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Biochemistry > Chapter 16: Lipid Metabolism > Flashcards

Chapter 16: Lipid Metabolism Flashcards

1
Q

what are the 4 ways fatty acids differ from one another?

A
  • length of hydrocarbon tails
  • number of carbon-carbon double bonds
  • positions of the double bonds in the chain
  • the number of branches
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2
Q

what is the purpose of fatty acid synthesis?

A

to make fatty acids

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3
Q

what are the starting molecules of fatty acid synthesis?

A
  • acetyl CoA

- malonyl ACP

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4
Q

what are the ending molecules of fatty acid synthesis?

A
  • palmitate (16)

- stearate (18)

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5
Q

how are fatty acids synthesized?

A
  • fatty acids are synthesized by the repetitive adding of 2-carbon units to the growing ends of a hydrocarbon chain
    • the growing chain is covalently attached to acyl carrier protein (ACP)
    • the linkage is a thioester
  • two main stages are initiation and elongation
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6
Q

what is the precursor for all fatty acid synthesis? what is it made from?

A

malonyl ACP, formed from acetyl CoA, is the precursor for all fatty acid synthesis

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7
Q

what are the steps of Malonyl ACP synthesis?

A
  1. Carboxylation of acetyl CoA in the cytosol to form malonyl CoA
  2. Transfer of malonyl from CoA to ACP
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8
Q

which enzyme catalyzes the carboxylation of acetyl CoA to form malonyl CoA?

A
  • biotin-dependent enzyme acetyl-CoA carboxylase

- uses a molecule of ATP

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9
Q

which enzyme catalyzes the transfer of malonyl from CoA to ACP?

A

malonyl CoA:ACP transacylase

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10
Q

what do malonyl ACP and acetyl ACP come together to form?

A

Acetoacetyl ACP

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11
Q

how is Acetoacetyl ACP formed?

A
  • *reaction catalyzed by 3-ketoacyl ACP synthase
    • a 2 carbon unit from acetyl ACP is transferred to the enzyme where it is covalently bound via a thioether linkage
    • the enzyme then catalyzes the transfer of this 2 carbon unit to the end of malonyl ACP, generating a 4 carbon intermediate (Acetoacetyl ACP) and CO2
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12
Q

what are the steps of initiation?

A
  1. An acetyl group gets transferred from CoA to ACP by acetyl CoA-ACP transacylase
  2. An acetyl (acyl) group next gets transferred to the K arm of the FAS complex
  3. A malonyl group gets transferred from CoA to ACP by malonyl CoA ACP transacylase - results in both arms of FAS occupied forming acylmalonyl ACP
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13
Q

what are the steps of elongation?

A
  1. 3-ketoacyl ACP reductase (KR) catalyzes the first reduction (ketone to alcohol)
    • requires NADPH
  2. 3-hydroxyacyl ACP dehydratase (DH) catalyzes the dehydration (removal of H20)
  3. Enoyl-ACP reductase (ER) catalyzes the second reduction (adds hydrogens)
    • requires NADPH
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14
Q

what is the final product of elongation?

A
  • *the final product is an acyl ACP that is two carbons longer
    • this acyl ACP becomes the substrate for elongation by KAS
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15
Q

what is ALWAYS the carbon donor for elongation?

A

malonyl ACP

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16
Q

why are only 16 and 18 carbon fatty acids made?

A

longer chain lengths do not fit in the binding site of the condensing enzyme

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17
Q

how are completed fatty acids released from ACP?

A

thioesterase (TE)

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18
Q

what are the enzymes of the fatty acid synthase?

A
  1. MAT: malonyl/acetyl transferase
  2. KAS: 3-ketoacyl ACP synthase
  3. KR: 3-ketoacyl ACP reductase
  4. DH: 3-hydroxyacyl ACP dehydratase
  5. TE: thioesterase
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19
Q

what happens after thioesterase releases free fatty acids, but subsequent modifications are needed?

A
  • *thioesterase releases free fatty acids, but subsequent modifications of these fatty acids require an activation step where they are converted to thioesters of CoA
    • ATP dependent reaction
    • catalyzed by acetyl-CoA synthetase
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20
Q

how are fatty acids longer than 16 or 18 carbons made?

A
  • made by extending palmitoyl CoA or stearoyl CoA in separate extension reactions
  • the enzymes that catalyze the reaction are called elongases and use malonyl CoA as the source of the 2 carbon extension unit
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21
Q

how are unsaturated fatty acids made in bacteria?

