Fatty acids are used in many tissues as fuel where they are oxidized to acetyl CoA, which is further metabolized in the citric acid cycle.
The liver processes glycerol either by the glycolytic or gluconeogenic pathway.
Regulation of Lipolysis
Lipolysis in adipocytes is triggered by a cAMP-dependent signaling cascade.
Epinephrine and glucagon trigger 7TM receptors that activate adenylate cyclase, leading to stimulation of protein kinase A (PKA).
PKA phosphorylates perilipin and hormone-sensitive lipase (HS lipase).
Phosphorylated perilipin restructures lipid droplets and releases the coactivator CGI-58 for adipose triacylglyceride lipase (ATGL).
Activated ATGL initiates lipolysis of TAG to DAG, followed by cleavage to MAG by HS lipase and glycerol and fatty acid by MAG lipase.
Triacylglycerol
Most fatty acids in humans are found in triacylglycerol.
Triacylglycerols constitute the major fuel store in the body.
The three hydroxyl groups of glycerol are esterified with fatty acids.
Mono- and diacylglycerol occur in minor amounts and are metabolic intermediates.
Example: 1-Palmityl 2,3-dioleoyl glycerol.
Glycerol Metabolism
Glycerol is absorbed by the liver.
Glycerol enters the liver, where it is metabolized by the glycolytic or gluconeogenic pathway.
Glycerol is phosphorylated and oxidized to dihydroxyacetone phosphate, which is isomerized to glyceraldehyde 3-phosphate.
Glycerol can be converted into pyruvate (glycolysis) or glucose (gluconeogenesis).
Fatty Acid Degradation Overview
Fatty acid oxidation yields acetyl CoA, which enters the Citric Acid Cycle (CAC).
The CAC generates CO2 and H2O.
Fatty Acid Activation
Fatty acids are linked to coenzyme A before they are oxidized.
Upon entering the cell cytoplasm, fatty acids are activated by attachment to coenzyme A.
Coenzyme A and the fatty acid form a thioester.
The activation reaction takes place on the outer mitochondrial membrane, catalyzed by acyl CoA synthetase.
The activation takes place in two steps:
The fatty acid reacts with ATP to form an acyl adenylate intermediate.
The sulphydryl group of CoA attacks the acyl adenylate to form acyl CoA.
The reaction is rendered irreversible by the action of pyrophosphatase.
Carnitine Shuttle
Carnitine carries acyl groups across the mitochondrial membranes.
Fatty acids are activated for oxidation in the cytosol, but they are oxidized in the mitochondrion.
Fatty acyl CoA cannot directly cross the inner mitochondrial membrane.
The acyl portion is transferred to the alcohol carnitine.
The reaction is catalyzed by carnitine palmitoyltransferase I (or carnitine acyltransferase I).
Acyl Carnitine Shuttle Mechanism
Acyl carnitine is shuttled across the inner mitochondrial membrane by a carnitine carrier protein (translocase).
The acyl group is transferred back to CoA by carnitine palmitoyl (acyl) transferase II.
The carrier returns carnitine to the cytoplasmic side in exchange for incoming acyl carnitine.
Carnitine Deficiency
Genetic deficiencies of the carnitine shuttle result in pathological conditions.
A number of diseases are caused by a deficiency in carnitine (malnutrition), carnitine transferase, or translocase (carnitine uptake defect).
Muscle, kidney, and heart are the tissues which are primarily impaired since they use fatty acids as a fuel.
Symptoms range from mild muscle cramping to severe weakness and even death.
Dietary carnitine therapy raises the plasma concentration of carnitine and forces its entry into tissues in a nonspecific manner, which is frequently beneficial.
Fatty Acid Degradation (β-oxidation)
Fatty acid degradation involves oxidation at the β-carbon atom, and hence the series of reactions is called the β-oxidation pathway.
The symbol \Delta^n is used to denote the position of the first carbon atom participating in a double bond. Thus, \Delta^2 designates a double bond between carbon 2 and 3.
Fatty acid degradation consists of four steps that are repeated:
Oxidation of the β carbon, catalyzed by acyl CoA dehydrogenase, generates trans-\Delta^2-enoyl CoA and FADH_2.
Hydration of trans-\Delta^2-enoyl CoA by enoyl CoA hydratase yields L-3-hydroxyacyl CoA.
Oxidation of L-3-hydroxyacyl CoA by L-3-hydroxyacyl dehydrogenase generates 3-ketoacyl CoA and NADH.
Cleavage of the 3-ketoacyl CoA by thiolase forms acetyl CoA and a fatty acid chain which is two carbons shorter.
Palmitate Oxidation
The complete oxidation of palmitate yields 106 molecules of ATP.
