Lipid breakdown and metabolism of ketone bodies
Content of subsection:
- Introduction to lipid breakdown and ketone body metabolism.
- Lipids as an energy source – TAG degradation in cells, β-oxidation of fatty acids.
- Formation and use of ketone bodies.
Introduction to lipid breakdown and ketone body metabolism
Triacylglycerols (TAGs) store large amounts of chemical energy. As an energy store, they are very advantageous because 1 g of anhydrous TAG stores six times more energy than 1 g of hydrated glycogen. The complete oxidation of 1 g of TAG yields approximately 38 kJ, while only 17 kJ are obtained from 1 g of carbohydrates or proteins. A 70 kg man stores approximately 400,000 kJ in his TAGs - the total weight of a TAG is around 10.5 kg. These supplies could allow us to survive several weeks of starvation. The main site of TAG accumulation is the cytoplasm of adipocytes.
Fatty acid oxidation
Individual types of fatty acid oxidation are indicated by Greek letters, which determine the carbon atom on which the reactions take place. β-oxidation taking place in the mitochondrial matrix is of major importance. Enzymes catalyzing the so-called ω- and α-oxidation occur on the membranes of the endoplasmic reticulum.
Conversion of fatty acids to glucose
Animals cannot convert fatty acids into glucose. Fatty acids represent a rich source of energy for gluconeogenesis, but glucose is not formed from their carbon atoms (with the exception of fatty acids with an odd number of C). Acetyl-CoA cannot be converted to either pyruvate or oxaloacetate - both carbons are split off as CO2 during the Krebs cycle. The pyruvate dehydrogenase reaction is irreversible. Interestingly, plants also have two other enzymes that allow them to convert AcCoA to OAA, in the so-called glyoxylate cycle.
Lipids as an energy source – TAG degradation in cells, β-oxidation of fatty acids
The utilization of lipids for energy production takes place in three basic phases:
- Lipid mobilization - hydrolysis of TAG to MK and glycerol and their transport blood.
- Activation of MK in the cytosol and their transport into the matrix mitochondria.
- β-oxidation - breaking down MK into acetyl~CoA, which enters the Krebs cycle, or ketone bodies are formed from it.
Lipid mobilization – lipolysis
The mobilization of stored lipids is enzymatically ensured by hormone-sensitive lipase' (HSL). It catalyzes the reaction:
- TAG → 3 MK + glycerol
Regulation of lipolysis
As the name suggests, the enzyme is under strict hormonal control. Its activity is stimulated by the ``phosphorylation of its molecule. Insulin as an anabolic hormone causes its inhibition, counterregulatory hormones (glucagon, catecholamines) or thyroid hormones on the contrary activate it.
Utilization of glycerol
Penetration of fatty acids into cells
The way of passing through the cell membrane depends on the length of the chain. Short chain fatty acids (↓12C) can penetrate by simple diffusion. Those with a longer chain use different transport systems in the membrane enabling their facilitated diffusion - for example FATP (fatty acid transport protein) or FAT/CD36 (fatty acid translocase).
Activation of fatty acids
Fatty acid activation occurs in the cytosol, on the outer mitochondrial membrane immediately after their entry into the cell. Without the activation, it is impossible to consider the involvement of their molecules in metabolism. Activation then simultaneously maintains their steady concentration gradient (analogous to glucose phosphorylation - see glycolysis). The principle of fatty acid activation is the ester linkage of a fatty acid binding molecule to the SH-group of coenzyme A via acyl-Coenzyme A synthetase (fatty acid thiokinase):
- MK + ATP + HS−CoA → acyl−CoA + AMP + 2 Pi
The activation of the fatty acid actually passes off in two stages. First, acyl adenylate (acyl-AMP) is formed and in the second phase AMP is exchanged for coenzyme A.
Entry of fatty acids into the mitochondria matrix
Beta oxidation of fatty acids
β-oxidation takes place only under aerobic conditions - it is closely related to the respiratory chain. The individual reactions of β-oxidation of fatty acids are catalyzed by four enzymes :
- Acyl~CoA−dehydrogenase – the prosthetic group is FAD;
- Enoyl~CoA−hydratase ;
- L-3-hydroxyacyl~CoA-dehydrogenase – the coenzyme is NAD + ;
The reactions can be summarized in the sequence of dehydrogenation – hydration – dehydrogenation – thiolytic cleavage . The first three reactions are analogous to those occurring in the Krebs cycle starting with succinate (see Krebs cycle):
Oxidation of succinate to fumarate using succinate dehydrogenase - FAD is the cofactor.
