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Fatty Acid Synthesis Flashcards

1
Q

When does synthesis of lipids occur?

Why?

A

FED STATE - high I/G after a meal

  • have an abundance of nutrients
  • also requires energy
  • conditions of excess caloric intake
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2
Q

(Most) biosynthetic reactions do not occur during the ________ state

A

Fasted

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3
Q

How are FA components of phospholipids?

A
  • Found in membranes

- Consist of 2 fatty acids + phosphate with polar head group on a glycerol backbone

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4
Q

How are FA components and properties of triglycerides?

A
  • 3 FA attached to glycerol backbone
  • Energy storage
  • Very hydrophobic
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5
Q

Functions of fatty acids (4)

A
  1. Components of phospholipids
  2. Components of triglycerides
  3. Second messengers
  4. Covalent modifications of proteins
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6
Q

How do FA covalently modify proteins?

A
  • Attaching a fatty acid to specific amino acid residue —> palmitate and myristate
  • Bring protein to a membrane and affect the protein’s activity
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7
Q

Major source of carbon for synthesis of lipids is _____________

Can also use?

A

Primarily excess dietary carbohydrates (glucose)

Can also use excess dietary protein

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8
Q

Primary tissue involved in synthesis of lipids

Also occurs in ?

A

Primary = liver

Also occurs in adipose tissue

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9
Q

Lipogenesis involves

A

Synthesis of fatty acids from glucose

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10
Q

Dietary carbohydrates —>

A

Dietary carbohydrates —> glucose —> acetyl CoA —> fatty acids —> triacylglycerols

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11
Q

Dietary proteins —>

A

Dietary proteins —> amino acids —> acetyl CoA —> fatty acids —> triacylglycerols

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12
Q

Insulin promotes conversion of acetyl CoA —>

A

Fatty acids

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13
Q

If take away insulin, what happens to acetyl CoA ?

A

Acetyl CoA builds up and goes to form ketone bodies

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14
Q

Untreated Type I diabetes

A

Overproduction of ketone bodies

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15
Q

Ketone bodies

A
  • Can be used for energy by anything but RBC
  • Occurs during periods of starvation
  • They are oxidized like fatty acids in the mitochondria
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16
Q

Early phase of low carb diet

A

Dec insulin -> any carbons go through acetyl CoA then make ketone bodies which are relied on for energy

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17
Q

5 Requirements for Fatty Acid Synthesis

A
  1. Acetyl CoA as starting substrate
  2. Reducing equivalents
  3. High I/G ratio (fed state)
  4. Energy
  5. Integration of 3 pathways
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18
Q

Acetyl CoA as starting substrate problem

A

It is not permeable to mitochondrial membrane and fatty acid synthesis occurs in the cytoplasm of liver cell

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19
Q

3 molecules that cannot cross mitochondrial membrane directly

A
  1. Acetyl CoA
  2. OAA
  3. NADH / NAD+
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20
Q

Reducing equivalents

A
  • Produce e- and H+

- Uses NADPH

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21
Q

NADPH is from:

A
  1. Pentose phosphate pathway

2. Malic enzyme

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22
Q

Why can NADH not be used as reducing equivalent in fatty acid synthesis?

A

NAD+ is needed to keep glycolysis going so that predominates there in the cytoplasm

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23
Q

Integration of what 3 pathways is required for fatty acid synthesis?

