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Biochem BLOCK 2 > Fatty acid synthesis > Flashcards

Fatty acid synthesis Flashcards

1
Q

sources of FA

A
  • dietary FA (most here)
  • excess dietary protein and carbs converted to FA for storage
  • De Novo synthesis
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2
Q

De Novo synthesis

A

in adults: most FA synthesis is in liver and lactating mammary glands, some in adipose

  • synthesized in the cytosol
  • cytosolic acetyl CoA is the carbon source for growing FA chain
  • energy source: ATP and NADPH
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3
Q

acetyl group of acetyl CoA

A
  • generated in the mitochondria

- required for FA production in the cytosol

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4
Q

CoA portion of the acetyl CoA..

A

cannot cross the mitochondrial membrane

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5
Q

production of cytosolic acetyl CoA

A

requires a process for generation and delivery of the acetyl-group of mitochondria acetyl-CoA to the cytosol
-charged, converted to citrate, this CAN cross the inner mitochondrial membrane to cytosol

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6
Q

ATP-citrate lyase

A

citrate to OAA and acetyl CoA to get in the cytosol

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7
Q

step 1 of de novo synthesis

A

mitochondrial acetyl CoA is generated from

  • oxidation of pyruvate
  • B-oxidation of long carbon chain CoA
  • catabolism of keno bodies and some amino acids
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8
Q

step 2 of de novo synthesis

-acetate exits mitochondria and enters the cytosol as citrate

A
  1. citrate is produced from condensation of OAA and the acetyl group of mitochondria acetyl CoA
  2. citrate accumulates to a high level in mitochondria when isocitrate dehydrogenase is inhibited by high ATP levels
  3. citrate exits mitochondria to the cytosol
  4. citrate is cleaved by ATP-citrate lyase to produce cytosolic acetyl CoA and OAA
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9
Q

What has to happen for citrate to accumulate in the mitochondria?

A

the accumulation of citrate in the mitochondria requires that high levels of ATP inhibit isocitrate dehydrogenase. This fatty acids are likely to be generated only in the well fed state

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10
Q

what is the regulatory step of de novo synthesis?

A

carboxylation of cytosolic acetyl COA to form malonyl CoA

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11
Q

acetyl CoA carboxylase

A

uses CO2 and energy from ATP hydrolysis to carboxylate the acetyl group of Acetyl CoA in the carboxylation of cytosolic acetyl CoA to form malignly CoA
-stored energy- C-C bonding in synthesis

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12
Q

what is important about carboxylation of cytosolic acetyl CoA to form Malonyl CoA?

A
  • this carboxylation and subsequent decarboxylation reaction provides the energy for carbon to carbon condensations to elongate the growing FA chain
  • carboxylation of acetyl CoA is the rate limiting and regulated step for FA synthesis
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13
Q

Where is energy stored for FA?

A

malonyl CoA

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14
Q

short term regulation of acetyl CoA carboxylase (ACC)

A
  • inactive ACC dimers are allosterically activated to its polymerized form by citrate
  • activated ACC is depolymerized by the end product long-chain fatty acyl CoA
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15
Q

what is ACC promoted by?

A

citrate

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16
Q

short term regulation of acetyl CoA carboxylase (ACC) (fasting)

A
  • AMP-activated protein kinase (AMPK) reversibly phosphorylates and inhibits ACC (fasting)
  • AMPK is allosterically activated by AMP and by phosphorylation by kinases, one of which is cAMP dependent PKA
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17
Q

What is ACC indirectly inhibited by?

A

epinephrine and glucagon

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18
Q

long term regulation of acetyl CoA carboxylase (ACC)

A
  • prolonged high calorie, high carb diets increase ACC synthesis which increase fatty acid synthesis
  • a low calorie or high-fat diet reduces FA synthesis by decreasing ACC synthesis
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19
Q

Fatty acid synthase (FAS)

A
  • a multifunctional dimeric enzyme in eukaryotes
  • 7 different enzymatic activities
  • a domain for covalently binding 4’-phosphopantetheine
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20
Q

FAS domain for binding 4’-phosphopantetheine

A
  • this domain function as an acyl carrier protein (ACP)

- ACP carries acyl units on its terminal thiol-group (-SH) during FA synthesis

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21
Q

At ACP site…

A
  • acetyl CoA deliver CoA to this
  • move 2-C molecule to holding site (cysteine)
  • opens up APC site for malonyl CoA, transfers 3-C
  • attack malignly group and decarboxylate that 3-C
  • 4-C structure @ ACP site
  • Keeps happening over and over again (about 7X)
  • needs a lot of NADPH
  • 16C FA
22
Q

What is required for chain elongation?

