Fatty Acid Metabolism & Red Blood Cell Metabolism

• Fatty acid chains polymerize in the cytoplasm but oxidize in the mitochondrial matrix.
• The separation of fatty acid metabolism sites prevents side reactions and allows pathway regulation.
• Acetyl-CoA, made in the matrix, moves to the cytoplasm for fat synthesis.
• FFAs, mobilized for oxidation, move into the mitochondrion.
• Fatty acid metabolic pathways are preceded by transport processes.

Red Blood Cell Metabolism

• Red blood cells lack mitochondria and cannot use FFAs for energy.
• They rely on anaerobic glycolysis for energy.

Pathway Reaction Steps: Acetyl-Coenzyme A to Palmitate

• Four reactions move acetyl-CoA from the mitochondrial matrix to the cytoplasm via the Acetyl-Coenzyme A Shuttle.

Citrate Synthase

• Acetyl-CoA from glucose condenses with oxaloacetate to form citrate.
• Citrate then transports from the mitochondrial membrane to the cytoplasm.

Citrate Cleavage Enzyme (Citrate Lyase)

• Citrate regenerates acetyl-CoA and oxaloacetate in the cytoplasm, which requires ATP and CoA.

Malate Dehydrogenase

• Oxaloacetate is reduced with NADH to produce malate.
• Malate can transport back into the mitochondrion, or undergo oxidative decarboxylation with malic enzyme.

Malic Enzyme

• Malate's oxidative decarboxylation produces pyruvate, CO2, and NADPH.
• Pyruvate transports back into the mitochondrion and converts back to oxaloacetate with pyruvate carboxylase.

Fat Oxidation in Mitochondria

• Mitochondria contain enzymes for fat oxidation AND glucose-derived aerobic energy.
• Impaired mitochondrial function can impair fat oxidation, leading to fat accumulation in tissues.
• Fatty acid polymerization initiates with the condensation of acetyl and malonyl groups to produce an acetoacetyl group in four reactions.
• Each enzyme function catalyzes individual domains of the fatty acid synthase multienzyme complex, a single polypeptide.

Acetyl-Coenzyme A-Acyl Carrier Protein Transacylase

• A 2-carbon acetyl group transfers from acetyl-CoA's phosphopantetheine group to acyl carrier protein (ACP)'s.
• The ACP transfers the acetyl group to 3-ketoacyl synthase (KS)'s cysteine thiol group.

Acetyl-Coenzyme A Carboxylase

• CO2 attaches to acetyl-CoA to produce malonyl-CoA, using ATP for energy.
• The same CO2 releases when the malonyl group condenses with the growing acyl chain.
• Acetyl-CoA carboxylase, like all carboxylases, needs biotin as a cofactor.

Malonyl-Coenzyme A-Acyl Carrier Protein Transacylase

• The malonyl group of malonyl-CoA transfers from CoA's phosphopantetheine to the phosphopantetheine in the ACP active site.

3-Ketoacyl Synthase

• The acetyl group (or longer acyl group) in the KS site condenses with malonyl-ACP, releasing terminal CO2, producing a 4-carbon 3-ketoacyl chain attached to the ACP.
• CO2 loss drives the reaction to completion; further 2-carbon additions to the acyl chain also come from malonyl-CoA.

β-Carbonyl Reduction

• Includes three reactions reduce the β-carbonyl on acyl-ACP.

3-Ketoacyl Reductase

• The 3-ketoacyl group reduces to a 3-hydroxyacyl group by NADPH.

Dehydratase

• Creates an unsaturated bond by water removal, similar to glycolysis' enolase reaction.

Enoyl Reductase

• The unsaturated bond reduces with NADPH.
• The reduced acyl intermediate then transfers to the free cysteine at the KS active site, restarts the cycle.

Elongation Cycle

• Repetitive condensation and reduction of malonyl-CoA units continue to produce palmitic acid.

Thioesterase

• When the growing acyl chain reaches 16 carbons, it releases from ACP as free palmitic acid.

Triglyceride Synthesis, Glycerol Kinase

• In the liver, glycerol phosphorylates with ATP.

Glycerol-3-Phosphate Dehydrogenase

• In liver and adipose tissue, glyceraldehyde 3-phosphate produced during glycolysis reduces to glycerol 3-phosphate.

Acyl-Coenzyme A Synthase (Fatty Acid Thiokinase)

• Fatty acids activate with CoA to acyl-CoA in an ATP-dependent reaction; AMP and pyrophosphate produce instead of ADP.
• Pyrophosphate's hydrolyzed to phosphate by pyrophosphatase, expending two high-energy bonds for each acyl-CoA.
• Two acyl-CoA molecules then esterify to glycerol 3-phosphate to produce a diacylphosphoglycerate.
• With phosphate removed, the third acyl group adds to form a triglyceride.