A
  • a double bond is added to a growing chain when it reaches 10 carbons in length
  • after desaturation, elongation continues normally
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22
Q

how are unsaturated fatty acids made in eukaryotes?

A
  • double bonds cannot be introduced during fatty acid synthesis since they employ the fatty acid synthase complex
  • eukaryotes use desaturase enzymes after synthesis to introduce double bonds
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23
Q

how are polyunsaturated fatty acids (PUFA) synthesized?

A

by the sequential action of different, highly specific desaturases

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24
Q

which desaturases do mammalian cells not contain?

A

mammalian cells do not contain desaturases that act beyond the C9 position

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25
Q

which fatty acids do we need from our diet?

A
  • Linolenic acid (omega 3)
  • linoleic acid (omega 6)

*interconvert using desaturases and elongases

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26
Q

an intermediate in the synthesis of triacylglycerols and glycerophospholipids

A

phosphatidate

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27
Q

how is phosphatidate formed?

A

by transferring the acyl group from fatty acid CoA molecules to the C-1 and C-2 positions of glycerol 3-phosphate

28
Q

what are the two general classes of Eicosanoids?

A
  1. Prostaglandins and thromboxanes

2. Leukotrienes

29
Q

what is the precursor of many Eicosanoids?

A

Arachidonate (20:4)

30
Q

how are prostaglandins synthesized?

A
  • **by the cyclization of arachidonate by an enzyme called prostaglandin H synthase (PGHS)
    • this is the cycloogygenase activity of this enzyme (COX-1)
    • this forms prostaglandin G2, the precursor for prostaglandin H2, which can be converted to prostacyclin, thromboxane A2, and a variety of prostaglandins
31
Q

in addition to the cycloogyenase activity of prostaglandin H synthase (PGHS), what other activity does it have?

A

lipooxygenase activity - leads to the production of leukotrienes

32
Q

how is ceramide generated?

A

starting with the condensation of serine and palmitoyl CoA

33
Q

what are the stages of cholesterol synthesis?

A
  1. Acetyl CoA to Isopentyl diphosphate
  2. Isopentyl diphosphate to squalene
  3. Squalene to cholesterol
34
Q

what happens during stage 1 of cholesterol synthesis (Acetyl CoA to Isopentyl diphosphate)?

A
  • the condensation of 3 acetyl CoA molecules leads to HMG CoA, which is reduced to mevalonate
  • mevalonate is then converted to isoprenyl diphosphate via two phosphorylations and a decarboxylation
35
Q

what happens during stage 2 of cholesterol synthesis (Isopentyl diphosphate to squalene)?

A
  • Isopentyl diphosphate is isomerized and then dimerized in a head to tail condensation reaction
  • subsequent condensation reactions generate squalene (C 30)
36
Q

how much ATP is needed to synthesize squalene?

A

20.5 ATP

37
Q

what happens during stage 3 of cholesterol synthesis (Squalene to cholesterol)?

A
  • the steps between squalene and the first fully cyclized intermediate (lanasterol) include the addition of a hydroxyl group followed by a concerted series of cyclizations
  • no ATP needed
38
Q

what is cholesterol a precursor of?

A
  • bile salts
  • hormones
  • the D vitamins
39
Q

how are fatty acids degraded?

A

beta oxidation

40
Q

what is the starting molecule of beta oxidation?

A

fatty acyl CoA

41
Q

what are the products of beta oxidation?

A
  • acetyl CoA
  • NADH
  • QH2
  • fatty acyl reduced by 2 carbons
42
Q

which carbon of the fatty acid is oxidized?/

A

the beta carbon

43
Q

where does fatty acid synthesis occur?

A

cytosol

44
Q

where does beta oxidation occur?

A

mitochondria and perioxisomes

45
Q

what are the four steps of beta oxidation?

A
  1. Fatty acyl CoA oxidized to trans-enoyl CoA
  2. Trans-enoyl CoA hydrated to form L-3-hydroxyacyl CoA
  3. L-3-hydroxyacyl CoA oxidized to form 3-ketoacyl CoA
  4. 3-ketoacyl CoA thiolysis to form acetyl CoA and a fatty acyl CoA shortened by two carbons
46
Q

what happens during the first step of beta oxidation (Fatty acyl CoA oxidized to trans-enoyl CoA)?

