Fatty Acid, Ketone Body, and Triacylglycerol Metabolism (Exam II)

Question 1

Why might individuals with nephrotic syndrome have puffiness around the eyes? Why might individuals with protein malnutrition have swollen legs?

Answer 1

Loss/decrease of serum albumin results in decreased oncotic pressure, and the flow of fluid out of vessels into
tissues causes swelling/edema.

Question 2

What should happen to the binding of Ca+2 to serum albumin under acidic conditions? Why?

Answer 2

It should decrease. Ionizable amino acid side chains now carry a positive charge.

Question 3

Is the unbound Ca+2 value normal in a patient with normal serum albumin and low serum total calcium?

Answer 3

No, the unbound Ca+2 is decreased and the patient is truly hypocalcemic.

Question 4

What is the trivial name, carboxyl-reference and omega (ω)-reference of: hexadecanoic acid

Answer 4

Palmitic acid.

16: 0
16: 0

Question 5

What is the trivial name, carboxyl-reference and omega (ω)-reference of: 9-hexadecenoic acid

Answer 5

palmitoleic acid

16: 1∆9
16: 1 (ω-7) or (n-7)

Question 6

What is the trivial name, carboxyl-reference and omega (ω)-reference of: octadecanoic acid

Answer 6

stearic acid

18: 0
18: 0

Question 7

What is the trivial name, carboxyl-reference and omega (ω)-reference of: 9-octadecenoic acid

Answer 7

oleic acid

18: 1∆9
18: 1 (ω-9) or (n-9)

Question 8

What is the trivial name, carboxyl-reference and omega (ω)-reference of: 9,12-octadecadienoic acid

Answer 8

linoleic acid

18: 2∆9,12
18: 2 (ω-6) or (n-6)

Question 9

What is the trivial name, carboxyl-reference and omega (ω)-reference of: 9,12,15-octadecatrienoic acid

Answer 9

α-linolenic acid

18: 3∆9,12,15
18: 3 (ω-3) or (n-3)

Question 10

What is the trivial name, carboxyl-reference and omega (ω)-reference of: 5,8,11,14-eicosatetraenoic acid

Answer 10

arachidonic acid

20: 4∆5,8,11,14
20: 4 (ω-6) or (n-6)

Question 11

List the essential features of a typical fatty acid

Answer 11

carboxylic acid group (COO-)

alkyl chain, typically linear/unbranched, even #C, long
if unsaturated: 1+ C=C in cis at 3 carbon interval

methyl terminus

Question 12

Is it possible for humans to synthesize linolenic acid starting from non-fatty acid precursors? Why?

Answer 12

Linoleic acid and α-linolenic acid contain unsaturation sites beyond carbons 9 and 10 (relative to the α-COOH group).

These two fatty acids cannot be synthesized from precursors in the body, and are thus considered the essential fatty acids; essential in the sense that they must be provided in the diet.

Question 13

Fatty acid synthesis: What 2C mitochondrial intermediate is the more immediate source of carbons?

Answer 13

malonyl CoA (via acetyl CoA) is source of the 2C units which are added carboxylate end of an acyl acceptor through a repetitive four-step sequence: condensation (decarboxylation), reduction, dehydration, reduction

Question 14

What is the subcellular location, and what two enzymes in this location are required for fatty acid synthesis?

Answer 14

Cytosolic process; especially important in liver, CNS, lactating mammary gland, adipose

Enzymes: Acetyl CoA carboxylase (ACC) & Fatty acid synthase (FAS)

Question 15

In what chemical form is carbon exported from mitochondria to the site of fatty acid synthesis?

Answer 15

Acetyl CoA is generated in the mitochondrial matrix but is needed in the cytosol, and CoA cannot cross the inner mitochondrial membrane. The solution is to transport acetate out as citrate.

Question 16

What reductant is used for fatty acid synthesis? What two processes ensure adequate supplies of the reductant?

Answer 16

NADPH.

1. Malic enzyme produces some of the NADPH: Malate + NADP+ → Pyruvate + CO2 + NADPH + H+
2. Pentose phosphate pathway

Question 17

What enzyme controls the rate of fatty acid synthesis, and how is it regulated? What coenzyme does it require?