Regulated Reactions: Acetyl-Coenzyme A Carboxylase

• The irreversible step in fatty acid synthesis (FAS), acetyl-CoA carboxylase, is controlled by two mechanisms.

Covalent Modification

• The active dephospho- form of acetyl-CoA carboxylase phosphorylates and inactivates by an AMP-activated protein kinase.
• Low energy charge diverts acetyl-CoA away from the citric acid cycle.

Allosteric Regulation

• The active dephospho- form of acetyl-CoA carboxylase regulates by citrate and palmitoyl-CoA.
• Citrate stimulation assures FAS abundance of 2-carbon units.
• Palmitoyl-CoA inhibition coordinates palmitate synthesis with triglyceride complex.

Unique Characteristics of Fatty Acid Synthesis

• In humans, the enzymes for fatty acid biosynthesis exist as a single polypeptide consisting of eight catalytic domains.
• The enzymatic activities form a complex that binds to the growing acyl chain for completion and release.
• The P domain contains the same phosphopantetheine group as in CoA.
• The phosphopantetheine attaches by a flexible arm, contacting the active sites.
• The fatty acid synthase complex is not subject to regulation, except by malonyl-CoA availability.

Compartmentation

• FAS does not compete with fatty acid oxidation by occurring in separate compartments of the cell.
• Cytoplasmic synthesis ensures NADPH and that product, palmitate, will not undergo oxidation.

Adipose Tissue Versus Liver

• Adipose tissue lacks glycerol kinase, found in the liver.
• The glycerol backbone for triglyceride assembly in adipose tissue comes from dihydroxyacetone phosphate in the glycolytic pathway.
• Uptake of glucose is essential for adipose synthesis of triglycerides.

Interface with Other Pathways, Elongation of Palmitate

• Longer fatty acids used in myelin synthesis elongate by enzymes in the endoplasmic reticulum, using malonyl-CoA as the 2-carbon donor and NADPH as the redox coenzyme.
• These extensions occur in the endoplasmic reticulum, not by the fatty acid synthase complex.

Desaturation of Fatty Acids

• Unsaturated fatty acids are components of cell membranes to maintain membrane fluidity.
• Phospholipids contain unsaturated fatty acids, but not all can be synthesized in the body.
• Fatty acid desaturase, an enzyme in the endoplasmic reticulum, introduces double bonds between carbons 9 and 10 in palmitate and in stearate, producing palmitoleic acid and oleic acid, respectively.
• Fatty acid desaturase requires O2 and either NAD+or NADPH.
• Humans can synthesize linoleic and linolenic acid and must obtain these essential fatty acids in the diet.
• Linoleic acid can serve as a precursor for arachidonate, sparing it as an essential fatty acid.
• Arachidonate is an important lipid and is a precursor to prostaglandins, thromboxanes, leukotrienes, and lipoxins..

Fatty Acid Metabolism Summary

• Fatty acid chains polymerize in the cytoplasm and oxidize in the mitochondrial matrix.
• The precursor for fat synthesis, acetyl-CoA, arises in the matrix and must first be transported to the cytoplasm for incorporation into a fatty acid.
• FFAs that have been mobilized for oxidation must be transported into the mitochondrion to undergo oxidation.
• FAS in eukaryotes occurs on a multifunctional enzyme complex contained within a single polypeptide.
• Humans lack the enzymes necessary to introduce double bonds beyond carbon 9, thus making linoleic acid (18:2:19,412) and linolenic acid (18:2:19,112,415) essential fatty acids in the diet.

Fatty Acid Mobilization and Oxidation

• To be oxidized, fatty acids are transported across the mitochondrial membrane by the carnitine cycle.
• Oxidation oxidizes the carbon of an acyl-CoA to form a carbonyl group, followed by release of acetyl-CoA.
• Odd-numbered fatty acids convert to succinyl-CoA after converted into beta oxidation
• Humans can not synthesize omega 6, and omega 3

Fatty Acid Transport into Mitochondria

• Fatty acids go accross the mitochondrial membrane via the carnitine cycle.
• Fatty acids activate to an acyl-CoA in the cytoplasm.

Carnitine Acyltransferase I

• An acyl group transfers to carnitine by the cytoplasmic form of the enzyme.
• This acylcarnitine then diffuses across the outer mitochondrial membrane.

Carnitine Acylcarnitine Translocase

• This membrane transporter then exchanges cytoplasmic acylcarnitine for mitochondrial carnitine.

