Lecture 24-Triacylglycerol & Fatty Acid Catabolism and Ketone Body Metabolism

Describe the structure and function of triacylglycerols and fatty acids

Fatty acids are chains of hydrogen-bearing carbon atoms that have a carboxylic acid atone end and a methyl group at the other end. The properties of fatty acids aredependent on chain length and degree of unsaturation.

Triacylglycerols Are the Storage Form of Fatty Acids

Fatty acids are stored as triacylglycerols (aka triacylglycerides, TAG) in which threefatty acids are esterified to one molecule of glycerol. TAG are stored in adipocytes inadipose tissue and are very energy rich

Explain how dietary triacylglycerols are digested, absorbed, and stored

Triacylglycerols from the diet form lipid droplets in the stomach. Bile salts, secreted by the gall bladder, insert into the lipid droplets, rendering them more accessible to digestion by lipases

Lipases, secreted by the pancreas (pancreatic lipases), convert the triacylglycerolsinto 2 fatty acids and monoacylglycerol. The digestion products are carried into the intestinal epithelium cells for absorption, where the triacylglycerols are reformed and packaged into chylomicrons, which travel through the blood so that the triacylglycerols can be absorbed by tissues.

Compare and contrast how dietary triacylglycerols are digested by pancreatic lipases to be stored as fuel with how endogenous triacylglycerols are broken down by lipolysis to be used by the tissues as fuel

Outline the three stages in which endogenous triacylglycerols are catabolized by listing the reactions and/or processes of each stage

1. Degradation of TAGs to release fatty acids and glycerol into the blood for transport to energy-requiring tissues (lipolysis)

2. Activation of the fatty acids and transport into the mitochondria for degradation (link fatty acid to CoA)

3. Degradation (beta oxidation or fatty acid oxidation) of the fatty acids to acetyl CoA for processing by the citric acid cycle

Explain how both activation and transport of fatty acids occurs by naming the enzymes involved, classifying the enzymes into the 6 enzyme classes, listing all reactants & products involved, and determining if the reactions are reversible or irreversible

Lipolysis

stored triacylglycerols are hydrolyzed by hormone-stimulated lipases

how does Lipolysis work?

1. Epinephrine and glucagon,a ctivate protein kinase Athrough the cAMP cascade(same as glycogen breakdown).

2. Protein kinase A phosphorylates perilipin, which is associated with the lipid droplet, and hormone-sensitive lipase (HSlipase).

3. Phosphorylation of perilipinresults in the activation of adipocyte triacylglyceride lipase(ATGL). ATGL initiates the breakdown of lipids (TAGà DAG+ free fatty acid).

4. Hormone-sensitive lipase (HSlipase) hydrolyzes DAG to MAG+ free fatty acid.

5. Monoacylglycerol lipase (MAGlipase) hydrolyzes MAG to freefatty acid + glycerol

mobilization of Triacylglycerols

Free fatty acids and glycerol are released into the blood. Fatty acids are transported to the target tissues in the blood bound to albumin. The glycerol released during lipolysis is absorbed by the liver for use in glycolysis or gluconeogenesis, depending on cell needs

Activation

Fatty acids are activated by linking them to coenzyme A

Fatty acid+ATP+CoA-SH→Fatty acyl-CoA+AMP+PPi

cofactor: Mg²⁺

ligase

irreversible

Upon entering the cell cytoplasm, fatty acids are activated by attachment to coenzyme Ain a reaction catalyzed by acyl CoA synthetase (aka fatty acid thiokinase). Thereaction proceeds through an acyl adenylate intermediate

Activation is rendered irreversible by the action of pyrophosphatase:

Cost of activation is 2 ATP equivalents

Transport

Carnitine carries long-chainactivated fatty acids into the mitochondrial matrix

After being activated by linkage to CoA, the fatty acid is transferred to carnitine, areaction catalyzed by carnitine acyltransferase I, for transport into the mitochondria.Acyl carnitine translocase transports the acyl carnitine into the mitochondria. In themitochondria, carnitine acyltransferase II transfers the fatty acid to CoA.

Fatty acyl-CoA+Carnitine↔Acyl-carnitine+CoA-SH

Transferase

reversible

Oxidation

Degradation involves 4 repeated steps, shortening fatty acid by 2 carbons/round

Fatty acid degradation consists of four steps that are repeated: an oxidation, a hydration, another oxidation, and thiolysis. Fatty acid degradation is also called β-oxidation because oxidation occurs at the β-carbon atom. Each round yields 1 FADH 2,1 NADH, and 1 acetyl CoA.