A

repeating the 7 steps of FA synthase

23
Q

initiation of elongation requires what?

A

transfer of butyryl to the cysteine reside holding sire of FAS

24
Q

What is the end product of FA synthesis

A

palmitoyl-S-CoA

25
Q

what is the rate limiting step and regulation step of FA synthesis

A

Acetyl CoA carboxylation to generate malonyl CoA

26
Q

how many times do the steps go through for FA synthesis?

A

7

-each round adds 2 carbons to the growing chain

27
Q

palmitoyl thioesterase

A

cleaves the thirster bond releasing saturated palmitate

28
Q

NADPH FA synthesis

A

functions as a biochemical reductant, a source of reducing agents for FA synthesis

29
Q

What is the predominant source of NADPH in FA synthesis?

A

HMP shunt

  • G6PDH: rate limiting, irreversible activitiy
  • G6PDH utilizes NADP+ as a coenzyme acceptor to oxidize glucose 6-P
  • from each HMP committed glucose 6-P, two NADPH are produced
30
Q

what is an additional source of NADPH in FA synthesis?

A
  • malate oxidation and decarboxylation by malic enzyme
  • ME oxidizes and decarboxylates malate to pyruvate
  • NADP+ functions as a coenzyme acceptor to oxidize malate generating NADPH
31
Q

Where does FA elongation take place at?

A

smooth ER

32
Q

How is palmitate elongated?

A

malonyl CoA gives 2-carbon units and the reduction of NADPH at the smooth ER

33
Q

brain tissue and FA elongation

A

special elongation capabilities to generate VLCFA (>22) that are require for brain lipids

34
Q

Where does desaturation of VLCFA occur?

A

smooth ER

contains desaturases

35
Q

What is a common mono unsaturated FA?

A

18: 1(9)
16: 1(9) is less common

36
Q

storage of FA as components of triacylglycerols

A
  • formation of mono, di, and triacylglycerol as FA are esterified via their carboxyl group to the carbons glycerol
  • esterified FA loose their charge, forming ‘neutral’ TAG molecules
37
Q

structure of TAG

A

carbon 1-saturated FA of varied length
carbon 2: unsaturated FA of varied length
carbon 3: saturated or unsaturated FA of varied length

*wanna make sure the terminal two are saturated

38
Q

TAG is…

A
  • only slightly soluble in water
  • unable to form micelles independently
  • able to coalesce and form nearly anhydrous, cytosolic oil droplets
39
Q

what is the primary storage place of TAG?

A

adipose tissue

40
Q

What is the site of FA synthesis?

A

liver

41
Q

synthesis of TAG requires the production of TAG building blocks

A
  • glycerol phosphate-the initial acceptor of activated FA during TAG synthesis
  • acyl CoA-free FA must be converted to the activated form
42
Q

synthesis of glycerol phosphate in the liver

A

glycerol kinase converts free glycerol to glycerol phosphate

43
Q

Where are the two pathways to produce glycerol phosphate?

A

liver and adipose tissue

  • produced from glucose via the glycolytic pathway
  • uses lots of energy, must be in the well fed state
  • glucose to DHAP
  • DHAP is reduced to glycerol phosphate by glycerol phosphate dehydrogenase
44
Q

GLUT 4 activity in adipocytes

A

insulin dependent

-when plasma glucose and insulin levels are low, adipose tissue cannot synthesize glycerol phosphate: no TAG production

45
Q

What is the conversion of free FA to acetyl CoA catalyzed by?

A

CoA synthase (thiokinase)

46
Q

synthesis of a TAG molecule

A
  • sequential addition of 2 FA from fatty acyl COA to glycerol phosphate by acyltransferase
  • removal of the phosphate group by a phosphatase
  • addition of the third FA by an acyltransferase
47
Q

Fates of TAG in the adipose tissue

A
  • TAG is stored as cytosolic lipid droplets

- TAG in this format is easily mobilized when fuel is required

48
Q

Fates of TAG in the liver tissue

A
  • small amounts of TAG are stored in the liver
  • TAG generated in the liver is packaged with other lipids and apoproteins to form lipoprotein particles called VLDL
  • Nascent VLDL are secreted to the blood and function to deliver endogenously synthesized lipids to the peripheral tissue
49
Q

chylomicrons deliver…

A

exogenous dietary acquired lipids

50
Q

VLDDs deliver…

A

de novo synthesized lipids

Biochem BLOCK 2 (14 decks)

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