Fatty Acid Catabolism and Ketogenesis PDF

Aurora Killi 31.08.21 Fatty acid catabolism and ketogenesis Week 2 Lessdrates Sources of fatty acids:...

Aurora Killi 31.08.21 Fatty acid catabolism and ketogenesis Week 2 Lessdrates Sources of fatty acids: et in ver convertscarbony to fats · " Fats consumed in the diet - the Fats stored in cells as lipids droplets - > - Fats synthesized in one organ for export to another Freusing Fats obtained by autophagy (during extremely stressful situations like starvation which degrades the cell´s own organelles) s old damaged cell parts Humans obtain fats in the diet, mobilize fats stored in adipose tissue, and, in the liver, convert excess dietary carbohydrates to fats for export to other tissues, while during starvation recycle lipids by autophagy. Industrialized countries: dietary TAGs are more than 40% of energy requirements during a day. - Nutritional guidelines say to only take up and consume 30% daily. - Resting skeletal muscle: TAGs provide more than 50% of the energy. - Liver and heart: 80% of energy come from TAGs. - Advantages: Fatty acids carry more energy per carbon than polysaccharides / because they are more reduced. In the carbohydrates each carbon - atom has an OH and H group attached. On the other hand, does - - fatty acids only bond with hydrogen making them more reduced. - - Fatty acids carry less water because they are nonpolar. That means - - that they are lighter in weight compared to carbohydrates when - - Fat storage in white adipose tissue stored in the body. - CH The fat droplet in the adipocytes is Glucose and glycogen are for short-term energy needs and quick very big and takes up almost all the - space in the cell so the nucleus is delivery, while fats are for long-term energy needs, good storage, - pushed to the side. and slow delivery - Lipolysis The process during which TAGs are broken down into fatty acids which can be oxidized further. TAGs are - - primarily stored in adipocytes which means that lipolysis takes place at a high rate in these cells. TAGs - - are stored in lipid droplets. - Place of action: Adipocytes, cytosol - TAGs are stored in adipocytes (in steroid-synthesizing cells of the adrenal cortex, ovaries, testis, liver, muscles, and other cells) TAGs are stored in cytosol in lipid droplets Mobilization of stored TAGs: 1. The mobilization of stored TAGs is initiated by hormones (glucagon, epinephrine). Either of these hormones binding to receptors on the adipocytes, initiates a second messenger cascade and adenylyl cyclase is activated by the G coupled receptor. 2. The activated adenylyl cyclase converts ATP into cAMP. cAMP activates protein kinase-A (PKA). We have seen this enzyme before in the carbohydrate metabolism, which also means that this also activates the breakdown of glycogen in other cells. PKA has two jobs: 3. Phosphorylates hormone sensitive lipase ↳ making it active 1  Aurora Killi 31.08.21 4. Phosphorylates the protein perilipin. Perilipin are protein that covers the lipid droplets. The droplets itself contains the lipids and it is surrounded by a phospholipid monolayer, and on top of these phosphor head there are perilipin. After phosphorylation of the perilipin, they change their structure 5. While this is happening the perilipin release a protein called CGI-58, this is then able to activate Adipose triacylglycerol lipase (ATGL). 6. Adipose triacylglycerol lipase is the first enzyme that starts to break down TAGs, which then is converted into diacylglycerols while one fatty acid is released. 7. Now the phosphorylated hormone sensitive lipase takes over the job for the further breakdown of diacylglycerides. The phosphorylated perilipin facilitates the binding of hormone sensitive lipase, which allows the further breakdown of diacylglycerides into monoacylglycerols while releasing one more fatty acid. 8. The enzyme monoacylglycerol lipase (MGL) releases the remaining fatty acid from the monoacylglycerol. 9. The fatty acids that are released are transported to the blood. 10. The protein serum albumin constitutes half of the protein that we find in the blood. Each serum albumin can non-covalently bound 10 fatty acids. 11. The bounded fatty acids are then transported to the myocytes, heart and renal cortex. They are further on metabolized into energy. Once the serum albumin passes by these tissues, the fatty acids are picked up by the fatty acids transported located in the cell membrane. This ensures the transport of them inside the cell for the corresponding catabolic processes and energy production. We need three different enzymes to break down TAGs from the lipid droplet, and release three fatty acid and one glycerol in the process: Adenylyl cyclase Adipose triacylglycerol lipase Monoacylglycerol lipase What happens to the Glycerol metabolism glycerol backbone Place of action: primarily in cytosol of liver, but the enzymes are expressed in all - - that is left behind? tissues. The glycerol metabolism of TAGs itself will release about 5% of the - energy that it contains in total. Glycerol from fats enters the gluconeogenesis