Calculate how many ATP are produced from the complete oxidation (to CO 2 and H 2 O) of any given even-chained saturated fatty acid

Palmitate 106

Stearate 120

Laurate 78

Oxidation of monounsaturated fatty acids

β-oxidation alone cannot degrade unsaturated fatty acids. Cis-Δ 3-enoyl CoAisomerase converts the cis-Δ 3-enoyl CoA double bond into trans-Δ 2-enoyl CoA, anormal substrate for β-oxidation

Oxidation of polyunsaturated fatty acids with odd-numbered double bonds

Fatty acids with odd-numbered double bonds are also handled by cis-Δ 3-enoylCoA isomerase, which converts the cis-Δ 3-enoyl CoA double bond into trans-Δ 2-enoyl CoA, a normal substrate for β-oxidation

Oxidation of polyunsaturated fatty acidswith even-numbered double bonds

Even numbers of double bonds require both cis-Δ 3 -enoyl CoA isomerase and 2,4-dienoylCoA reductase. 2,4-dienoyl CoA reductase is an enzyme that uses NADPH to reduce the 2,4-dienoyl intermediate to trans-Δ3-enoyl CoA. cis-Δ3-Enoyl CoA isomerase then converts trans-Δ3-enoyl CoA into the trans-Δ2 form, a customary intermediate in the β-oxidation pathway.

Oxidation of odd-chained fatty acids

β Oxidation of fatty acids with odd numbers of carbons generates propionyl CoA in the last thiolysis reaction

Propionyl CoA carboxylase, a biotin enzyme, adds a carbon to propionyl CoA to formmethylmalonyl CoA (reaction similar to pyruvate carboxylase). Succinyl CoA, a citric acid cycle component, is subsequently formed

Oxidation of long-chained fatty acids and branched-chain fatty acids

Oxidation of very long-chain fatty acids and branched chain fatty acids can occur inperoxisomes. These organelles are small membrane-bounded compartments that arepresent in the cells of most eukaryotes. Fatty acid oxidation in these organelles, which haltsat octanoyl CoA, serves to shorten very long chains (C26) to make them better substrates of β-oxidation in mitochondria.

The first dehydration in peroxisomal fatty acid degradation requires a flavoprotein dehydrogenase that generates H 2O 2, which is converted into water and oxygen by catalase. Subsequent steps are identical to β-oxidation but are carried out by different enzyme isoforms

Ketone Bodies

In fasting or diabetes, oxaloacetate is used to form glucose in the gluconeogenic pathway. In these situations, acetyl CoA can't enter TCA and thus is diverted to formacetoacetate and D-3-hydroxybutyrate. These two molecules as well as acetone are often referred to as ketone bodies. Abnormally high levels of ketone bodies are present in the blood of untreated diabetics.

Ketone bodies

• Acetoacetate (fuel)

• D-3-hydroxybutyrate (fuel)

• Acetone (waste)

Ketone bodies are synthesized in the

liver

Acetoacetate is formed from acetyl CoA in three steps. The next ketone body, D-3-Hydroxybutyrate, is formed by the reduction of acetoacetate. Because it is a β-ketoacid, acetoacetate also undergoes a slow, spontaneous decarboxylation to acetone,a waste product.

Degradation of ketone bodies as fuel

In tissues using ketone bodies (like heart, kidney, and the brain during starvation), D-3-hydroxybutyrate is first oxidized to produce acetoacetate and NADH for use inoxidative phosphorylation. Acetoacetate, which is ultimately metabolized to two molecules of acetyl CoA using 2 steps, which then enter the citric acid cycle.

Excessive ketone bodies in the blood is dangerous

Ketosis (ketoacidosis) occurs when there are high concentration of ketone bodies in the blood, and this can be life threatening. For example, diabetic ketosis occurs in diabetic patients that are unable to produce insulin. The liver in these patients cannot absorb glucose; it cannot provide oxaloacetate to process fatty acid-derived acetylCoA. Adipose cells continue to release fatty acids, resulting in the formation of large amounts of acidic ketone bodies.

Calculate how many ATP can be produced from the complete oxidation (to CO 2 and H 2 O) of any given ketone body

Acetoacetate 19 ATP

β-Hydroxybutyrate 21.5 ATP

acetone No significant direct ATP yield