- Oxidation of succinate to fumarate using succinate dehydrogenase - FAD is the cofactor.
- Addition of water to the double bond in fumarate results in malate catalyzed by fumarate hydratase.
- Oxidation of malate to oxaloacetate using the enzyme malate dehydrogenase – the cofactor is NAD + .
- 1. Acyl~CoA-dehydrogenase - first oxidation
- This enzyme catalyzes the formation of a double bond between the 2nd (α) and 3rd (β) carbons of the fatty acid chain. This is a stereospecific reaction that produces trans-enoyl-CoA. The electron acceptor is FAD . There are different types of dehydrogenases in cells, which differ in the length of the MK chain that they oxidize:
- short MK (4–6 C),
- medium MK (6–10 C),
- long MK (12–18 C).
- 2. Enoyl-CoA-hydratase
- This enzyme catalyzes the hydration of the trans double bond formed in the first step. A hydroxyl group is formed – L-3-hydroxyacyl-CoA.
- 3. Hydroxyacyl-CoA-dehydrogenase
- This enzyme catalyzes the oxidation of the hydroxyl group at the third (β) carbon to a keto group. Electrons are accepted by the coenzyme NAD + .
- 4. β-ketothiolase
- The final step of one turn of β-oxidation is thiolytic cleavage catalyzed by β-ketothiolase. It involves an attack of the SH− group of the coenzyme on the β-keto carbon of the fatty acid chain. The reaction leads to the formation of AcCoA and acyl-CoA, which is two carbons shorter.
One turn of β-oxidation
β-oxidation is a cyclic process, one turn of which can be written as:
Acyl−CoA + FAD + NAD+ + HS−CoA → acyl−CoA (2 C shorter) + FADH2 + NADH+H+ + AcCoA
The intermediate product (acyl−CoA 2 C shorter) enters the next round of β-oxidation. Most fatty acids have an even number of Cs, so the last turn converts butyryl-CoA into two molecules of AcCoA.
Yield of complete oxidation of palmitate
To give an idea of the overall yield of fatty acid oxidation, here is the equation and energy balance of the complete oxidation of palmitate:
Palmitoyl~CoA + 7 FAD + 7 NAD+ + 7 HSCoA + 7 H2O → 8 AcCoA + 7 FADH2 + 7 NADH+H+
As you can read in the article on the respiratory and ATP production, article on the respiratory chain and ATP production , we cannot determine the exact amount of ATP produced in the respiratory chain during the oxidation of nutrients. Therefore, please consider the following numbers only as approximate and generally correct quantities. We present them here so that you can compare them with the oxidation of other nutrients, eg glucose. In the respiratory chain, 2.5 (3) ATP is obtained from one NADH and 1.5 (2) ATP from one FADH 2 , which in total represents:
- 7 × FADH2 = 10, 5 (14) ATP,
- 7 × NADH = 17, 5 (21) ATP
- Oxidation of 8 AcCoA in the Krebs cycle = 80 (96) ATP.
The total profit stopped at a total of 108 (131) ATP. But we used 2 ATP to activate the fatty acid , so the net gain is 106 (129) ATP .
Fontana J., Trnka J., Maďa P., Ivák P. et al.: Transformation of substances and energy in the cell. In: Functions of cells and the human body : Multimedia scripts.
Regulation of beta-oxidation of fatty acids
Regulation of β-oxidation takes place at the level of entry of fatty acids into mitochondria – more precisely at the level of the carnitine transporter carnitine acyltransferase I (CAT I). This enzyme is inhibited by the intermediate product of fatty acid formation - malonyl~CoA. We are talking about the so-called cross regulation. The principle is that fatty acid synthesis takes place in the cytosol, just like the reaction catalyzed by CAT I. Malonyl~CoA is formed as a product of the first reaction of fatty acid formation. Cross-regulation prevents simultaneous synthesis and degradation of MK. Inzulin inhibits β-oxidation, while counterregulatory hormones activate it.Kategorie:Biochemie Kategorie:FBLT
Fatty acids with an odd number of C
Breakdown of unsaturated fatty acids
Most of the unsaturated fatty acids in the human body and in food have a cis configuration of double bonds. Their degradation in β-oxidation proceeds by the process described above until their double bond comes into contact with enoyl-CoA hydratase. This requires only trans isomers – it is therefore necessary to convert the cis isomer to trans using an isomerase.
