Lecture 23a

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Questions and Answers

What is the primary source of odd-numbered fatty acids in the diet?

Dairy products

Marine organisms

Most plants (correct)

Meat from ruminants

What is the intermediate product formed during the b-oxidation of odd-numbered fatty acids?

Propionyl-CoA (correct)

.Acetyl-CoA

Methylmalonyl-CoA

Succinyl-CoA

What is the enzyme required for the carboxylation of propionyl-CoA?

Dehydrogenase

Propionyl-CoA carboxylase (correct)

Pyruvate carboxylase

Acetyl-CoA carboxylase

What is the final product of the oxidation of propionyl-CoA?

Succinyl-CoA Signup and view all the answers

Which enzyme catalyzes the hydration of an alkene to form an alcohol in the β-oxidation pathway?

Enoyl-CoA hydratase Signup and view all the answers

What is the net result of the reaction catalyzed by thiolase in step 4 of the β-oxidation pathway?

Thiolysis of a carbon-carbon bond Signup and view all the answers

Which of the following is NOT a characteristic of the trifunctional protein?

It is a soluble enzyme in the matrix Signup and view all the answers

How many molecules of acetyl-CoA are produced from one palmitoyl-CoA molecule during the β-oxidation pathway?

8 Signup and view all the answers

What is the total ATP yield from the oxidation of one palmitoyl-CoA molecule to CO2 and H2O?

106 Signup and view all the answers

What is the role of the isomerase enzyme in the oxidation of unsaturated fatty acids?

It converts cis double bonds to trans double bonds Signup and view all the answers

How many ATP equivalents are consumed during the activation of palmitate?

2 Signup and view all the answers

What is the result of the reaction catalyzed by β-hydroxyacyl-CoA dehydrogenase in step 3 of the β-oxidation pathway?

The formation of β-ketoacyl-CoA and NADH Signup and view all the answers

What is the energy yield from the oxidation of a monounsaturated fatty acid, compared to a saturated fatty acid?

It is lower Signup and view all the answers

What is the function of enoyl-CoA isomerase in the oxidation of oleic acid?

To reposition the double bond, converting the cis isomer to a trans isomer Signup and view all the answers

What is the purpose of the auxiliary enzyme ∆3,∆2-enoyl-CoA isomerase in the oxidation of polyunsaturated fatty acids?

To isomerize the double bond, converting the cis isomer to a trans isomer Signup and view all the answers

What is the difference between the oxidation of monounsaturated and polyunsaturated fatty acids?

The oxidation of polyunsaturated fatty acids requires two additional enzymes, enoyl-CoA isomerase and NADPH-dependent 2,4-dienoyl-CoA reductase Signup and view all the answers

What is the primary function of the mitochondrial respiratory chain in Stage 3 of fatty acid oxidation?

Providing energy for ATP synthesis by oxidative phosphorylation Signup and view all the answers

What is the net result of each cycle in the β-oxidation pathway?

Removal of one acetyl moiety and shortening of the chain by two carbons Signup and view all the answers

How many molecules of acetyl-CoA are formed from one palmitoyl-CoA molecule?

8 Signup and view all the answers

What is the characteristic of the C-C bond between methylene groups in fatty acids?

Quite stable Signup and view all the answers

Where are the enzymes responsible for Steps 2, 3, and 4 of the β-oxidation pathway located in fatty acids longer than C12?

Mitochondrial inner membrane Signup and view all the answers

What is the name of the multienzyme complex responsible for Steps 2, 3, and 4 of the β-oxidation pathway in fatty acids longer than C12?

Trifunctional protein Signup and view all the answers

What is the result of the dehydrogenation of alkane to alkene catalyzed by Acyl-CoA Dehydrogenase?

Formation of a trans double bond Signup and view all the answers

What is the function of Electron-Transferring Flavoprotein (ETF) in the dehydrogenation reaction?

To transfer electrons from FAD to the electron-transport chain Signup and view all the answers

What is the effect of a recessive mutation in Medium-chain Acyl-CoA Dehydrogenase (MCAD) on fatty acid catabolism?

Defect in ketone bodies formation Signup and view all the answers

What is the direction of electron transfer from Acyl-CoA to Ubiquinone?

Acyl-CoA to ETF: Ubiquinone oxidoreductase to Ubiquinone Signup and view all the answers

What is the location of the Acyl-CoA Dehydrogenase enzymes in the mitochondria?

