T16 - Fatty Acid Synthesis and Oxidation Flashcards
What molecule is the precursor for de novo fatty acid synthesis?
acetyl-CoA
How long are the majority of fatty acids in humans?
16-20 carbons long
What is the relationship between saturation and melting point?
more saturated → higher melting point (i.e. a solid)
What is the function of palmitic acid in humans?
production of fatty acid synthase
What is the function of oleic acid in humans?
comprises bulk of fatty acids in TGs
What is the function of linoleic acid in humans?
essential fatty acid
What is the function of linolenic acid in humans?
essential fatty acid
What is the function of arachidonic acid in humans?
precursor of prostaglandins and other eicosanoids
In what tissue(s) are fatty acids primarily synthesized in humans?
bulk of synthesis in liver, with significant amount in adipose tissue
Where in the cell does fatty acid synthesis occur?
begins in the cytoplasm and generates up to 16 carbons in length (no double bonds) in cytoplasm → further elongation + desaturation occurs in ER
Where does fatty acid shortening take place? (2)
peroxisomes and mitochondria
What are the three instrumental characteristics of the de novo synthesis pathway of fatty acids?
- 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
Write out the overall reaction for the synthesis of palmitic acid (16:0).
8 acetyl-CoA + 14 NADPH + 14 H+ + 7 ATP → palmitic acid + 8 CoASH + 7 Pi + 7 ADP + 6 H2O
Why is the bicarbonate ion required in fatty acid synthesis?
required for conversion of acetyl-CoA to malonyl CoA by action of biotin-requiring acetyl-CoA carboxylase
What are the two cytosolic enzymes required for palmitic acid synthesis from acetyl-CoA?
acetyl-CoA carboxylase (ACC)
fatty acid synthase (FAS)
Considering the overall chemical equation for the synthesis of palmitic acid, describe what specific processes the two cytosolic enzymes accomplish.
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
What is the committed step in fatty acid biosynthesis?
activation of acetyl-CoA by carboxylation to form malonyl-CoA (mediated by acetyl-CoA carboxylase)
Acetyl-CoA carboxylase requires what cofactors to function?
biotin
ATP
bicarbonate ion donor
Differentiate between mammalian and bacterial acetyl-CoA carboxylase.
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
What are the three distinct components of acetyl-CoA carboxylase?
biotin carboxylase
biotin carrier protein
transcarboxylase
Describe the biotin requirement of acetyl-CoA carboxylase.
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
(T/F) In humans, the components of the fatty acid synthase (FAS) protein are encoded by separate genes.
False. The entire protein is encoded by one gene.
Describe how reaction intermediates are associated with fatty acid synthase.
all reaction intermediates bound to -SH groups:
(1) either a cysteine resiude on beta-keto acyl-CoA synthase, or:
(2) a 4’-phosphopantetheine moiety
Describe the significance of the 4’-phosphopantetheine moeity in fatty acid synthase.
represents post-translational modification of fatty acid synthase by transfer of 4’-phoshopantetheine group from CoA
What is Acyl Carrier Protein (ACP)?
the region of the fatty acid synthase protein that contains the 4’-phosphopantetheine group
Describe the structure/arrangement of the fatty acid synthase enzyme.
two identical subunits in a head-to-tail arrangement
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
What is the product of fatty acid synthase?
palmitic acid (16:0)
Where are modifications (elongation, introduction of double bonds) of palmitic acid carried out?
ER
What is the carbon donor for fatty acid elongation?
malonyl-CoA (gives 2 carbons at a time)
How many reaction steps are there in fatty acid elongation?
4 reaction steps, each catalyzed by separate enzymes:
- condensing enzyme
- keto reductase
- dehydratase
- enol reductase
What is the first reaction of fatty acid elongation?
condensation, mediated by one of 7 mammalian condensation enzymes (ELOVL 1-7 enzymes)
What is the regulated step in fatty acid elongation?
the first condensation reaction
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
What provides reducing power in fatty acid elongation?
NADPH
What enzyme introduces double bonds to fatty acids?
desaturases
Where in the cell are desaturases located?
ER
Describe desaturases. (2)
employ oxygen as terminal electron acceptor
possess short electron transport chain that contains cytochrome b5
What provides reducing equivalents for desaturase action?
NADH (or NADPH in some systems)
What are the most common desaturases?
stearoyl-CoA grouping of desaturases:
Δ-9 desaturase
Δ-5 desaturase
Δ-6 desaturase
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)
What are the preferred substrates for stearoyl-CoA desaturase (Δ5, Δ6)?
Δ5 and Δ6 use essential fatty acids as their substates
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
Draw a diagram that illustrates the role of cytochrome b5 in the desaturation of fatty acids.
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
Naturally occurring fatty acids have what double bond configuration?
cis
What is the carbon numbering convention for fatty acids?
carbon #1 is always the carboxyl carbon
What are the two families of essential fatty acids?
omega-3 fatty acids
omega-6 fatty acids
Omega-3 fatty acids are derived from
linoleic acid (18:3)
Omega-6 fatty acids are derived from
linoleic acid (18:2)
The elongation and desaturation of linoleic acid produces
linoleic acid → [elongation, desaturation] → arachidonic acid → prostaglandins, leukotrienes
The elongation and desaturation of linolenic acid produces
linolenic acid → [elongation, desaturation] → DHA + EPA
What are the predominant fatty acids in fish/fish oils?