A
  1. Glycolysis
  2. TCA Cycle (citrate)
  3. Pentose Phosphate Pathway
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24
Q

Glucose can be oxidized in 2 ways:

A
  1. Glycolysis- produce pyruvate and NAD+ picks up electrons to give NADH
  2. Pentose phosphate pathway- produce a 5-C sugar and NADP+ picks up electrons to give NADPH
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25
Both ways to oxidize glucose occurs during? Which predominates?
During the fed state. Which pathway predominates depends on the respective Kms on the enzymes in each pathway: Lower Km = higher affinity so gets glucose first
26
Overall Reaction
8 acetyl CoA + 7 ATP (activation step) + 8 ATP (citrate lyase step) + 14 NADPH —> Palmitate + 14 NADP+ + 8 CoA + 6 H2O + 15 ADP + 7 Pi
27
Fatty acids are built from
2-C units (acetyl CoA)
28
Starting point for all fatty acids =
16-C saturated fatty acid Called palmitate
29
____ acetyl CoA total are required
8
30
Activation step
- Consumes 7 ATP total - Needs to occur for every acetyl CoA molecule except the first one - Acetate units are activated for transfer by conversion to malonyl-CoA (3-C)
31
Malonyl- CoA serves as __________
“ Active 2-C donor “
32
What drives chain growth?
1. Decarboxylation of malonyl Co-A | 2. Reducing power of NADPH
33
How many glucose are required for 1 palmitate (16-C)?
4 glucose
34
Problem once pyruvate gets into mitochondria?
1. Need to keep TCA cycle going but OAA is very limiting.. Reaction that produces OAA is not favorable and only reason TCA cycle goes forward is because whatever OAA is made is immediately bound to citrate synthase to make a binding site for acetyl CoA and form citrate 2. Also need to produce enough citrate for it to exit mitochondria
35
How is the glucose conversion to Acetyl Co-A problem solved?
2 enzymes: Pyruvate carboxylase Pyruvate Decarboxylation
36
Pyruvate carboxylase
Converts pyruvate —> 4-C OAA
37
Acetyl CoA _________ modifies pyruvate carboxylase
Positively allosteric
38
Pyruvate dehydrogenase
Converts pyruvate —> Acetyl CoA
39
Acetyl CoA _________ modifies pyruvate dehydrogenase What else regulates PDC?
Negative allosteric Insulin activates via dephosphorylation
40
Support pyruvate dehydrogenase is the lower Km enzyme....
Acetyl CoA will be in slight excess —> if it builds up, PDC slows down —> any pyruvate entering mitochondria is diverted to pyruvate carboxylase and also acetyl CoA positively modifies the pyruvate carboxylase —> make enough OAA to condense with acetyl CoA to form citrate that can exit the mitochondira
41
ATP-Citrate Lyase reaction
Citrate + ATP + CoA-SH + H2O —> Acetyl CoA + ADP + Pi + OAA
42
Acetyl CoA is made in the __________ but __________
Made in the mitochondria but is not permeable to the mitochondrial membrane
43
Acetyl CoA is transferred out of the _______ into the ________ as ________
Out of the mitochondria into the cytoplasm as citrate
44
Enzyme that catalyzes the split of citrate back into acetyl CoA and OAA
ATP- citrate lyase
45
Even though you hydrolyze an ATP for every citrate that exits the mitochondria and used in cytoplasm for FA synthesis there are 2 benefits:
1. Make NAD+ for glycolysis | 2. Make NADPH for FA synthesis (in addition to pentose phosphate pathway)
46
What happens to the acetyl CoA from ATP-citrate lyase reaction?
Goes towards fatty acid synthesis
47
What happens to the OAA from ATP-citrate lyase reaction?
Must return to the mitochondria for TCA cycle to continue BUT it is impermeable to the membrane
48
2 enzymes involved in getting OAA back into the mitochondria (as pyruvate)
1. Cytoplasmic malate dehydrogenase | 2. Malic enzyme
49
Cytoplasmic malate dehydrogenase reaction
