28 Questions
What is the primary source of odd-numbered fatty acids in the diet?
Most plants
What is the intermediate product formed during the b-oxidation of odd-numbered fatty acids?
Propionyl-CoA
What is the enzyme required for the carboxylation of propionyl-CoA?
Propionyl-CoA carboxylase
What is the final product of the oxidation of propionyl-CoA?
Succinyl-CoA
Which enzyme catalyzes the hydration of an alkene to form an alcohol in the β-oxidation pathway?
Enoyl-CoA hydratase
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
Which of the following is NOT a characteristic of the trifunctional protein?
It is a soluble enzyme in the matrix
How many molecules of acetyl-CoA are produced from one palmitoyl-CoA molecule during the β-oxidation pathway?
8
What is the total ATP yield from the oxidation of one palmitoyl-CoA molecule to CO2 and H2O?
106
What is the role of the isomerase enzyme in the oxidation of unsaturated fatty acids?
It converts cis double bonds to trans double bonds
How many ATP equivalents are consumed during the activation of palmitate?
2
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
What is the energy yield from the oxidation of a monounsaturated fatty acid, compared to a saturated fatty acid?
It is lower
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
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
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
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
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
How many molecules of acetyl-CoA are formed from one palmitoyl-CoA molecule?
8
What is the characteristic of the C-C bond between methylene groups in fatty acids?
Quite stable
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
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
What is the result of the dehydrogenation of alkane to alkene catalyzed by Acyl-CoA Dehydrogenase?
Formation of a trans double bond
What is the function of Electron-Transferring Flavoprotein (ETF) in the dehydrogenation reaction?
To transfer electrons from FAD to the electron-transport chain
What is the effect of a recessive mutation in Medium-chain Acyl-CoA Dehydrogenase (MCAD) on fatty acid catabolism?
Defect in ketone bodies formation
What is the direction of electron transfer from Acyl-CoA to Ubiquinone?
Acyl-CoA to ETF: Ubiquinone oxidoreductase to Ubiquinone
What is the location of the Acyl-CoA Dehydrogenase enzymes in the mitochondria?
Inner mitochondrial membrane
What is the consequence of MCAD deficiency on energy metabolism in cells?
Cells rely more on glucose for energy
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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