Carnitine Acyltransferase II

• The mitochondrial form of this enzyme then transfers the acyl group back to CoA.
• Medium-chain/short-chain fatty acids enter the mitochondrion directly, activating them in the mitochondrial matrix by acyl-CoA synthetases rather than the cycle noted above.

β-Oxidation of Acyl-Coenzyme A

• Oxidation at the beta-carbon of the fatty acid occurs with reduction of flavin adenine dinucleotide (FAD) at the A2 position to produce A2-trans-enoyl-CoA
• Electrons from FADH2 transfer to ubiquinone in the electron transport chain through varying lengths of fatty acids.
• The succinate dehydrogenase reaction catalyzes a separate reaction in the citric acid cycle.

Methylmalonyl-Coenzyme A Mutase

• Methylmalonyl-CoA converts Vitamin B12-dependently to succinyl-CoA, which then enters the citric acid cycle.

Enoyl-Coenzyme A Reductase

• The hydroxyl group forms through the hydration of the A2-trans-enoyl double bond.

3-Hydroxyacyl-Coenzyme A Dehydrogenase

• The hydroxyl group oxidizes, reducing NAD+ to NADH to produce a beta-keto group.

B-Ketothiolase

• Acetyl-CoA cleaves at the B-keto group, attaching CoA which then enters the B-oxidation cycle.
• The acetyl-CoA in the matrix then becomes available to the citric acid cycle for further oxidation.

Ketone Bodies Formation

• A third acetyl-CoA condenses with acetoacetyl-CoA to form beta-hydroxy-beta-methylglutaryl-CoA (HMG-CoA).

HMG-CoA Lysase

• HMG-CoA hydrolyzes to produce acetyl-CoA and acetoacetate.

B-Hydroxybutyrate Dehydrogenase

• Acetoacetate then reduces to form beta-hydroxybutyrate.

Acetone Formation

• Acetoacetate degrades in anonenzymatic reaction to produce acetone; accumulation can result in a fruity breath odour.

Succinyl-Coenzyme A: Acetoacetate-Coenzyme A Transferase

• In tissues, acetoacetate converts to acetyl-CoA via succinyl-CoA.
• Acetoacetate which metabolizes in the mitochondrial matrix is reciprocally metabolized in the citric acid cycle.

Regulated Reactions of Hormone-Sensitive Lipase

• Hormone-sensitive lipase in adipose tissue regulates fatty acid oxidation.
• High mobilization of fat contributes to ketosis because of conditions like starvation in type 1 diabetes.
• Minimal insulin in the blood and glucagon promote the form of hormone-sensitive lipase.
• The glycerol carries to the liver, where it enters gluconeogenesis.
• Fatty acids carries via serum albumin to tissues to catabolize energy but the liver uses energy from fat mobilization .

Peroxisomal Oxidation of Fatty Acids

• Very long chain fatty acids degrade in peroxisomes via B-oxidation to produce no NADH or FADH2; rather, H2O2 produces before catalase degrades it.
• Octanoyl-CoA and acetyl-CoA are final products then metabolized normally in mitochondria.

ω-Oxidation of Fatty Acids

• Oxidation at the terminal carbon (w-carbon) carries out by enzymes in the endoplasmic reticulum producing cytochrome p450, NADPH, & molecular O2 with normal B-oxidation acting at fatty acid ends.

α-Oxidation of Fatty Acids

• fatty acids (>20 carbons) & branched-chain fatty acids are metabolized via a-oxidation, which releases the carboxyl as one at a time - mostly in brain/nervous tissue.

Peroxisomal Oxidation of Fatty Acids

• Oxidation at the terminal carbon (w-carbon) carries out by enzymes in the endoplasmic reticulum producing cytochrome p450, NADPH, & molecular O2 with normal B-oxidation acting at fatty acid ends.

Adrenoleukodystrophy

• is a neurologic disorder from defective peroxisomal oxidation of fatty acids: reduced plasmalogens, adrenocortical insufficiency, and abnormalities in the white matter of the cerebrum can form.

Jamaican Vomiting Sickness

• Raw Ackee fruit contains hypoglycin inhibits both the medium- and short-chain acyl-CoA dehydrogenases, inhibiting and leading to nonketotic hypoglycemia.

Zellweger Syndrome

• is associated with the absence of peroxisomes in the liver and kidneys, resulting an accumulation of very long chain fatty acids.

Carnitine Deficiency

• Causes muscle aches/weakness, elevated blood FFAs, and low ketone production.
• Nonketotic hypoglycemia results because gluconeogenesis can't supported fat oxidation.

Refsum Disease

• (a-oxidation-deficient) accumulates phytanic acid in the brain, causing neurologic symptoms.
• Phytanic acid is a branched-chain fatty acid found in plants and in dairy products.