OAA + ADP + NADH —> Malate + NAD+ - Favorable - NAD+ can go to glycolysis
50
Malic enzyme reaction
Malate + NADP+ —> pyruvate + NADPH - Pyruvate enters mitochondria - NADPH goes toward FA synthesis
51
Rate-Limiting Step of Fatty Acid Synthesis
- The activation step Acetyl CoA + ATP + HCO3- —> Malonyl CoA + ADP + Pi + H+ Enzyme: Acetyl CoA Carboxylase (ACC)
52
Substrate for fatty acid synthesis
Acetyl CoA (cytoplasm)
53
Acetyl CoA Carboxylase (ACC) Type of reaction?
Catalyzes the carboxylation of acetyl CoA (2-C) to form malonyl CoA (3-C) * Irreversible
54
ACC requires ?
Biotin is a coenzyme
55
2-steps of ACC reaction
1. Biotin accepts HCO3- | 3. Add C from HCO3- to form malonyl CoA “active 2-C donor”
56
How is Acetyl CoA Carboxylase is regulated?
Hormonally AND allosterically
57
Hormonal regulation of ACC
Dephosphorylated and active in FED state Phosphorylated and inactive in FASTED state
58
Mechanism of ACC being dephosphorylated and active in fed state
Insulin signaling activates protein phosphatase 2A which activates ACC
59
Mechanism of ACC being phosphorylated and inactive in fed state
Glucagon signaling activates AMP-dependent protein kinase (AMPK) which inactivates ACC by directly phosphorylating it
60
AMPK activated by? Inhibited by?
Activated by AMP Inhibited by ATP So during times of low energy charge (glucagon predominates during fasted state), AMPK is activated and goes to inactivate ACC by phosphorylating it
61
ACC is normally a _____ —> ends up _________ to become active
Normally a dimer —> polymerizes to become active Active as a polymer.
62
Positive allosteric modifier of Acetyl CoA Carboxylase How?
Citrate Activates ACC by facilitating the polymerization of the inactive dimer.
63
Negative allosteric modifier of Acetyl CoA Carboxylase How?
Palmitoyl CoA Palmitoyl CoA is generated at end of synthesis of FA —> when it builds up it causes depolymerization
64
Citrate’s effect on phosphorylated ACC Importance:
Can take phosphorylated ACC —> add citrate to it —> restore part of ACC’s activity - Advantage for efficiency/conservation in terms of using carbon skeletons because hormonal control is still slower than changes in metabolite flux - Metabolite flux changes rapidly - concentrations matter
65
What enzyme extends the fatty acid chain?
Fatty acid synthase
66
The intermediates are ______ as a ______ to either ____ or _____ during the entire biosynthesis
Covalently bonded as a thioester bond to either ACP or condensing enzyme
67
3 reaction types in fatty acid extending chain
1. Condensation 2. Reduction 3. Dehydration
68
Condensation
Put 2 molecules together
69
Reduction
Requires NADPH
70
Dehydration
Get rid of all double bonds to make saturated fatty acid
71
4 important structural domains of fatty acid synthase
1. Condensing domain 2. Modification domain 3. Thioesterase (TE) 4. Acyl carrier protein
72
Modification domain
Where reduction and dehydration reactions occur
73
Thioesterase
Split fatty acid chain and release it from complex
74
Acyl carrier protein (ACP)
Has an SH group to form thioester bond to link intermediates to ACP with growing chain throughout entire biosynthesis
75
2 important steps to occur before elongation
1. Priming reaction | 2. Condensation
76
Priming reaction
Acetyl CoA + ACP —> Acetyl-ACP + CoA Malonyl CoA + ACP —> Malonyl-ACP + CoA
77
Explain what happens during priming reaction. Enzyme?
First load acetyl CoA and then load malonyl CoA onto the acyl carrier protein. Enzyme: Malonylacetyl transferase (MAT)
78
Important prosthetic group on ACP
- from vitamin phosphopantetheine - identical to portion of CoA - SH group is point of attachment
79
ACP does what?
Shuttles intermediates (similar to what lipoamide arm does)