Inner mitochondrial membrane Signup and view all the answers

What is the consequence of MCAD deficiency on energy metabolism in cells?

Cells rely more on glucose for energy Signup and view all the answers

Study Notes

Dehydrogenation of Alkane to Alkene

Catalyzed by 3 isoforms of Acyl-CoA Dehydrogenase (AD) on the inner mitochondrial membrane:
- Very-long-chain AD (12-18 carbons)
- Medium-chain AD (6-14 carbons)
- Short-chain AD (4-8 carbons)
Results in trans double bond, different from naturally occurring unsaturated fatty acids
Electrons from bound FAD transferred directly to the electron-transport chain via Electron-Transferring Flavoprotein (ETF)

Medium-Chain AD Deficiency

Genetic defect in FA catabolism due to recessive mutation in MCAD
Leads to hypoketotic hypoglycemia and liver dysfunction

Oxidation of Odd-Numbered Fatty Acids

Many plants and some marine organisms synthesize odd-numbered fatty acids
Oxidation of odd-numbered fatty acids from food by β-oxidation leads to the formation of 3-carbons Propionyl-CoA
Propionyl-CoA is converted to succinyl-CoA and enters the citric acid cycle

Oxidation of Propionyl-CoA

Same reaction as pyruvate carboxylase or acetyl-CoA carboxylase
Deoxyadenosyl Cobalamine is required for the oxidation of propionyl-CoA

Hydration of Alkene

Catalyzed by two isoforms of enoyl-CoA hydratase:
- Soluble short-chain hydratase (crotonase family)
- Membrane-bound long-chain hydratase, part of trifunctional complex
Acts on trans double bonds
Water adds across the double bond, yielding an alcohol

Second Dehydrogenation Step

Catalyzed by two isoforms of β-hydroxyacyl-CoA dehydrogenase:
- Soluble short-chain DHase (crotonase family)
- Membrane-bound long-chain DHase, part of trifunctional complex
The enzyme uses NAD cofactor as the hydride acceptor
Only L-isomers of hydroxyacyl CoA act as substrates

Thiolytic Cleavage

Catalyzed by acyl-CoA acetyltransferase (thiolase) via covalent mechanism
The carbonyl carbon in β-ketoacyl-CoA is electrophilic
Active site thiolate acts as a nucleophile and releases acetyl-CoA
Terminal sulfur in CoA-SH acts as a nucleophile and picks up the fatty acid chain from the enzyme

Trifunctional Protein

Catalyzes the three last steps for fatty acids longer than 12 carbons
Trifunctional protein is a hetero-octamer
Four α subunits have enoyl-CoA hydratase activity, β-hydroxyacyl-CoA dehydrogenase activity
Four β subunits have long-chain thiolase activity
May allow substrate channeling
Associated with the inner-mitochondrial membrane

The 4-Step Cycle Repeats

Each round produces an acetyl-CoA and shortens the chain by two carbons
Fatty acid catabolism for energy:
- Repeating the above four-step process six more times (7 total) results in eight molecules of acetyl-CoA
- FADH2 is formed in each cycle (7 total)
- NADH is formed in each cycle (7 total)
- Acetyl-CoA enters the citric acid cycle and further oxidizes into CO2
- This makes more GTP, NADH, and FADH2

Total ATP Yield

β-oxidation dehydrogenases: 7FADH2 + 7NADH (= 28 ATP)
8 Acetyl-CoA oxidation in the TCA cycle: 80 ATP
Total ATP yield: 108 ATP

Oxidation of Unsaturated Fatty Acids

Naturally occurring unsaturated fatty acids contain cis double bonds
Are not a substrate for enoyl-CoA hydratase
Two additional enzymes are required:
- An isomerase: converts cis double bonds starting at carbon 3 to trans double bonds
- A reductase: reduces cis double bonds not at carbon 3

Oxidation of Monounsaturated Fatty Acids

Requires one additional step
An auxiliary enzyme (∆3,∆2-enoyl-CoA isomerase) is needed
Energy yield is less since unsaturated fatty acids are already partially oxidized

Oxidation of Polyunsaturated Fatty Acids

Requires two additional steps
A first auxiliary enzyme (∆3,∆2-enoyl-CoA isomerase) is needed
A second auxiliary enzyme is needed (the second double bond requires reduction/isomerization)
Energy yield is less since unsaturated fatty acids are already partially oxidized

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