Lipid Metabolism Flashcards
When and where does fatty acid oxidation occur?
- Primarily in the mitochondrial matrix
- Between meals, during fasting, during increased energy demand
Compare the energy yields of glycolysis and fatty acid oxidation.
- More energy per mole from FAs than glucose because they are more reduced.
How are long chain fatty acids transported into mitochondria?
Part 1: activated transport from cytosol to intermembrane space
- FA => fatty acyl CoA. FA activation by acyl CoA synthetase; requires two ATP equivalents + CoA
- Transport across outer membrane
Part 2: Translocation from intermembrane space to matrix
- fatty acyl CoA => fatty acylcarnitine by carnitine palmitoyl transferase I (CPTI). Requires carnitine and releases CoA
- transport across inner membrane by carnitine acylcar-nitrine translocase
Part 3: Conversion back to fatty acyl CoA
- fatty acylcarnitine => fatty acyl-CoA. Conversion by carnitine palmitoyl-transferase II; requires CoA and releases carnitine.
- Carnitine translocated back to intermembrane space by carnitine acylcar-nitrine translocase.
How are short and medium chain FAs transported from the cytosol into the mitochondria?
- Diffusion across outer membrane
- Transport across inner membrane by the monocarboxylate transporter
- Activation to fatty acyl-CoA by acyl-CoA synthetase
Describe the structure of a fatty acyl coA.
- SCoA attached to carbonyl carbon
- Carbons starting after carbonyl carbon numbered a, B, etc…
- Last carbon = omega carbon
What are the steps of B-oxidation?
- Oxidation:
- Oxidation by acyl CoA dehydrogenase forms double bond between a and B carbons.
- Generates 1 FAD(H2) ~ 1.5 ATP - Hydration
- Hydroxyl group added to B carbon by enoyl coA hydratase.
- Requires water. - Oxidation
- Hydroxyl group on B carbon oxidized to a ketone by B-hydrozy acyl CoA dehydrogenase.
- Generates 1 NADH ~ 2.5 ATP - Cleavage
- Acetyl CoA is cleaved from the FA chain by B-keto thiolase, shortening the chain by 2 C.
- Requires CoASH
Steps repeat in a spiral until the final cleavage where a 4 chain FA is broken into 2 x acetyl CoA
Where do odd chain FAs come from and how are they oxidized differently than even chain FAs?
- Dietary
- B-oxidation:
- until cleavage of acetyl CoA results in propionyl CoA (3C) - Carboxylation:
- Propionyl CoA carboxylated by propionyl CoA carboxylase
- Requires 2 ATP equivalents, biotin, and CO2 - Epimerization
- Stereochemical rearrangement of D-methyl malonyl CoA to L form by methyl maolonyl CoA epimerase or racemase - Rearrangement
- L-methyl malonyl CoA rearranged to form succinyl CoA by methyl malonyl CoA mutase
- Requires B12
- Succinyl CoA then enters the TCA cycle
What is the ATP yield from B oxidation of palmitate?
C16 = 7 cycles (2 x 7 + 2 = 16)
7 FADH2 ~ 10.5 ATP from oxidative phosphorylation
7 NADH2 ~ 17.5 ATP from oxidative phosphorylation
8 Acetyl CoA = 80 ATP from TCA cycle
-2 ATP from FA activation
106 mol ATP / mol palmitate
How is B-oxidation regulated?
By energy state!
- Transcriptionally:
- Use of FAs as fuel increases transcription of FA metabolism genes - Inhibition of CPTI during fatty acid synthesis
- FA synthesis activated with high insulin
- AMP-PK inhibited during FA synthesis; phosphorylation
by AMP-PK necessary to activate acetyl CoA carboxylase
- Malonyl CoA inhibits CPTI - ETC: high energy/reducing power inhibits B-ox
- high ATP/ADP ratio inhibits ETC
- inhibited ETC increases NADH/FAD(H)2
- high NADH/FAD(H)2 inhibit B-oxidation
What is the function of the peroxisome?
Oxidation of very long chain FAs
Reoxidation of FAD used in B-oxidation of very long chain FAs
- FAD(H)2 oxidized by O2 forming H202
- H202 decomposed to water + O2 by catalase
Synthesis of plasmalogens
Detox of phenols, formaldehyde, alcohols
How are very long chain FAs oxidized?
Oxidation begins in the peroxisome:
- Activation
- VLCFA activated to VLCFA CoA by VLACS (very long chain acyl coA synthetase) - Transport of activated VLCFA into peroxisome
- Oxidation of activated VLCFA CoA until chain is 4-6C
- Step 1 = oxidation by oxidase; generates H202
- Later steps produce NADH and Acetyl CoA - Addition of carnitine to acetyl-CoA and short acyl-CoAs
- Diffusion from peroxisome to be taken up by mitochondria for further oxidation
What is a-oxidation?
Pathway for oxidation of branched chain FAs occurring in the peroxisomes
Occurs only at branch sites, then normal B-oxidation can occur in peroxisomes/mitochondria
What is omega-oxidation?
Alternate pathway for FA oxidation that occurs in the ER when there is a defect in B-oxidation.
- Oxidation of omega carbon (farthest from carbonyl)
- addition of hydroxyl group - Further oxidation to form carboxylic acid on omega carbon
- Product = dicarboxylic acid; more soluble than normal FAs and can be released into the bloodstream
Where and how are ketone bodies oxidized?
