T16 - Fatty Acid Synthesis and Oxidation Flashcards
1
Q
What molecule is the precursor for de novo fatty acid synthesis?
A
acetyl-CoA
2
Q
How long are the majority of fatty acids in humans?
A
16-20 carbons long
3
Q
What is the relationship between saturation and melting point?
A
more saturated → higher melting point (i.e. a solid)
4
Q
What is the function of palmitic acid in humans?
A
production of fatty acid synthase
5
Q
What is the function of oleic acid in humans?
A
comprises bulk of fatty acids in TGs
6
Q
What is the function of linoleic acid in humans?
A
essential fatty acid
7
Q
What is the function of linolenic acid in humans?
A
essential fatty acid
8
Q
What is the function of arachidonic acid in humans?
A
precursor of prostaglandins and other eicosanoids
9
Q
In what tissue(s) are fatty acids primarily synthesized in humans?
A
bulk of synthesis in liver, with significant amount in adipose tissue
10
Q
Where in the cell does fatty acid synthesis occur?
A
begins in the cytoplasm and generates up to 16 carbons in length (no double bonds) in cytoplasm → further elongation + desaturation occurs in ER
11
Q
Where does fatty acid shortening take place? (2)
A
peroxisomes and mitochondria
12
Q
What are the three instrumental characteristics of the de novo synthesis pathway of fatty acids?
A
- no free intermediates between acetyl-CoA and end product (most commonly palmitic acid), with the exception of malonyl-CoA;
- free -SH groups required
- HCO3- required, but carbon from HCO3- does not appear in end product
13
Q
Write out the overall reaction for the synthesis of palmitic acid (16:0).
A
8 acetyl-CoA + 14 NADPH + 14 H+ + 7 ATP → palmitic acid + 8 CoASH + 7 Pi + 7 ADP + 6 H2O
14
Q
Why is the bicarbonate ion required in fatty acid synthesis?
A
required for conversion of acetyl-CoA to malonyl CoA by action of biotin-requiring acetyl-CoA carboxylase
15
Q
What are the two cytosolic enzymes required for palmitic acid synthesis from acetyl-CoA?
A
acetyl-CoA carboxylase (ACC)
fatty acid synthase (FAS)
16
Q
Considering the overall chemical equation for the synthesis of palmitic acid, describe what specific processes the two cytosolic enzymes accomplish.
A
acetyl-CoA carboxylase:
7-acetylCoA → 7 malonyl-CoA, coupled to 7 ATP + 7 CO2 → 7 ADP + 7 Pi
fatty acid synthase:
1-acetyl-CoA → palmitate, coupled to 7-malonyl-CoA (see above) + 14 NADPH + 14 H+ → 14 NADP+ + 7 CO2 + 8 CoASH
17
Q
What is the committed step in fatty acid biosynthesis?
A
activation of acetyl-CoA by carboxylation to form malonyl-CoA (mediated by acetyl-CoA carboxylase)
18
Q
Acetyl-CoA carboxylase requires what cofactors to function?
A
biotin
ATP
bicarbonate ion donor
19
Q
Differentiate between mammalian and bacterial acetyl-CoA carboxylase.
A
mammalian = biotin carboxylase, biotin carrier protein, transcarboxylase all associated with single multifunctional polypeptide encoded by one gene
bacterial = all three are encoded by separate genes and are separate proteins
20
Q
What are the three distinct components of acetyl-CoA carboxylase?
A
biotin carboxylase
biotin carrier protein
transcarboxylase
21
Q
Describe the biotin requirement of acetyl-CoA carboxylase.
A
ACC contains biotinyl group covalently attached to epsilon-amino group of lysyl residue
CO2 initially liked to biotin using energy from ATP hydrolysis
then CO2 is transferred to acetyl-CoA
22
Q
(T/F) In humans, the components of the fatty acid synthase (FAS) protein are encoded by separate genes.
A
False. The entire protein is encoded by one gene.
23
Q
Describe how reaction intermediates are associated with fatty acid synthase.