or glycolysis In the beginning of the glycerol metabolism energy needs to be invested, but - we regain more than enough ATP to cover this cost - Allows limits anaerobic catabolism of fats The transport of the glycerol from adipocytes to the liver is ensured by passive diffusion through aquaporins. Glycerol released in adipocytes is - always transported to the liver where it is primarily used for the - gluconeogenesis. However, there can be a small intramuscular storage of - fats, in form of glycerol and this is always used in the glycolysis. Transport of fatty acids into the cell requires a conversion to fatty acyl-CoA 2  Aurora Killi 31.08.21 To transport fatty acids into the matrix of mitochondria for further oxidation, they need to be activated. - This means that an CoA group is added to the fatty acid with the help of a group is isoenzymes called fatty - - - acyl-CoA synthetase: takes in Cytosol - Place 1. The carboxylate ion is adenylated by ATP, to form a fatty acyl-adenylate and pyrophosphate PPi, which is immediately hydrolyzed into two molecules of Pi. 2. The thiol group of coenzyme A attacks the fatty acyl-adenylate, while releasing adenosine monophosphate (AMP) and forming the thioester fatty acyl-CoA 3. The fatty acyl-CoA is used either for oxidation of fatty acid in the matrix of mitochondria or for synthesis of membrane lipids. Both reaction 1 and 2 are exergonic reaction which ensures that the conversion happens at a high rate. Acyl-Carnitine/Carnitine transport Place of action: Mitochondrial membrane in all cells that use fatty acids for energy Substrate: Fatty acid Product: Acyl-CoA ATP count: 1 ATP used Protein: Carnitine-Acylcarnitine translocase When this conversion has happened, there is a transport system that ensures that the acyl-CoA can be transported into the matrix of mitochondria. This transporter is called Acyl-Carnitine/Carnitine transporter. Carnitine as a molecule is an amino acid derivate and ensures the transport of the acyl-CoA. Step 1 (cytosol) Acyl-CoA is synthesized in the cytosol, and on the outer mitochondrial membrane there is an enzyme called carnitine acyltransferase 1. This enzyme performs transfer reaction by replacing the Co-A group with carnitine forming the acyl-Carnitine. Step 2 (intramembranous space and matrix) Acyl-carnitine is transported through the outer mitochondrial membrane and into the intermembranous space. It is further transported in the matrix with the help of the acyl-carnitine/carnitine transporter. Step 3: After transported in the matrix of mitochondria, an antiport will transport one molecule of carnitine outside, but first the carnitine molecule needs to be released from the acyl-carnitine by the reverse process of the one that happened in the cytosol. Carnitine acyltransferase 2 is located on the matrix side of the inner mitochondrial membrane and ensures this reaction. Acetyl-CoA is added, while carnitine is released. Fatty acyl- CoA will continue to the beta-oxidation. This transport is typical for fatty acids longer than 12C, usually starting from 14C and up. This means that the short chain and medium chain fatty acid can diffuse through the inner mitochondrial membrane. This is an exception form the rule of the impermeable inner membrane of mitochondria. 3  Aurora Killi 31.08.21 The role of acyl-carnitine/carnitine transporter: Most of the dietary in our food and the fats that the body synthetized on its own are long chained fatty acids. From the picture we can see that all fatty acids found in meat, most of the ones found in olive oil and some of the ones we get from plants are long chained fatty acids. This emphasizes the importance of this transport system! Stages of fatty acid oxidation: 1. β-oxidation: Stage 1 consists of oxidative conversion of two-carbon units into acetyl-CoA β-oxidation with concomitant generation of NADH + H+ and FADH2. Involves oxidation of β carbon to thioester of fatty acyl-CoA 2. Citric acid cycle: Stage 2 involves oxidation of acetyl-CoA into CO2 via citric acid cycle with concomitant generation of NADH + H+ and FADH2. 3. ETC: Stage 3 generates ATP from NADH + H+ and FADH2 via the respiratory chain β-oxidation Place of action: Every cell with mitochondria (matrix), but at different rates. It is the most typical oxidation - of fatty acids inside of our cells. Exception: the brain has a high preference for glucose (not fats) as energy source. The fats need more oxygen for their oxidation. The delivery of oxygen to the brain is limited, which determines the preference of glucose as energy source. through barrier Substrate: Acyl-CoA blood-brain Product: Acetyl-CoA ATP count: 2 reducing cofactors generated → 4 ATP Each pass removes one acetyl-CoA molecule, which later activates the citric acid cycle and will gain most of - the energy from the fatty acids. This β-oxidation is necessary due to the single bond between the - - methylene groups that we find in the fatty acids are stable. This makes the fatty acids hard to break down and release the acyl-coA. The c-c bonds between the carbonyl group, that you find