Fatty Acid Metabolism Flashcards

Question

Mitochondrial β-oxidation Overview

Answer 1

For long unbranched saturated even C fatty acids. * Occurs in the mitochondrial matrix * Oxidative * Requires FAD and NAD⁺ * Exergonic * Remove 2C fragments from carboxylate-end forming acetyl CoA * Repetitive process * Dehydrogenation (FAD) * Hydration * Dehydrogenation (NAD⁺) * Thiolysis * Key regulated enzyme * CPT I * Inhibited by malonyl CoA

Answer 2

Repetitive four-step sequence of reactions. 1. **First dehydrogenation** of the _fatty-acyl CoA_ catalyzed by *chain-length specific acyl CoA dehydrogenases* using _FAD_ forms an _enoyl CoA_ with trans double bond between C2 and C3 (α and β carbons) and generates FADH₂. 2. **Hydration** of the _trans Δ²-enol CoA_ by *enoyl CoA hydratases* adds water across the double bond to form _β-hydroxyacyl CoA_. 3. **Second dehydrogenation** of the _β-hydroxyacyl CoA_ by *β-hydroxyacyl CoA dehydrogenase* using _NAD_ oxidizes the hydroxyl group at the β-carbon to a keto group forming a _β-ketoacyl CoA_. 4. **Thiolytic cleavage** between the α and β carbons by *β-ketoacyl CoA thiolase* using CoA releases _Acetyl CoA_ from the carboxy-terminus leaving a fatty acyl_n-2C CoA 5. Cycle is repeated until the entire FA is degraded into acetyl CoA fragments. For LCFA only, steps 2-4 are catalyzed by mitochondrial trifunctional protein (TFP).

Answer 3

* Acetyl CoA * Oxidized in the TCA cycle: CO₂, FADH₂, NADH, and GTP are produced * Used for ketogenesis in liver * FADH₂ * Oxidized by ETC at CoQ for ATP synthesis * NADH * Oxidized by ETC at Complex I for ATP synthesis * NADH and ATP can be used in gluconeogenesis by the liver

Answer 4

* Substrate availability * as [substrates] increase β-oxidation increases * Malonyl CoA availability * as [malonyl CoA] decreases β-oxidation increases * Because malonyl CoA inhibits CAT-1 * Acetyl CoA/CoA ratio * as ratio increases, β-oxidation decreases * Due to decreased thiolase activity becauses needs free CoA to continue * In liver, acetyl CoA also allosterically activates pyruvate carboxylase of gluconeogenesis ⇒ tells liver its time to make glucose. * In muscle, acetyl CoA increases citrate concentration which inhibits glycolysis at PFK-1, conserving glucose.

Answer 5

7 FADH₂ x 2 ATP/FADH₂ = 14 ATP 7 NADH x 3 ATP/NADH = 21 ATP 8 Acetyl CoA: If completely oxidized through the ETC 24 NADH x 3 ATP/NADH = 72 ATP 8 FADH₂ x 2 ATP/FADH₂ = 16 ATP 8 GTP _= 8 ATP_ = 131 ATP Cost of activation _- 2 ATP_ About 130 ATP per molecule of Palmatate or 8 ATP / carbon

Answer 6

Odd # FA from exogenous sources oxidized for energy. 3C product at the end. Requires additional enzymes. 1. _Propionyl CoA_ (3C) converted to _L-methylmalonyl CoA_ by *propionyl CoA carboxylase* via the addition of a carbon dioxide in the form of HCO₃^- to the α-carbon. * Enzyme requires biotin and ATP * Third carboxylase encountered 2. _L-methylmalonyl CoA_ converted to succinyl CoA by methylmalonyl CoA mutase * Enzyme requires Vit B₁₂ coenzyme. 3. Succinyl CoA can: * Enter the TCA cycle ⇒ Anaplerotic reaction * OR BE USED IN THE LIVER TO MAKE GLUCOSE!!! * Only time you can make glucose from FA is with propionyl to succinyl CoA!!!

Answer 7

Problem: Intermediate formed is not a substrate for the enoyl hydratase which requires a trans Δ² double bond. Solution: Use additional enzymes to convert the product into an acceptable substrate. **Double bonds at odd-numbered C requires 3,2-enoyl CoA Isomerase.** **Double bond at even-numbered C requires 2,4-dienoyl CoA reductase.** Energy yield for unsaturated FA less that saturated FA because fewer H's.