A
all reaction intermediates bound to -SH groups:
(1) either a cysteine resiude on beta-keto acyl-CoA synthase, or:
(2) a 4’-phosphopantetheine moiety
24
Q
Describe the significance of the 4’-phosphopantetheine moeity in fatty acid synthase.
A
represents post-translational modification of fatty acid synthase by transfer of 4’-phoshopantetheine group from CoA
25
What is Acyl Carrier Protein (ACP)?
the region of the fatty acid synthase protein that contains the 4'-phosphopantetheine group
26
Describe the structure/arrangement of the fatty acid synthase enzyme.
two identical subunits in a head-to-tail arrangement
27
What are the six enzymes associated with fatty acid synthase?
beta ketoacyl-ACP transferase
malonyl/acetyl transferase
3-hydroxyacyl-ACP dehydrase
enoyl-CoA reductase
beta ketoacyl-ACP reductase
thioesterase
28
What is the product of fatty acid synthase?
palmitic acid (16:0)
29
Where are modifications (elongation, introduction of double bonds) of palmitic acid carried out?
ER
30
What is the carbon donor for fatty acid elongation?
malonyl-CoA (gives 2 carbons at a time)
31
How many reaction steps are there in fatty acid elongation?
_4 reaction steps, each catalyzed by separate enzymes:_
1. condensing enzyme
2. keto reductase
3. dehydratase
4. enol reductase
32
What is the first reaction of fatty acid elongation?
condensation, mediated by one of 7 mammalian condensation enzymes (ELOVL 1-7 enzymes)
33
What is the regulated step in fatty acid elongation?
the first condensation reaction
34
Differentiate between the condensing enzyme and the other three enzymes (keto reductase, dehydratase, enol reductase) in fatty acid elongation.
There are multiple copies of the condensing enzyme, and the condensing enzyme shows specificty for fatty acid substrates, unlike the other three enzymes
35
What provides reducing power in fatty acid elongation?
NADPH
36
What enzyme introduces double bonds to fatty acids?
desaturases
37
Where in the cell are desaturases located?
ER
38
Describe desaturases. (2)
employ oxygen as terminal electron acceptor
possess short electron transport chain that contains cytochrome b5
39
What provides reducing equivalents for desaturase action?
NADH (or NADPH in some systems)
40
What are the most common desaturases?
stearoyl-CoA grouping of desaturases:
Δ-9 desaturase
Δ-5 desaturase
Δ-6 desaturase
41
Stearoyl-CoA desaturase (Δ9) preferentially acts on what molecule(s)? In other words, what are the preferred substrates for this enzyme?
preferentially desaturates palmitic acid (16:0) to palmitoleic acid (16:1)
preferentially desaturates stearic acid (18:0) to oleic acid (18:1)
42
What are the preferred substrates for stearoyl-CoA desaturase (Δ5, Δ6)?
Δ5 and Δ6 use essential fatty acids as their substates
43
What is the limitation on _where_ a double bond can be introduced in animal fatty acids?
animals lack the capability of introducing a double bond _within 7 carbons_ of the methyl/omega end
44
Draw a diagram that illustrates the role of cytochrome b5 in the desaturation of fatty acids.
45
Where does _shortening_ of very long chain fatty acids occur?
very long chain fatty acids (20-26 carbons) are shortened in peroxisomes
**note:** this is NOT the same as beta-oxidation, and therefore the enzymes used for shortening are not the same as those used for beta-oxidation
46
Naturally occurring fatty acids have what double bond configuration?
cis
47
What is the carbon numbering convention for fatty acids?
carbon #1 is always the carboxyl carbon
48
What are the two families of essential fatty acids?
omega-3 fatty acids
omega-6 fatty acids
49
Omega-3 fatty acids are derived from
linoleic acid (18:3)
50
Omega-6 fatty acids are derived from
linoleic acid (18:2)
51
The elongation and desaturation of linoleic acid produces
linoleic acid → [elongation, desaturation] → **arachidonic acid** → prostaglandins, leukotrienes
52
The elongation and desaturation of linolenic acid produces
linolenic acid → [elongation, desaturation] → **DHA + EPA**
53
What are the predominant fatty acids in fish/fish oils?
