron to circularize in water. Apoproteins on the chylomicron surface help to hold the shape.

45
Q

How are newly synthesized triacylglycerides made in the liver ER transported?

A
  1. TAs and apolipoproteins made in the liver ER are secreted in VLDLs.
  2. VLDLs are delivered to muscle and adipose.
  3. Before cell uptake, lipoprotein lipase liiberates FAs from monoacylglycerol
  4. the components are taken up by adipose where they are reassembled for storage. In muscle, fatty acids are oxidized for energy.
46
Q

In what structure are VLDLs synthesized?

A

Apolipoproteins and VLDLs are made in the lumen of the ER.

47
Q

Describe transport of cholesterol.

A
  1. cholesterol is packaged into HDLs
  2. HDLs are delivered to tissues.
48
Q

What do all lipid-transporting vehicles have in common?

A

They all make the lipid water-soluble.

49
Q

What enzyme catalyzes the commited step of leukotriene synthesis?

A

5-lipoxygenase.

CORE II Block 1 (5 decks)

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