Answer 8

* Peroxisomes required for β-oxidation of VLCFA. * Only peroxisomes have fatty acyl CoA synthetases for FA ≥ 24 C. * Transport into peroxisomes uses an ABC transporter instead of carnitine. * Repetitive 4-step process similar to mitochondrial β-oxidation results in chain shortening to MCFA. * First enzyme is a flavin oxidase which uses O₂ as the electron acceptor producing H₂O₂ * OK because peroxisomes have catalase * Less energy yield because FAHD₂ is not produced * Oxidation process stops at MCFA * MCFA then linked to carnitine (not CoA because peroxisomes don't have ETC enzymes) * MCFA-carnitine complex travel to the mitochondria where they are oxidized into acetyl CoA

Answer 9

* Rare disorder of peroxisomal VLCFA β-oxidation * Due to defect in the ABCD1 transporter which brings VLCFA into the peroxisomes. * Results in the accumulation of VLCFA especially in the brain and adrenals * Childhood and adult forms * Childhood cerebral form * Boy develops normally until school age (~ 7 y/o) * Shows dementia * Progressive neurological loss to a vegetative state * Adrenal insufficiency * Typically fatal by age 10

Answer 10

* Occurs at the α-carbon instead of the β-C * Shortens FA by 1C * Important in the oxidation of branched-chain phytanic acid ⇒ 20C product of chlorophyll degradation * Methyl group on C3 of phytanic acid prevents β-oxidation * 4-step process results in the hydroxylation of C2 of BCFA with release of C1 as CO₂ * Enzyme #2: α-hydroxylase is a site of pathology * Generates pristanic acid which gets activated and undergoes β-oxidation

Answer 11

* Phytanic acid storage disease * Autosomal recessive * Due to deficiency of the _α-hydroxylase_ enzyme involved in peroxisomal _α-oxidation of branched chain FA_ * Symptoms first seen in teens * Full progression can take 30 years * Treatment: dietary restriction of phytanic acid

Answer 12

* 4C water-soluble organic acids * PKa ~ 3.5 so easily ionizes in plasma leading to acidosis * Made primarily by the liver from acetyl CoA * Used by peripheral tissues to generate acetyl CoA and ultimately ATP * Very important for the brain during fasting because can easily cross the blood brain barrier * Ketone bodies are not linked to CoA so easily leaves the mitochondria after production.

Answer 13

Synthesis of ketone bodies. Occurs in mitochondria of hepatocytes. Driven by [acetyl CoA]. 1. _2 Acetyl CoA_ condensed to form _acetoacetyl CoA_ (4C) by *thiolase* with the release of a CoA. 2. _Acetoacetyl CoA_ (4C) condensed with another _acetyl CoA_ (2C) to form _β-hyroxyl-β-methylglutaryl CoA_ (HMG CoA - 6C) by *HMG-CoA Synthase* releasing another CoA. 3. _HMG CoA_ is cleaved by *HMG-CoA lyase* to _acetoacetate_ (4C) releasing an acetyl CoA. 4. _Acetoacetate_ can: * Be converted to _β-hyroxybutyrate_ by *β-hyroxybutyrate dehydrogenase* with concominant conversion of NADH to NAD⁺. * Undergo spontaneous decarboxylation to acetone + CO₂ * Metabolic dead end * Acetone excreted in the breath (& urine) and can be used as a diagnostic measure of DKA Ketone bodies readily exit the mitochondria after production. Presence of HMG-CoA lyase in the liver allows production of ketone bodies. Release of CoA supports continued β-oxidation. High availability of NADH from β-oxidation pushes acetoacetate to β-hydroxybutyrate.

Answer 14

* Catabolism and utilization of ketone bodies for energy * Occurs in mitochondria of nonhepatic tissues (esp brain) * Liver lacks acetoacetate:succinyl CoA transferase so unable to use KB for energy 1. _β-OH-butyrate_ ⇔ _acetoacetate_ while NAD⁺ → NADH + H⁺ by *β-hydroxybutyrate dehydrogenase* 2. _Acetoacetate_ ⇔ _acetoacetyl CoA_ while Succinyl-CoA → Succinate by *acetoacetate:succinyl-CoA CoA transferase (Thiophorase)* * **Acetoacetyl CoA trapped in the mitochondria 3. _Acetoacetyl CoA_ + CoA ⇔ _2 Acetyl CoA_ by *Thiolase* 4. Acetyl CoA → TCA cycle → ATP

Answer 15

* Defects in ketogenesis * Caused by deficiency of mitochondrial HMG CoA synthase or lyase * Results in hpoketotic hypoglycemia * Defects in ketolysis * Due to deficiency in transferase * Results in ketoacidosis

Answer 16

**Fasting Provoked Hypoketotic Hypoglycemia** * Most common defect in mitochondrial FA β-oxidation * Autosomal recessive * Single missense mutation cause more than 90% of cases * A disorder of fasting adaptation * β-oxidation of FA ≤ 12 C is greatly reduced * Insufficient ketogenesis → hypoketosis * Hypoglycemia due to: * increased use of glucose in the absence of ketones as an alternate fuel * decreased production of glucose due to increased acetyl CoA suppression of glyconeogenesis * MCFA are pushed to the alternate path of _ω-oxidation in the ER_ * Involves hydroxylation of ωC followed by oxidation to form a carboxyl group at the ω end producing _medium-chain dicarboxylic acids_ * Up-regulation of this "rescue" path is a red flag * Treatment: * Avoid fasting * IV glucose in acute episode * Carnitine supplementation to aid in excretion of metabolites * Newborn screening