Midterm II: Fatty Acid Oxidation (Ben) Flashcards by Kiva Mcfetridge

What are longer free fatty acids bound to for transport in the blood?

How many FAs per unit of this transport molecule?

Albumin

10 FAs per albumin monomer

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How does transport of shorter chain fatty acids differ from longer?

SCFAs are more water-soluble and exist in unionized or anion forms

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What is the first step of catabolism of a free fatty acid?

What is special about this step energetically?

Fatty Acid Activation

with CoA and ATP via acyl-CoA synthetase
it is the ONLY step of FA oxidation which uses ATP

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What is the enzyme which “activates” fatty acids for oxidation and where is it found (4 locations)?

Acyl-CoA Synthetase

Found:

in the ER
in peroxisomes
in mitochondria
on mitochondrial outer membrane

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What is the overal reaction of FFA activation?

Include the enzyme.

FFA + CoA + ATP

—- acyl-CoA synthetase —->

acyl-CoA + PPi + AMP

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What happens to the PPi formed from cleaved ATP during fatty acid activation and why?

it is hydrolyzed by inorganic pyrophosphatase

to ensure that the reaction is carried out fully and is irreversible in vivo

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What is the site of fatty acid activation?

the outer mitochondrial membrane

(where acyl-CoA synthetase is found)

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What happens to acyl-CoA in the intermembrane space?

(before it can enter the mitochondrial matrix)

Carnitine Palmitoyltransferase-I

(or Carnitine Acyltransferase-I)

catalyzes transfer of acyl group from CoA to carnitine to form acylcarnitine

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How does acylcarnitine penetrate the inner mitochondrial membrane?

via Carnitine-Acylcarnitine Translocase

exchanges carnitine for acylcarnitine across the membrane

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What happens to acylcarnitine once it is inside the mitochondrial matrix?

Start with the enzyme, then describe the process.

Carnitine Palmitoyltransferase-II

(or Carnitine Acyltransferase-II)

transfers acyl group to CoA reforming Acyl-CoA and liberating carnitine

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Where is Carnitine Palmitoyltransferase-I located?

across the outer mitochondrial membrane

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Where is Carnitine-AcylCarnitine Translocase located?

across the inner mitochondrial membrane

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Where is Carnitine Palmitoyltransferase-II located?

on the inside of the inner mitochondrial membrane

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Why is β-oxidation called β-oxidation?

because it is a cycle of successive cleavages of Acyl-CoA molecules at…

the C-C bond between the α(2) and β(3) carbons

(2nd and 3rd Cs from the -COOH end)

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What is the collective name for the enzymes which perform β-oxidation?

And where are they found?

Fatty Acid Oxidase

found in the mitochondrial matrix and inner mitochondrial membrane

(adjacent to to the respiratory chain)

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Energetically, what are the important products (3) of β-oxidation?

How are they used?

FADH₂ - oxidative phosphorylation > ATP
NADH - oxidative phosphorylation > ATP
Acetyl-CoA - citric acid cycle > ATP + NADH + FADH₂

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What is the first step of β-oxidation?

Substrates/reactants?

Enzyme?

Products?

Removal of one hydrogen each from α and β carbons…

Reactants: Acyl-CoA + FAD

Enzyme: Acyl-CoA Dehydrogenase

Products: Δ²-trans-Enoyl-CoA + FADH₂

What is the 2nd step of β-oxidation?

Substrates/reactants?

Enzyme?

Products?

Water is added to saturate the double bond…

Reactants: Δ²-trans-Enoyl-CoA + H2O

Enzyme: Δ²-Enoyl-CoA Hydratase

Products: L-β-Hydroxy-acyl-CoA

What is the 3rd step of β-oxidation?

Substrates/reactants?

Enzyme?

Products?

What is special about the enzyme?

Further dehydrogenation of the 3-carbon…

Reactants: L-β-Hydroxyacyl-CoA + NAD⁺

Enzyme: β-Hydroxacyl-CoA Dehydrogenase

Product: β-Ketoacyl-CoA + NADH + H⁺

The enzyme is stereospecific for the L-isomer.

What is the 4th step of β-oxidation?

Substrates/reactants?

Enzyme (2 names)?

Products?

How is one of the end products different from what entered β-oxidation?

Cleavage at the 2,3 position…

Reactants: β-Ketoacyl-CoA + CoA-SH

Enzyme: Thiolase (or Acyl-CoA Acetyltransferase)

Products: Acetyl-CoA + Acyl-CoA

Acyl-CoA product here is 2 carbons shorter than acyl-CoA which entered the cycle.

How does a fatty acid chain’s length affect how it is transported into the mitochondria?

> 12C - with carnitine
< 12 C - without carnitine, activated in mitoch.

Which β-oxidation enzyme has 3 different isoforms?

What for?

What are they?

Which one is frequently deficient?