Answer 17

Acetyl CoA carboxylase (ACC). It converts acetyl CoA, carbon dioxide & ATP to malonyl CoA (3C)

Regulation:
allosteric effectors: ⊕ citrate Ө end product

covalent regulation: ⊕ dephosphorylation Ө phosphorylation (phosphorylated by AMP activated protein kinase (AMPK). PKA also plays a role.)

under long-term control: change in rate of synthesis: ↑ CHO ↑ expression via trans-acting ChREBP at cis-acting ChoRE. ↑ insulin also ↑ expression via a different TF (SREBP)

Coenzyme: biotin shuttles CO2 to acceptors (acetyl CoA here)

Question 18

List the steps of the repetitive process carried out by fatty acid synthase.

Answer 18

1. Condensation/Decarboxylation
   β-ketoacyl-ACP synthase (KS) reacts priming acetyl-ACP with chain-extending malonyl-ACP
2. Reduction
   β-ketoacyl-ACP reductase (KR) reduces the carbon 3 ketone to a hydroxyl group
3. Dehydration
   β-Hydroxyacyl ACP dehydratase (DH) removes water
4. Reduction
   Enoyl-ACP reductase (ER) reduces the C2-C3 double bond.

After 7 cycles, palmitate (16 carbon) cleaved from ACP domain by thioesterase.

Question 19

What is the major product of mammalian fatty acid synthase?

Answer 19

Palmitic acid (16:0) is primary product

Question 20

What two general types of fatty acid modification are possible in humans? What limitations to desaturation exist in humans?

Answer 20

Elongation: ER (and mitochondria):
Mechanistically like de novo synthesis. CoA esters used: no ACP; substrates of > 10C; product is typically 18C (longer in brain)

Desaturation: ER:
Fatty acid and NADH get oxidized as O2 is reduced to 2 H2O. We can’t insert C=C between C10 and ω-C

Question 21

Fatty acids are activated for metabolism (oxidation, complex lipid synthesis, etc.) in what subcellular locations? This activation involves esterification to what substance? How many high energy bonds of ATP are invested in the activation of each fatty acid molecule?

Answer 21

Fatty acids are stored as triglycerides/triacylglycerols (TAG) primarily in adipocytes of white adipose tissue (↑ I/G). TAG are stored within a single, large (>90% of cell volume) droplet coated with a monolayer containing PL, cholesterol, and proteins such as perilipin.

TAG is composed of 3 fatty acids esterified to glycerol.

For each molecule of fatty acid activated, one molecule of coenzyme A and one molecule of adenosine triphosphate (ATP) are used, equaling a net utilization of the two high-energy bonds in one ATP molecule

Question 22

How is the mobilization of stored fat (TAG) from adipose tissue mediated and regulated? What is the role of perilipin and HSL?

Answer 22

TAG stored in adipose are degraded to fatty acids (3) plus glycerol by adipose lipases.

Hormone sensitive lipase is phosphorylated by PKA (in response to catecholamines primarily) and active;

Phosphorylation of perilipin by PKA allows P-HSL to bind to P-perilipin on the lipid droplet.

Phosphorylated perilipin changes conformation, exposing the stored lipids to hormone-sensitive lipase-mediated lipolysis

Catecholamines (+) while insulin (-).

Question 23

The transport of LCFA into the mitochondrial matrix for oxidation involves esterification to what compound? Does this transport require additional expenditure of metabolic energy? Do MCFA or SCFA require this compound for entry?

Answer 23

Activation involves formation of CoA ester; required for participation of fatty acid in metabolism. CoA derivatives can’t cross inner mitochondrial membrane. Transfer fatty acid to carnitine. Requires CPT-I and II (carnitine palmitoyl transferases). Requires acylcarnitine/carnitine translocase.

1. Fatty Acid → Fatty Acyl CoA (acyl CoA synthetase)
   * Diffuse through outer mitochondrial membrane*
2. Fatty Acyl CoA + Carnitine → CoA + Fatty Acid-Carnitine (CPT I)
   * Diffuse through inner mitochondrial membrane using acylcarnitine/carnitine translocate*
3. Fatty Acid-Carnitine + CoA → Fatty Acid-CoA + Carnitine

The activation requires one mole of ATP.