Produced by the liver, but used (and oxidized) in peripheral tissues
- Oxidation
- D-B-hydroxybutyrate oxidized by D-B-hydroxybutyrate dehydrogenase (hydroxyl to ketone)
- Generates 1 NADH - Activation
- Acetoacetate activated by succinyl CoA:acetoacetate CoA transferase.
- Requires succinyl CoA; generates succinate - Cleavage
- Formation of 2 acetyl CoA by thiolase
- Requires CoASH
How and where is alcohol metabolized?
Mainly occurs in the liver:
- Oxidation
- ethanol oxidized by alcohol dehydrogenase (hydroxyl to aldehyde)
- cytosolic
- generates 1 NADH - Oxidation
- acetyaldehyde oxidized by acetaldehyde DH to form acetate
- mitochondrial
- generates 1 NADH
- inhibited by disulfiram: buildup of toxic acetaldehyde => positive punishment to stop drinking - Activation
- acetyl coA formed by activation of acetate by acetyl CoA synthetase
- requires COSH and 2 ATO equivalents
Also occurs in ER (microsomal ethanol oxidizing system):
- Oxidation
- cytochrome P450 enzyme oxidizes ethanol to acetaldehyde
- requires NADPH + O2
What disorders are associated with FA oxidation?
impaired LCFA oxidation - carnitine related issues
hypoketotic hypoglycemia - MC fatty acyl CoA DH deficiency
dicarboxylic aciduria - when B-ox is impaired and omega oxidation is primary oxidation
Zellweger syndrome - peroxisome defect and can’t oxidize VLCFAs
How are acetyl CoA and malonyl CoA generated for FA synthesis
- Conversion of glucose to mitochondrial acetyl CoA
- pyruvate from glycolysis converted to OAA by pyruvate carboxylase and acetyl CoA by PDH in mitochondria - Transport of mitochondrial acetyl CoA to cytosol
- OAA and acetyl CoA condense to form citrate via citrate synthase
- high levels of citrate => export to cytosol
- citrate lyase breaks cytosolic citrate into OAA + acetyl CoA
- OAA recycled to pyruvate by cytosolic malate DH and malic enzyme
- regulation: citrate lyase and malic enzyme upregulated by demand - Synthesis of malonyl CoA
- acetyl CoA carboxylated by acetyl CoA carboxylase
- requires ATP, CO2, and biotin
- malonyl coA can be converted back to acetyl-CoA by MCD
How is acetyl CoA carboxylase regulated?
Feedback/feedforward regulation:
- activated by citrate
- inhibited by palmityol CoA
Covalent modification:
- phosphorylated enzyme = inactive
- AMP-activated protein kinase phosphorylates - activated by low energy levels
- Phosphatase dephosphorylates - activated by insulin
How are the FA anabolic and catabolic pathways co-regulated?
CPTI is inhibited by malonyl CoA during FA synthesis to block B-oxidation
How are FAs synthesized from acetyl CoA and malonyl CoA?
Catalyzed by FA synthase on the cytosolic side of the ER. Enzyme is a homodimer with 7 active sites per monomer - palmitate (C16) can be synthesized w/o substrate-enzyme release
FA synthase has an acyl carrier protein (-SH) and a condensing site (cys-SH)
- Acyl carrier site picks up acetyl CoA, which is then transferred to the condensing site
- Malonyl CoA picked up by acyl carrier site
- Condensation (nucleophilic attack and decarboxylation):
- basically, malonyl CoA decarboxylated and acetyl transferred to end of malonyl coA - Reduction
- ketone reduced
- requires 1 NADPH - Dehydration
- reduction of alcohol to alkene - Reduction
- alkene reduced to alkane
- requires 1 NADPH - Chain transferred to condensing site
- New acetyl CoA binds acyl carrier protein and cycle repeats
How are FAs desaturated?
Primarily desaturated at C9, C6, C5 (counting from a-C); omega 3 and 6 must come from diet
- Oxidation by fatty acyl CoA desaturase
- requires O2 and 2 H+
- requires oxidation of 2 cyt b5 (Fe 2+ -> Fe 3+) - Reduction of cyt b5 by cyt b5 reductase
- Requires 1 NADH
When and how are ketone bodies synthesized?
In the mitochondria of hepatocytes during fasting or carb restriction
- Condensation
- 2 acetyl coA condensed by thiolase - Addition of acetyl CoA to acetoacetyl CoA
- HMG CoA synthase - Removal of acetyl CoA from HMG CoA
- HMG CoA lysase - Conversion of acetoacetate into (a) D-B-hydroxybutyrate and (b) acetone
(a) D-B-hydroxybutyrate DH; requires NADH
(b) spontaneous decarboxylation
How is ketone body synthesis regulated?
Increased when fatty acids levels are elevated during fasting, starvation, or low carb diet
High levels of NADH inhibit isocitrate DH in the TCA cycle => acetyl CoA buildup
Acetyl CoA => ketone bodies
Which are the ketogenic amino acids and how are they used?
Ketogenic amino acids can be used to generate acetyl CoA and acetoacetate and are thus a fuel source during starvation (when all fat stores are depleted)
trp, thr, lys, ile, lys, leu, phe, tyr