80
Condensation reaction
Acetyl-ACP + Malonyl-ACP —> acetoacetyl-ACP + ACP+ CO2
81
Process of condensation reaction
Acetyl CoA binds to carrier protein —> moves over to condensing enzyme freeing up its site on the carrier protein —> ACP binds malonyl CoA —> condensing enzyme catalyzes a decarboxylation —> CO2 that was added by acetyl CoA carboxylase comes off —> 2 carbons that originated from acetyl CoA will end up where the CO2 was lost from (at the bottom-defining the omega end of the fatty acid)
82
Decarboxylation reaction purpose
It is favorable — makes the condensation favorable by providing the energy for it
83
Synthesize fatty acid from _____ end to ______ end
Omega end to Carboxy terminus end
84
Growing end of the fatty acid is on the
Acyl carrier protein
85
Elongation involves a step wise reduction of ________ to __________
Keto group at C-3 to a methylene group (CH2)
86
Acetyl CoA goes in as acetyl CoA when?
ONLY THE FIRST ONE that makes up the omega end
87
The liver being the primary site of FAS also explains why
Glycolysis increases in the fed state in the liver
88
ATP in the fed state
Make a lot of it but it is rapidly consumed. As always, cell tries to maintain ATP levels so energy charge is around 0.85
89
Biosynthetic reactions occur when
You have plenty of carbon sources coming in from diet so that you can use some of them for energy and put the others into storage (which also requires energy to do).
90
Order of reaction type in FAS
Condensation —> reduction —> dehydration —> reduction
91
Products formed (in order) during fatty acid synthesis
Acetyl-ACP —> malonyl ACP —> acetoacetyl ACP —> 3-hydroxbutyryl ACP —> crotonyl ACP —> butyryl ACP —> butyryl ACP then condenses with malonyl ACP to begin another round
92
Introduction of double bonds occurs in
The endoplasmic reticulum
93
Mammals cannot add double bond beyond?
C-9
94
2 essential fatty acids
Linoleate (omega-6) Linolenate (omega-3)
95
Linoleate (omega-6) structure and use?
18:2 ; double bond at carbon 9 and 12 Used to synthesize arachidonic acid (20:4)
96
Arachidonic acid use
Precursor for prostaglandins, leukotrienes, thromboxanes
97
Aspirin does what?
Inhibits synthesis of prostaglandins by occupying site on prostaglandin synthase which prevents arachidonic acid from getting access to active site
98
Lineolenate (omega-3) structure
18:3 | Double bonds at carbon 9,12,15
99
Pentose Phosphate Pathway has?
2 branches- oxidative and nonoxidative
100
Oxidative branch of pentose phosphate pathway chemistry Reaction?
Going from 6 carbons to 5 carbons - Oxidation / reduction - Occurs in 3 steps Glucose-6-Phosphate + 2 NADP+ —> ribulose-5-phosphate + 2 NADPH + CO2
101
Step 1 of oxidative branch of pentose phosphate pathway Enzyme?
OXIDATION Glucose-6-phosphate + NADP+ —> 6-phosphoglucono-delta-lactone + NADPH - OH group becomes C=O - NADP+ picks up electrons to form NADPH - Rate limiting step! Enzyme: glucose-6-phosphate dehydrogenase
102
Step 2 of oxidative branch of pentose phosphate pathway
6-phosphoglucono-delta-lactone + H2O —> 6-phosphogluconate + H+ - Goes from closed structure to open structure - C=O —> CO2 with resonance
103
Step 3 of oxidative branch of pentose phosphate pathway Enzyme:
DECARBOXYLATION 6-phosphogluconate + NADP+ —> Ribulose-5-phosphate +NADPH + CO2 Enzyme: 6-phosphogluconate dehydrogenase
104
Non oxidative branch of pentose phosphate pathway primary function
Produce 5-C sugars (ribulose-5-phosphate) for nucleotide biosynthesis - where ribose and deoxyribose come from
105
Which branch of pentose phosphate pathway is favored?
Depends on needs of cell

Biochem (15 decks)

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