MCFA and SCFA get activated inside the matrix so carnitine is not required for their oxidation.

Question 24

For mitochondrial β-oxidation of fatty acids: List the steps of the repetitive process.

Answer 24

First dehydrogenation (chain-length specific acyl CoA dehydrogenases; FAD linked); form enoyl CoA with trans double bond between C2 and C3 (α and β carbons).

Hydration (enoyl CoA hydratases); add water to the double bond; form β-hydroxyacyl CoA. Requires a trans ∆2 substrate.

Second dehydrogenation (β-hydroxyacyl CoA dehydrogenases; NAD linked); oxidize the hydroxyl group at β-C (C3) to a keto group; form β-ketoacyl CoA.

Thiolytic cleavage (β-ketoacyl CoA thiolases); cleavage occurs between the α and the β-carbon; the 2C’s at carboxy-terminus are split off as acetyl CoA.

Question 25

For mitochondrial β-oxidation of fatty acids: What reducing equivalents are produced?

Answer 25

FADH2 and NADH. Oxidized by ETC at CoQ and Complex I, respectively, for ATP synthesis; NADH and ATP used in gluconeogenesis

Question 26

For mitochondrial β-oxidation of fatty acids: What 2C product is made? What are its fates?

Answer 26

acetyl CoA (from 2C at carboxylate end)

oxidized in TCA cycle: CO2, FADH2, NADH, GTP are produced OR used for ketogenesis in liver

Question 27

What is the pathway by which the 3C final product of the β-oxidation of odd-numbered fatty acids is metabolized? What coenzymes are required?

Answer 27

Carboxylate to 4C, convert to succinate.

1. Propionyl CoA carboxylate, ATP, biotin & bicarbonate create 4-carbon molecule (L-methylmalonyl CoA)
2. Methylmalonyl CoA mutate, B12 and 5’-deoxyadenosylcobalamin convert to succinyl CoA.

Question 28

Regulation of mitochondrial β-oxidation of fatty acids

Answer 28

substrate availability: as ↑, β-oxidation ↑

malonyl CoA availability: as ↓, β-oxidation ↑ (since ↑malonyl CoA Ө CAT-1)

acetyl CoA/CoA ratio: as ↑, β-oxidation ↓ due to ↓thiolase activity.

Question 29

Why are additional enzymes are required for the oxidation of PUFA?

Answer 29

Problem: intermediate formed is not a substrate for enoyl hydratase, which requires trans ∆2.

Solution: use additional enzyme(s) to convert it to an acceptable substrate. Double bond at odd-numbered C requires 3,2-enoyl CoA isomerase; at even, 2,4-dienoyl CoA reductase

Energy yield from unsaturated fa is less than from saturated

Question 30

What is the ATP yield from the oxidation of 18:0?

Answer 30

~146 ATP (8 ATP/carbon).

ATP Yield from oxidation of stearic acid:
(18/2 - 1) = 8 NADH = 24 ATP
8 FADH2 = 16 ATP

9 Acetyl CoA:

27 NADH = 81 ATP
9 FADH2 = 18 ATP
9 GTP = 9 ATP

Total: 148
Minus 2 ATP: 146 ATP

Question 31

What role do peroxisomes play in β-oxidation? What is special about the first oxidation reaction there?

Answer 31

An additional organelle, the peroxisome, is required for the β-oxidation of VLCFA

Only peroxisomes have fatty acyl CoA synthetases for fa > 24C.

Differences
Transport into peroxisome doesn’t require carnitine and uses an ABC-transporter instead.

First oxidative step transfers electrons directly to O2, generating H2O2, so oxidase and not dehydrogenase.

Process stops at MCFA (linked to carnitine, not CoA), that go to mitochondria; a chain-shortening process.

Question 32

What is the function of peroxisomal α-oxidation?

Answer 32

occurs at α-C (C-2), not β-C (C-3) and shortens by 1C.