The reaction for one round of β-oxidation is:
Cn-acyl CoA + FAD + NAD^+ + H2O + CoA → C{n-2}-acyl CoA + FADH2 + acetyl CoA + NADH + H^+
The stoichiometry of the full oxidation of palmitoyl CoA (C{16}-acyl CoA) is:
Palmitoyl CoA + 7 FAD + 7 NAD^+ + 7 H2O + 7 CoA → 8 acetyl CoA + 7 FADH_2 + 7 NADH + 7 H^+
Calculations:
8 acetyl CoA → 80 ATP (CAC: 1 acetyl CoA = 10 ATP)
When monounsaturated fatty acids are degraded by β-oxidation, cis-\Delta^3-enoyl CoA is formed, which cannot be processed by acyl CoA dehydrogenase.
Cis-\Delta^3-enoyl CoA isomerase converts the double bond into trans-\Delta^2-enoyl CoA, a normal substrate for β-oxidation.
Polyunsaturated Fatty Acids
When polyunsaturated fatty acids are degraded by β-oxidation, cis-\Delta^3-enoyl CoA isomerase is also required.
2,4-Dienoyl CoA is generated, but cannot be processed by the normal enzymes.
2,4-Dienoyl CoA is converted into trans-\Delta^3-enoyl CoA by 2,4-dienoyl CoA reductase.
Cis-\Delta^3-enoyl CoA isomerase converts this trans-\Delta^3-enoyl CoA to trans-\Delta^2-enoyl CoA, a normal substrate.
Odd-Chain Fatty Acids
β-Oxidation of fatty acids with odd numbers of carbons generates propionyl CoA in the last thiolysis reaction.
Propionyl carboxylase, a biotin enzyme, adds a carbon to propionyl CoA to form methylmalonyl CoA.
Succinyl CoA, a citric acid cycle component, is subsequently formed from methylmalonyl CoA by methylmalonyl CoA mutase, a vitamin B12 requiring enzyme.
Ketone Bodies
Acetyl CoA formed from fatty acid degradation cannot enter the citric acid cycle (CAC) when fat and carbohydrate metabolism are not balanced (i.e., no glucose).
If glucose is unavailable, oxaloacetate levels will drop, and as a consequence, acetyl CoA cannot enter the CAC.
Acetyl CoA is diverted to form ketone bodies.
Ketone Body Formation
3-hydroxybutyrate and acetone are derived from acetoacetate, which is formed in three steps:
Two molecules of acetyl CoA condense to form acetoacetyl CoA. This reversible reaction is catalyzed by thiolase.
Acetoacetyl CoA reacts with acetyl CoA and H_2O to form 3-hydroxyl-3-methylglutaryl CoA (HMG-CoA), catalyzed by HMG-CoA synthase.
3-hydroxy-3-methyl-glutaryl CoA is cleaved into acetyl CoA and acetoacetate by hydroxymethylglutaryl CoA cleavage enzyme.
Acetoacetate is reduced to D-3-hydroxybutyrate by hydroxybutyrate dehydrogenase. Product formation of this reaction is determined by the NADH/NAD^+ ratio inside mitochondria. Acetoacetate can also undergo slow, spontaneous decarboxylation to acetone.
Ketone Body Metabolism
3-hydroxybutyrate is oxidized to acetoacetate, which is activated by the transfer of CoA from succinyl CoA. Acetoacetyl CoA is cleaved into two molecules of acetyl CoA.
Tissue Utilization of Ketone Bodies
The liver lacks 3-ketoacyl CoA transferase and has therefore acetoacetate available to supply other organs.
During fasting or in untreated diabetics, the liver converts fatty acids into ketone bodies, which are a fuel source in a number of tissues.
Ketone body production is especially important during starvation, when ketone bodies become the predominant fuel.
Under normal conditions the brain utilizes glucose as fuel, but during starvation it can switch to metabolize acetoacetate, which can make up 75% of the fuel needs.
Ketone Body Supply
The liver supplies ketone bodies to other organs during fasting or diabetes.
Clinical Insight: Diabetic Ketosis
Diabetes is a condition characterized by the absence (type 1) or resistance (type 2) to insulin. As a consequence, glucose cannot enter cells. All energy must be derived from fats, leading to the production of acetyl CoA.
Acetyl CoA builds up because oxaloacetate, which can be generated from glucose, is not available to replenish the citric acid cycle.
Moreover, fatty acid release from adipose tissue is enhanced in the absence of insulin function.
An overproduction of ketone bodies can occur, which are released into the bloodstream.
Ketone bodies are moderately strong acids, and excess production can lead to acidosis.
The increased blood acidity impairs tissue function, most importantly in the central nervous system.