DHA and EPA, which are derived from linolenic (omega-3) acid
54
Essential fatty acids are important in the production of prostaglandins. What is the function of prostaglandin? (4)
regulates heart rate/blood pressure/blood clotting
immune function
fertility/conception
neural development in children
55
De novo synthesis of fatty acids requires acetyl-CoA and NADPH. How can acetyl-CoA be produced? (3)
from the oxidation of pyruvate
from the oxidative products of most amino acids
citrate (cytosolic source)
56
Write out the metabolic pathway of linoleic (omega 6) acid. (6)
linoleic acid → [delta6-desaturase] → gamma-linoleic acid → [ELOVL5] → di-homo gamma-linoleic acid → [delta5-desaturase] → **arachidonic acid** → [ELOVL2] → docosatetraenoic acid → [ELOVL2] → tetracosatetraenoic acid
57
Write out the metabolic pathway of linolenic (omega 3) acid.
linolenic acid → [delta6-desaturase] → octadecatetraenoic acid → [ELOVL5] → eicosatetraenoic acid → [delta5-desaturase] → eicosapentaenoic acid → [ELOVL2] → docosapentaenoic acid → [ELOVL2] → tetracosapentaenoic acid
58
De novo synthesis of fatty acids requires acetyl-CoA and NADPH. How can NADPH be produced? (2)
two dehydrogenases of PPP
product of reaction catalyzed by malic enzyme
59
Where does the conversion of pyruvate and AAs into acetyl-CoA take place?
mitochondria
Acetyl-CoAs produced in the mitochondria can't cross the inner mitochondrial membrane
60
What is the cytosolic source of acetyl-CoA? How does this molecule become acetyl-CoA?
citrate that came from the mitochondria
cleaved by ATP-citrate lyase in the cytosol to form acetyl-CoA
61
How is short term/acute regulation of fatty acid biosynthesis achieved? (2)
by controlling activity of acetyl-CoA carboxylase via (1) effects of citrate and (2) control by phosphorylation
62
Describe how citrate regulates acetyl-CoA carboxylase.
citrate causes polymerization of acetyl-CoA carboxylase into large aggregates with _high_ catalytic activity
63
Describe how phosphorylation regulates acetyl-CoA carboxylase.
glucagon + epinephrine → increase cAMP → cAMP-dependent kinase activates AMP dependent kinase → phosphorylation of ACC → _decrease_ in ACC activity
64
Describe how long-term regulation of fatty acid synthesis is accomplished. (3)
(1) fasting state signaling
(2) insulin signaling
(3) glucose signaling
65
In the fed state, do mammals prefer to burn off carbohydrates or fatty acids?
in fed state, mammals prefer to burn off carbohydrates and _store the fatty acids_ as triglycerides
66
Describe how long-term regulation of fatty acid synthesis is accomplished via fasting-state signaling.
synthesis inhibited during fasting state (pre-synthesized/ingested fatty acids burned off for energy)
67
Describe how long-term regulation of fatty acid synthesis is accomplished via insulin signaling.
insulin signaling → increase in SREPB-1c expression in liver → activation of genes for fatty acid synthesis → activation of ACC2 in mitochondria that reduces fatty acid oxidation via increased malonyl CoA production and inhibited CPT1 protein
68
Describe how long-term regulation of fatty acid synthesis is accomplished via glucose signaling.
glucose → activates ChREBP → ChREBP activates liver pyruvate kinase (LPK) → LPK catalyzes conversion of PEP to pyruvate → citrate production from Krebs cycle → fatty acid synthesis source
69
Lipolysis is mediated by what three enzymes?
adipose triglyceride lipase (ATGL)
hormone-sensitive lipase (HSL)
monoacylglycerol lipase (MAL)
70
Describe the function/activity of adipose triglyceride lipase.