DHA and EPA, which are derived from linolenic (omega-3) acid
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
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)
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
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
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
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
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
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
Describe how citrate regulates acetyl-CoA carboxylase.
citrate causes polymerization of acetyl-CoA carboxylase into large aggregates with high catalytic activity
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
Describe how long-term regulation of fatty acid synthesis is accomplished. (3)
(1) fasting state signaling
(2) insulin signaling
(3) glucose signaling
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
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)
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
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
Lipolysis is mediated by what three enzymes?
adipose triglyceride lipase (ATGL)
hormone-sensitive lipase (HSL)
monoacylglycerol lipase (MAL)
Describe the function/activity of adipose triglyceride lipase.
favors TAG substrates
catalyzes rate-limiting first step of lipolysis
Describe the function/activity of hormone-sensitive lipase.
believed to function as diacylglycerol lipase in vivo
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
What hormone inhibits mobilization of fat?
insulin — makes sense because insulin is released in the fed state
In the context of lipolysis, what enzymes does insulin activate?
phosphodiesterase (cAMP → 5’ AMP)
hexokinase (glucose → G6P)
LPL (breakdown of dietary lipids for storage)
What is the major triggering event/signal for lipolysis to begin?
when insulin levels fall in the presence of basal levels of lipolytic hormones
What is the metabolic complication most directly related to alterations in fatty acid synthesis/oxidation?
non-alcoholic fatty liver disease (NAFLD)
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
What are the two primary metabolic alterations associated with non-alcoholic fatty liver disease?
development of insulin resistance
→→ which results in hyperinsulinemia and hyperglycemia
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
What is the first step in fatty acid oxidation?
activation
Which enzyme mediates activation of fatty acids for oxidation?
fatty acyl-CoA synthetases
The biosynthesis of CoA requires (3)
4 mol ATP
pantothenic acid
cysteine
Pantothenic acid is also known as
vitamin B5
What are the five distinct functional groups of CoA?
3’-phosphoadenosine
diphosphate, organophosphate anhydride
pantoic acid
beta-alanine
beta-cystamine
(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.
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)
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
Each round of beta oxidation produces
1 mol NADH
1 mol FADH2
1 mol acetyl-CoA (enters TCA → oxidized to CO2)
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
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
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
The oxidation of palmitate yields how many ATPs?
129
What is the efficiency of energy conservation in fatty acid oxidation under standard conditions?
40% efficiency
What is the peroxisome analog in plants?
glyoxysomes
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
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
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
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
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
What are the three most commonly encountered defects in fatty acid oxidation?
carnitine deficiency
CPT deficiency
acyl-CoA dehydrogenase deficiency
CPTII deficiency is characterized by (2)
muscle weakness following strenuous exercise
myoglobinuria (result of muscle breakdown → myoglobin excretion in kidneys)
Describe the characteristics of CPTI deficiency.
early manifestation
can cause life-threatening non-ketotic hypoglycemia
How are acyl-CoA dehydrogenase (long, medium, short) deficiences inherited?
autosomal recessive inheritance
Which of the acyl-CoA dehydrogenase deficiencies is best characterized?
medium (MCAD) deficiency — thought to be most common of all inborn errors of metabolism
When does MCAD usually manifest?
within 2 years of life, after a 12 hour fasting period
What are the symptoms of MCAD deficiency?
vomiting
lethargy
hypoketotic hypoglycemia
dicarboxylic aciduria
What provides a diagnostic clue for MCAD deficiency?
excessiveurinary excretion of glycine esters and carnitie
What is recommended of MCAD-deficient patients do avoid complications?
avoid prolonged periods of starvation
Why does MCAD deficiency lead to hypoglycemia?
block in hepatic fatty acid oxidation leads to slowdown of gluconeogenesis
impaired fatty acid oxidatio in muscle
How do cells deal with accumulated medium chain acyl-CoA chains?
alternative metabolic pathways such as omega-oxidation or transesterification to glycine or carnitine
What three molecules comprise the grouping known as “ketone bodies?”
acetoacetate
beta-hydroxybutyrate
acetone
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
The brain accounts for what percentage of glucose utilization in the body?
60-70%
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]
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
The first step of ketogenesis is what kind of chemical reaction?
a condensation reaction
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
In ketogenesis, acetoacetate is enzymatically converted to D-3-hydroxybutyrate. What is an alternative fate of acetoacetate?
can undergo spontaneous decarboxylation to acetone
Where in the cell does ketogenesis occur?
mitochondria [of liver cells]
Of the three ketone bodies, which two are the primary ones exported into circulation?
acetoacetate
D-3-hydroxybutyrate
Describe how NADH levels influence which ketone bodies are preferentially synthesized.
if NADH is high → higher proportion of D-3-hydroxybutyrate
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
Which step of ketogenesis is dependent on NADH?
conversion of acetoacetate to beta-hydroxybutyrate via D-3-hydroxybutyrate dehydrogenase
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
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
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)
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
What happens in diabetic ketoacidosis in Type I DM patients? Draw out the pathway of events.
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
Describe the components of the ketogenic diet.
high in fat and protein, but low in carbohydrates
(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.
Fatty acids are not oxidized in which two tissues?
brain — fatty acids can’t cross blood/brain barrier
erythrocytes — don’t have mitochondria
(T/F) Peroxisome oxidation does NOT release energy.
True.
What is the source of carnitine used to transport long chain fatty acids into the mitochondria? (2)
synthesized from lysine
ingested from diet
Which of the two carnitine palmitoyltransferase proteins is more regulated?
CPTI is more regulated than CPTII