Breakdown of very long chain fatty acids
Oxidation of very long chain fatty acids (more than 18 carbons) takes place in peroxisomes. The first step is catalyzed by flavoprotein dehydrogenase, which transfers electrons to O2 – H2O2 is formed :
- FADH2 from the first step is reoxidized not in the respiratory chain, but by reaction with O2: FADH2 + O2 → FAD + H2O2
- Peroxisomal catalase decomposes H2O2: 2 H2O2 → 2 H2O + O2
α-oxidation and ω-oxidation
Those are minor fatty acid oxidation pathways. In ω-oxidation, reactions happen on the terminal carbon of the chain. In α-oxidation, oxidation is on the α-carbon.
Omega oxidation occurs in the enoplasmic reticulum. The terminal methyl group is hydroxylated, which is further oxidized to a carboxyl group. A dicarboxylic acid is formed, which can be degraded to dicarboxylic acid with 6-10C, which is already sufficiently soluble in water.
Formation and utilization of ketone bodies
Formation and function of ketone bodies
Ketone (bodies) include acetoacetate, β-hydroxybutyrate and acetone. The main site of their formation is the mitochondria of hepatocytes. Ketones represent a water-soluble transport form of acetyls. It is formed when there is an excess of acetyl~CoA produced by liver beta-oxidation – the liver „pre-chews“ fatty acids and provides the body with ketone bodies as an alternative source of energy.
The entry of AcCoA into the Krebs cycle depends on the availability of oxaloacetate. It is produced by the carboxylation of pyruvate. During starvation or diabetes mellitus OAA is consumed in the process of gluconeogenesis. The lack carbohydrates leads to a decrease in the amount of OAA and thus to a slowing down of the Krebs cycle. It could be said that "fats burn in the fire of carbohydrates"..
The environment of the organism
Before we get to the specific reactions of the formation of ketone bodies – ketogenesis, we will describe the situation in the organism under which it takes place. At the beginning is the activation of lipolysis through hormone-sensitive lipase (HSL). After the activation of lipolysis, plasma concentrations of fatty acids increase, which enter the liver cells to an increased extent. In them, they undergo β-oxidation, which produces an excess of AcCoA. It cannot be used sufficiently in other pathways and therefore enters ketogenesis. Therefore, the source of carbon atoms in ketogenesis is only acetyl~CoA.
The course of the formation of ketone bodies
The course of the formation of ketone bodies can be described by the following reactions:
- Condensation of two molecules of AcCoA → acetoacetyl~CoA .
- Reaction with another AcCoA → 3-hydroxy-3-methylglutaryl~CoA (HMG~CoA).
- Cleavage of HMG~CoA → AcCoA and acetoacetate.
- Reversible conversion of acetoacetate and β-hydroxybutyrate.
- Decarboxylation of acetoacetate.
β-Ketothiolase catalyzes the last step of β-oxidation of fatty acids – thiolytic cleavage. During the formation of ketone bodies, the reaction is reversed and one molecule of acetoacetyl~CoA is formed from two molecules of AcCoA. The reaction takes place in the matrix of mitochondria.
This enzyme catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA. Condensation takes place on the third carbon of acetoacetyl~CoA to form 3-hydroxy-3-methylglutaryl-CoA. This important intermediate occurs not only in the metabolism of ketone bodies, but also occurs during the synthesis of cholesterol.
This enzyme catalyzes the cleavage of HMG-CoA into acetoacetate and AcCoA. This creates the first ketone body.
This enzyme catalyzes the mutua reversible conversion of two ketone bodies – acetoacetate and β-hydroxybutyrate. The cofactor is NAD+. During the massive formation of ketone bodies β-hydroxybutyrate is quantitatively the most important ketone body in the blood, i.e. most of the acetoacetate is converted to it.
Decarboxylation of acetoacetate
Part of the acetoacetate molecules spontaneously i.e. non-enzymatically decarboxylates into acetone, which has no use in the human body and is excreted by breathing or urine.
Activation and utilization of ketone bodies
Regulation of ketogenesis
Regulation of ketogenesis takes place at four levels:
- Hormone-sensitive lipase – lipolysis in adipose tissue.
- Carnitine acyltransferase I – entry of fatty acids into mitochondria, where their β-oxidation takes place.
- Routing of acetyl-CoA from β-oxidation to ketogenesis and not to the Krebs cycle .
- Mitochondrial HMG-CoA synthase.
A high level of ketone bodies in the blood signals the presence of a large amount of acetyl-CoA. Its consequence is the inhibition of lipolysis.
Concentration of ketone bodies in plasma and ketoacidosis