Important in the oxidation of branched-chain phytanic acid, a 20C product of chlorophyll degradation in ruminant bacteria; found in milk and animal fat

Methyl group on C3 of phytanic acid prevents β-oxidation

Question 33

Where are ketone bodies made? What is the building block? What mitochondrial enzyme is key? What are the products? Which products are metabolically useful? Are these products actually ketones? What is the consequence of their overproduction? Why can’t liver use ketone bodies?

Answer 33

Ketone bodies: 4C, H2O-soluble molecules made primarily by liver from the acetyl CoA produced by β-oxidation; used by peripheral tissues to generate acetyl CoA and, ultimately, ATP

Synthesis of ketone bodies (in mitochondria of hepatocytes): ketogenesis. Driven by [acetyl CoA].

Presence of HMG-CoA lyase in liver mitochondria allows ketogenesis. Release of CoA supports continued β-oxidation. Use of acetyl CoA keeps acetyl
CoA/CoA ratio low in liver so thiolase remains active.

The “KB” acetoacetate and β-hydroxybutyrate are organic acids

Succinyl CoA transferase is essentially absent from liver, so liver can make but cannot use KB (stops at acetoacetate).

Question 34

Where and how are ketone bodies degraded? What is the immediate product?

Answer 34

High availability of NADH from β-oxidation pushes acetoacetate to β-hydroxybutyrate. Acetoacetate and Succinyl CoA transferase produce acetoacetyl CoA which undergoes a thiolase to become 2 Acetyl CoA.

Question 35

X-ALD

Answer 35

X-linked adrenoleukodystrophy (X-ALD, 1:17,000 males): defect in the ABC (ABCD1) transporter that allows entry of VLCFA into peroxisome; childhood and adult forms.

In childhood cerebral form, boy develops normally until school age then shows dementia, progressive neurologic loss to a vegetative state, and adrenal insufficiency. Typically fatal by age 10 years.

Result in accumulation of VLCFA, especially in brain (but also in adrenals with X-ALD).

Question 36

Refsum disease

Answer 36

Refsum disease (phytanic acid storage disease) is due to a deficiency of the α-hydroxylase

AR: see progressive peripheral neuropathy, retinitis pigmentosa, nerve deafness.

Signs first seen in teens; full progression can take thirty years.

Treatment: dietary restriction of phytanic acid.

Question 37

Zellweger syndrome

Answer 37

Inability to import peroxisomal matrix proteins due to defects in the receptors (PEX proteins) that recognize proteins with peroxisomal targeting sequences.

Zellweger syndrome (cerebrohepatorenal syndrome):

1) AR, rare
2) VLCFA accumulate; phytanic acid accumulates
3) severe neurologic defects, progressive hepatic and renal dysfunction, skeletal dysmorphias, facial abnormalities similar to Down syndrome; patients rarely survive first year of life.

Question 38

Ketoacidosis such as DKA

Answer 38

ketolysis defects → ketoacidosis

Ketoacidosis also seen in untreated type 1 diabetes mellitus: diabetic ketoacidosis or DKA. With DKA the problem is increased production of KB.

Question 39

MCAD deficiency

Answer 39

Dependence of ketogenesis on mitochondrial β-oxidation: MCAD deficiency

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common defect
in mitochondrial fa β-oxidation

Typically the patient is less than two years of age with a viral infection that results in decreased appetite. Initially glycogen stores meet the baby’s energy needs. Fats are mobilized but their use is decreased. Ketone body levels remain low. Lethargy and vomiting develop. Glucose levels fall. Lethargy progresses to coma. ~ 20% die during first attack.

MCAD is a disorder of fasting adaptation. β-oxidation of fa

Question 40

Hypoketotic hypoglycemia in disorders of mitochondrial β-oxidation, including carnitine deficiency

Answer 40

ketogenesis defects → hypoketotic hypoglycemia (mitochondrial HMG CoA synthase and lyase)

Carnitine deficiency
i) decreased synthesis (ex. premature babies)
ii) defect in membrane transporter in muscle, heart,
kidney; uptake decreased so excretion increased
iii) severity varies
iv) treatment involves avoidance of fasting, a diet low in
LCFA, and dietary supplementation with MCTG and
carnitine.