favors TAG substrates
catalyzes rate-limiting first step of lipolysis
71
Describe the function/activity of hormone-sensitive lipase.
believed to function as diacylglycerol lipase in vivo
72
Describe the pathway/cascade that leads to activation of hormone-sensitive lipase diacylglycerol lipase activity.
glucagon/epinephrine/beta-corticotropin bind to cell surface receptor → activate adenylate cyclase → increase in cAMP → activate PKA → phoshporylates and _activates_ hormone-sensitive lipase
73
What hormone inhibits mobilization of fat?
insulin — makes sense because insulin is released in the fed state
74
In the context of lipolysis, what enzymes does insulin activate?
phosphodiesterase (cAMP → 5' AMP)
hexokinase (glucose → G6P)
LPL (breakdown of dietary lipids for storage)
75
What is the major triggering event/signal for lipolysis to begin?
when insulin levels fall in the presence of basal levels of lipolytic hormones
76
What is the metabolic complication most directly related to alterations in fatty acid synthesis/oxidation?
non-alcoholic fatty liver disease (NAFLD)
77
Fatty acids in the liver are derived from what four sources?
(1) hydrolysis of adipose tissue TGs
(2) hydrolysis of dietary TGs
(3) uptake of CMrs
(4) synthesis form acetyl-CoA
78
What are the two primary metabolic alterations associated with non-alcoholic fatty liver disease?
development of insulin resistance
→→ which results in hyperinsulinemia and hyperglycemia
79
In non-alcoholic fatty liver disease, what causes excess fat to accumulate in the liver? (4)
rate of hepatic uptake of FFAs is unregulated and directly proportional to plasma FFA concentration
plasma FFA concentration increases because of increased adipocyte mass and increased hydrolysis of triglycerides (via hormone-sensitive lipase)
in insulin-resistant states, insulin-mediated activation of hepatic SREBP-1c is always on, contiuously stimulating fatty acid biosynthesis
once hyperglycemia develops, elevated glucose activates ChREBP and continuously stimulates fatty acid biosynthesis
80
What is the first step in fatty acid oxidation?
activation
81
Which enzyme mediates activation of fatty acids for oxidation?
fatty acyl-CoA synthetases
82
The biosynthesis of CoA requires (3)
4 mol ATP
pantothenic acid
cysteine
83
Pantothenic acid is also known as
vitamin B5
84
What are the five distinct functional groups of CoA?
3'-phosphoadenosine
diphosphate, organophosphate anhydride
pantoic acid
beta-alanine
beta-cystamine
85
(T/F) Fatty acids or fatty acyl CoA derivatives can freely diffuse across the inner mitochondrial membrane.
**False.** The inner mitochondrial membrane is impermeable to fatty acyl-CoA derivatives.
86
How is transport of fatty acids across the inner mitochondrial membrane accomplished?
via transesterification to **carnitine:**
catalyzed by sequential action of carnitine palmitoyltransferase I (CPTI, outer mitochondrial membrane) and carnitine palmitoyltransferase II (CPTII, inner mitochondrial membrane)
87
The carnitine palmitoyltransferase system is *not* required for which grouping of fatty acids?
short and medium-chain fatty acids — they can directly cross into the inner mitochondrial membrane and undergo beta oxidation
88
Each round of beta oxidation produces
1 mol NADH
1 mol FADH2
1 mol acetyl-CoA (enters TCA → oxidized to CO2)
89
How is the acetyl-CoA produced by each round of beta oxidation further processed?
enters TCA and oxidized to CO2, concurrently producing 3 mol NADH, 1 mol FADH2, and 1 ATP
90
List the sequence of intermediates and enzymes in beta oxidation.
acyl-CoA → [acyl-CoA dehydrogenase (variants include long chain, medium chain, and short chain)] → enoyl CoA → [enoyl-CoA hydratase] → L-hydroxyl CoA → [beta-hydroxyacyl-CoA dehydrogenase] → ketoacyl CoA → [beta-ketothiolase] → shortened (by 2) acyl-CoA + acetyl-CoA
91
What is the formula to calculate the number of ATP molecules produced by oxidation of an n-carbon fatty acid?
assuming even number of carbons:
17\*(n/2) - 2
92
The oxidation of palmitate yields how many ATPs?