Acyl-CoA Dehydrogenase (the 1st step)

for different lengths of fatty acids
LCFAs (C 12-18)
MCFAs (C 4-14) - frequently deficient (MCAD)
SCFAs (C 4-8)

How many ATP are ultimately formed per NADH produced in β-oxidation?

Per FADH₂?

NADH - 2.5 ATP

FADH₂ - 1.5 ATP

Where are > 12 C FAs oxidized?

And < 12 C?

> 12 C - on the inner mitochondrial membrane

< 12 C - via soluble enzymes in the mitochondrial matrix

What is special about the last 3 enzymes of β-oxidation? And what are the 3 enzymes?

they are a tightly associated _trifunctional protein_ * enoyl-CoA hydratase * β-hydroxyacyl-CoA dehydrogenase * thiolase

What results from the breakdown of an _odd-chain_ fatty acid? Where can this go and what can it become?

The remaining 3C residue is... **_Propionyl-CoA_** ...which is converted to **_succinyl-CoA_** and enters the citric acid cycle - in this way, it is the _only_ glucogenic part of any fatty acid

How are _very long chain fatty acids_ oxidized? What products result from this?

VLCFAs (C20/22) are oxidized **_in peroxisomes_** - forming **_acetyl-CoA**_ and _**H₂O₂_** - BUT this β-oxidation sequence ends at **_O_****_ctanoyl-CoA_** and is continued in mitochondria

What is the first step of glycerol conversion to a glycolysis intermediate in the liver? Enzyme + product + special reactant

Phosphorylation of glycerol... ## Footnote Reactants: **Glycerol + ATP** Enzyme: **Glycerol Kinase** Products: **Glycerol-3-phosphate + ADP**

What is the 2nd step of glycerol conversion to a glycolysis intermediate in the liver? (Enzyme + products + special reactants)

Dehydrogenation of G-3-P... ## Footnote Reactants: **G-3-P + NAD⁺** Enzyme: **Glycerol-3-phosphate Dehydrogenase** Products: **DHAP + NADH + H⁺**

What _two hormones_ induce increased fatty acid oxidation in the fasting/low-glucose state? How?

**Epinephrine/Adrenaline + Glucagon** Induce increased oxidation via: * **Phosphorylation/Inhibition of _Perilipin_** (lipid droplet "protecting" protein) * **Activation of _Hormone-sensitive Lipase_**

High levels of what _3 molecules_ **inhibit** fatty acid oxidation? (think products of oxidation + substrates for FA synthesis...)

1. **Malonyl-CoA** 2. **NADH** 3. **Acetyl-CoA**

What effect does **Malonyl-CoA** have on fatty acid oxidation? How?

it _inhibits_ it via... inhibition of _Carnitine Acyltransferase I_ which keeps fatty acids from entering the mitochondria

What effect does increased **NADH** have on fatty acid oxidation? How?

it _inhibits_ it via... inhibition of **_3-hydroxyacyl-CoA Dehydrogenase_**... the 3rd enzyme in beta-oxidation (which it is a product of... so simple allosteric inhibition)

What effect does increased **Acetyl-CoA** have on fatty acid oxidation? How?

it _inhibits_ it via... inhibition of **_thiolase_**... (the last enzyme of beta-oxidation) which it is a product of... so simple allosteric inhibition

What is an important mechanism for _long-term_ upregulation of fatty acid oxidation? Levels of what _general_ group of metabolic molecules are increased by this mechanism?

**PPAR-α** (Peroxisome Proliferator-Activated Receptor α) - a nuclear receptor protein - increases transcription of _fatty acid oxidation enzymes_

What are some endogenous and exogenous ligands of the PPAR system?

endogenous: **_fatty acids_ (arachidonic + other PUFAs)** exogenous: **_fibrates_** (drug class for dyslipidemia)

In what tissues is **PPARα** found? And what genes does it upregulate transcription of? and downregulate?

**Liver, Muscle, Adipose Tissue** Upregulates: * **Fatty Acid Transporters** * **Peroxisomal FA Oxidation Enzymes** * **CAT I + II** * **Lipoprotein Lipase** * **Apo A1/2** * **Acyl-CoA Synthase + Dehydrogenase** Downregulates: **Apo CIII**

What is the most common genetic defect of fatty acid oxidation?

**Acyl-CoA Dehydrogenase** **deficiency** - specifically the _medium-chain_ isoform

What are the symptoms of _Acyl-CoA dehydrogenase deficiency_? And treatment?

Symptoms: * **Hypoglycemia** * **Decreased ketogenesis** * **Hepatic lipid accumulation** * **Vomiting + drowsiness** Treatment: * **Frequent carbohydrate-rich meals** * **Carnitine supplementation**

What 3 enzymes break down triglycerides after _perilipin_ deactivation?

1. **_Adipocyte Triglyceride Lipase_** **(ATGL)** - TG ---\> DG + FA 2. **_Hormone Sensitive Lipase_** (**HSL)** - DG ---\> MG + FA 3. **_Monoacylglycerol Lipase_** (**MGL)** - MG ---\> FA + Glycerol