A

-oxidation of fatty acyl CoA to trans-enoyl CoA
-accompanied by reduction of FAD to FADH2
~acyl-CoA dehydrogenase
-pass electrons along to Fe-S
-electrons move onto Q to form QH2

47
Q

what enzyme is used in the hydration of trans-enoyl CoA to form L-3-hydroxyacyl CoA?

A

2-enoyl-CoA hydratase

48
Q

what happens during the third step of beta oxidation (L-3-hydroxyacyl CoA oxidized to form 3-ketoacyl CoA)?

A
  • L-3-hydroxyacyl CoA oxidized to form 3-ketoacyl CoA
  • NAD+ reduced to NADH
  • L-3-hydroxyacyl CoA dehydrogenase
49
Q

what happens during the fourth step of beta oxidation (3-ketoacyl CoA thiolysis to form acetyl CoA and a fatty acyl CoA shortened by two carbons)?

A
  • 3-ketoacyl CoA thiolysis to form acetyl CoA and a fatty acyl CoA shortened by two carbons
  • 3-ketoacyl CoA thiolase
  • HS-CoA goes in, acetyl CoA comes out
50
Q

what does the carnitine shuttle system?

A
  • long chain fatty acyl CoA formed in the cytosol cannot diffuse across the inner mitochondrial membrane to the matrix where beta oxidation occurs
  • carnitine shuttle system actively transports the long-chain fatty acids
51
Q

how is bacteria beta oxidation unusual?

A

bacteria synthesize odd-chain fatty acids that are degraded by beta oxidation except that the final product is a 3-carbon acyl CoA - propionyl CoA

52
Q

what happens to propionyl CoA?

A

can be converted to succinyl CoA and eventually oxaloacetate

53
Q

do saturated and unsaturated fatty acids require the same enzymes for oxidation?

A

unsaturated fatty acids require 2 enzymes in addition to those usually needed for fatty acid oxidation

54
Q

where does most lipid biosynthesis in eukaryotic cells occur?

A
  • **endoplasmic reticulum
  • the biosynthetic enzymes are membrane bound with their active sites oriented toward the cytosol, so they can have access to water-soluble cytosolic compounds
55
Q

what are the principle hormone regulators of fatty acid metabolism?

A
  • glucagon
  • epinephrine
  • insulin
56
Q

what is the key regulatory enzyme for fatty acid synthesis?

A

acetyl-CoA carboxylase (step 1 of making malonyl CoA)

57
Q

how does high insulin affect fatty acid metabolism?

A
  • inhibit the hydrolysis of stored triaglycerols

- stimulate the formation of malonyl CoA by acety CoA carboxylase

58
Q

how do high glucagon and epinephrine affect lipid metabolism?

A
  • glucagon and epinephrine are high in the fasted state and will signal the mobilization of fatty acids to serve as fuel
  • high glucagon inhibits fatty acetyl-CoA carboxylase, slowing storage
59
Q

where does digestion of dietary lipids occur?

A

mainly in the small intestine where suspended fat particles are coated with bile salts, which help them become soluble

60
Q

how/where are triaglycerols broken down?

A

triacylglycerols are broken down in the small intestine by lipases

61
Q

why can’t triacylglycerols, cholesterol, and cholesteryl esters be transported int he blood or lymph as free molecules?

How is this problem fixed?

A
  • they are insoluble in water
  • instead, these lipids assemble with phospholipids and amphipathic lipid-binidng proteins to form spherical macromolecules called lipoproteins
62
Q

what happens to acetyl CoA produced in the liver from fatty acid oxidation?

A

most is routed to the krebs cycle, but some can follow an alternate pathway

63
Q

what happens during fasting when glycolysis is decreased and the gluconeogenic pathway is active?

A
  • the pool of oxaloacetate becomes temporarily depleted and the amount of acetyl CoA exceeds the capacity of the krebs cycle
  • the excess acetyl CoA is used to form ketone bodies
64
Q

what are common ketone bodies?

A
  • beta-hydroxybutyrate
  • acetoacetate
  • acetone
65
Q

where are ketone bodies made?

A

ketone bodies are synthesized in the liver during starvation, becoming substitutes for glucose

66
Q

where are ketone bodies oxidized?

A

ketone bodies are oxidized in the mitochondria in skeletal muscle and the intestine during starvation

Biochemistry (9 decks)

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