129
93
What is the efficiency of energy conservation in fatty acid oxidation under standard conditions?
40% efficiency
94
What is the peroxisome analog in plants?
glyoxysomes
95
Describe the steps of peroxisomal fatty acid oxidation.
initial dehydrogenation carried out by cyanide-insensitive oxidase system → generates H2O2 → H2O2 eliminated by catalase → then remaining steps are same as those in mitochondrial fatty acid oxidation
96
What are three important differences between mitochondrial and peroxisomal fatty acid oxidation?
(1) mitochondrial = acyl-CoA dehydrogenase; peroxisomal = cyanide-insensitive, H2O2-producing oxidase system
(2) peroxisome has specificity for long chain fatty acids
(3) peroxisome may shorten long chains to 8 carbons and then send to mitochondria to finish oxidation
97
What effect does malonyl-CoA have on fatty acid oxidation, and how does it create this effect?
malonyl-CoA **reduces** fatty acid oxidation by inhibiting CPTI, the protein that shuttles fatty acids into mitochondria
98
What effect does insulin have on fatty acid oxidation, and how does it create this effect?
insulin stimulates fatty acid synthesis → production of malonyl-CoA → inhibition of CPTI → reduced fatty acid oxidation
99
What effect does glucose have on fatty acid oxidation, and how does it create this effect?
[same effect as insulin]:
insulin stimulates fatty acid synthesis → production of malonyl-CoA → inhibition of CPTI → reduced fatty acid oxidation
100
What are the three most commonly encountered defects in fatty acid oxidation?
carnitine deficiency
CPT deficiency
acyl-CoA dehydrogenase deficiency
101
CPTII deficiency is characterized by (2)
muscle weakness following strenuous exercise
myoglobinuria (result of muscle breakdown → myoglobin excretion in kidneys)
102
Describe the characteristics of CPTI deficiency.
early manifestation
can cause life-threatening non-ketotic hypoglycemia
103
How are acyl-CoA dehydrogenase (long, medium, short) deficiences inherited?
autosomal recessive inheritance
104
Which of the acyl-CoA dehydrogenase deficiencies is best characterized?
medium (MCAD) deficiency — thought to be most common of all inborn errors of metabolism
105
When does MCAD usually manifest?
within 2 years of life, after a 12 hour fasting period
106
What are the symptoms of MCAD deficiency?
vomiting
lethargy
hypoketotic hypoglycemia
dicarboxylic aciduria
107
What provides a diagnostic clue for MCAD deficiency?
excessiveurinary excretion of glycine esters and carnitie
108
What is recommended of MCAD-deficient patients do avoid complications?
avoid prolonged periods of starvation
109
Why does MCAD deficiency lead to hypoglycemia?
block in hepatic fatty acid oxidation leads to slowdown of gluconeogenesis
impaired fatty acid oxidatio in muscle
110
How do cells deal with accumulated medium chain acyl-CoA chains?
alternative metabolic pathways such as omega-oxidation or transesterification to glycine or carnitine
111
What three molecules comprise the grouping known as "ketone bodies?"
acetoacetate
beta-hydroxybutyrate
acetone
112
Under what conditions does acetyl-CoA get shunted into ketone body synthesis? (3)
normally, acetyl-CoA has to be bound to oxaloacetate to enter the Krebs cycle
in fasting conditions, oxaloacetate is converted to glucose (gluconeogenesis) for use by brain
no oxaloacetate left to bind to acetyl-CoA, so acetyl-CoA gets shunted to ketone body synthesis
113
The brain accounts for what percentage of glucose utilization in the body?
60-70%
114
Why can't the brain use fatty acids as an energy source? What becomes the energy source of the brain in starving conditions?
fatty acids can't cross the blood/brain barrier
therefore, in starving/fasting conditions (or in high-fat diet or type I diabetes), brain must use _amino acids_ or _ketone bodies_ for energy [both of which *can* cross the blood/brain barrier]
115
List ot the ketogenesis/ketone body synthesis pathway.
**2x acetyl-CoA** → [acetoacetyl-CoA thiolase a.k.a. beta-ketothiolase] → acetoacetyl-CoA → [HMG-CoA synthase] → HMG-CoA → [HMG-CoA lyase] → **acetoacetate** → [D-3-hydroxybutyrate dehyroganse] → **D-3-hydroxybutyrate**
116
The first step of ketogenesis is what kind of chemical reaction?
a condensation reaction
117
Where is the enzyme HMG-CoA synthase primarily found? What reaction does this enzyme mediate?
primarily found in the liver
mediates acetoacetyl-CoA → HMG-CoA in ketogenesis
118
In ketogenesis, acetoacetate is enzymatically converted to D-3-hydroxybutyrate. What is an alternative fate of acetoacetate?
can undergo spontaneous decarboxylation to acetone
119
Where in the cell does ketogenesis occur?
mitochondria [of liver cells]
120
Of the three ketone bodies, which two are the primary ones exported into circulation?
acetoacetate
D-3-hydroxybutyrate
121
Describe how NADH levels influence which ketone bodies are preferentially synthesized.
if NADH is high → higher proportion of D-3-hydroxybutyrate
122
Acetone is one of the three ketone bodies, but is not released into circulation. How is it excreted?
acetone is a volatile product that is excreted via lungs
123
Which step of ketogenesis is dependent on NADH?
conversion of acetoacetate to beta-hydroxybutyrate via D-3-hydroxybutyrate dehydrogenase
124
Explain why low carbohydrate levels are correlated with low oxaloacetate levels.
when carbohydrate levels are low, oxaloacetate is converted to glucose by the liver via gluconeogenesis
125
Describe the relative utilization of ketone bodies by the heart, muscle, and brain.
in early stages of starvation, _heart_ and _muscle_ will use ketone bodies to preserve glucose for brain usage
126
How are ketone bodies uptaken and utilized by extrahepatic tissue? Write out the pathway.
**D-3-hydroxybutyrate** → [D-3-hydroxybutyrate dehydrogenase] → acetoacetate → [beta-ketoacyl-CoA-transferase] → acetoacetyl-CoA → [acetoacetyl-CoA thiolase a.k.a. beta-ketothiolase] → **2x acetyl-CoA**
(essentially, you're reversing the reactions of ketogenesis)
127
What is the significance of the enzyme beta-ketoacyl-CoA-transferase? (2)
enzyme used to convert acetoacetate to acetoacetyl-CoA so that ketone bodies can be used by extrahepatic tissue
this enzyme is _not_ present in the liver — only in extrahepatic tisse
128
What happens in diabetic ketoacidosis in Type I DM patients? Draw out the pathway of events.
129
What is the effect of ketone body accumulation on blood pH?
**blood pH drops** because ketone bodies are acidic — body tries to compensate with HCO3- buffer system (leads to **hyperventilation**), but that system is quickly overwhelmed
130
Describe the components of the ketogenic diet.
high in fat and protein, but low in carbohydrates
131
(T/F) Biotin deficiency is a complication that can affect a patient's ability to synthesize fatty acids.
**False.** It is impossible to become deficient in biotin.
132
Fatty acids are *not* oxidized in which two tissues?
brain — fatty acids can't cross blood/brain barrier
erythrocytes — don't have mitochondria
133
(T/F) Peroxisome oxidation does NOT release energy.
**True.**
134
What is the source of carnitine used to transport long chain fatty acids into the mitochondria? (2)
synthesized from lysine
ingested from diet
135
Which of the two carnitine palmitoyltransferase proteins is more regulated?
CPTI is more regulated than CPTII