in the last step of the β- - oxidation is easier to break and thus release acyl-CoA. - mostFA Long fatty acid oxidation is performed by a single trifunctional protein. The first enzyme of this protein is located separately, but still attached to the inner mitochondrial membrane, whereas the rest of the - - three reaction and enzymes is a three functional enzyme. - The medium chain and short chain fatty acids are processed by soluble free enzymes which catalyzes - - - - conversion of these fatty acids into acyl-coA acetyl-cost - - Step 1: dehydrogenation Enzyme: Acyl-coA dehydrogenase Product: Trans-∆2-Enoyl-CoA Cofactor: FAD and released FADH2 There is a creation of one double bond between the 𝛼 and β carbon atom, - - that is why it is called β-oxidation. This step is catalyzed by the enzyme acyl- - coA dehydrogenase. This enzyme has three different isoenzyme which can - catalyze: - Longs chains fatty acids (12-18 carbons) Medium chains fatty acids (4-14 carbons) > prefers - - - Short chains fatty acids (4-8) - 4  Aurora Killi 31.08.21 Formation of double bond between the alpha and beta carbon atom. This step is catalyzed by acyl-CoA dehydrogenase. It has three different isoenzymes that can catalyze long chain, medium chain, and short chain fatty acids. The results in the formation of a trans double bond, which is different from the naturally occurring unsaturated fatty acids. This step releases FADH 2, which is a membrane bound protein. - Therefore, there is a need for electron-transferring flavoprotein (ETF). It is not the same as the second - complex of the electron transport chain, but it is a similarly working protein that can accept electrons from - 12 different dehydrogenases. The FADH2 molecule produced in this step is transported to the electron - - transport chain with the help of the electron-transferring flavoprotein. 1.5 ATPs are created per 1 FADH2. - - Step 2: Hydration Enzyme: Enoyl-CoA hydratase (first part of the trifunctional protein complex) Enzyme class: Lyase - Product: L-β-hydroxy-acyl-CoA Cofactor: None ( Through two isoforms of the enzyme enoyl-CoA hydratase we ensure the addition of a water molecule. The enoyl-CoA hydratase is the first part of the - - trifunctional complex. Water is added across the double bond yielding - - alcohol on the beta carbon atom. This reaction is analogous to the fumarase - - reaction in the citric acid cycle (same stereospecificity) Step 3: Dehydrogenation Enzyme: β-hydroxyacyl-CoA dehydrogenase Enzyme class: ? Product: β-Ketoacyl-CoA Byproduct: NAD and releases NADH+H The NAD is cofactor used and NADH+H released and can activate the first complex of ETC. Usually is hydroxyl group on the secondary alcohol is dehydrogenated with the help of NAD. Step 4: Transfer of fatty acid chain and release of Acetyl-CoA Enzyme: Acyl-CoA acetyltransferase (thiolase) Enzyme class: Transferase? Product: Acyl-CoA (myristoyl-CoA) Byproduct: CoA-SH The enzyme acyl-CoA acetyltransferase doesn’t have different isoenzyme. - This enzyme through the covalent mechanism binds the substrate to the thiolate active site where a thiolase group is present. This thiolase group on the active side perform a nucleophilic - attack and in this way is the Acetyl-CoA released. The rest of the molecule is still attached to the enzyme. Here comes another nucleophilic attack performed by the CoA itself. The CoA picks up the remaining part of fatty acids, reduced by the two carbon atoms, and this forms shorter acyl-coA. The net reaction is thiolysis of the carbon-carbon bond. & 5  2012 Acetyl-10A - 20 10 Acery Aurora Killi - 1 - B OX - 31.08.21 - - Each round of β-oxidation produces one acetyl-CoA and shortens the chain by two carbon atoms. However, when the last round of the β-oxidation comes, there are 2- Acetyl-CoA molecules released. This means that the number β-oxidation rounds - are always one less than the number of Acetyl-CoA molecules released. A fatty acid with 20 C will give 10 Acetyl-CoA molecules by 9 rounds of the β-oxidation. - Acetyl-CoA: number of C-atoms divided by 2 𝛃-oxidation cycles: Number of acetyl-CoA and subtract 1 - 16 C - ATP count: Number of ATP gained from fatty acids & depends on the length of the carbon chain. One - - I round of the β-oxidation yields 1 FADH2 and 1 NADH - · + H+, thus one round itself will create 2.5 + 1.5 ATP = - E 4 ATP. In addition, we have 10 ATP that we generate form one citric acid cycle. As β-oxidation rounds are one less than the acetyl-CoA created then the citric acid cycle will have one more additional round. In this table you can see how much FADH2 and NADH + H+ the fatty acid Palmitoyl-CoA (16 carbon molecule) creates. When we add all of it together is the final number 108 ATP molecules, but there is a loss of 2 ATP for each fatty acid transport inside the matrix of mitochondria by the Acyl-Carnitine/carnitine – transporter. That transporter needs a double phosphorylation (2 ATP) to bring the fatty acid inside. 4 9 Genetic defects in fatty acyl-CoA dehydrogenases > First Step of Box. - Most common is the mutation in gene coding for medium-chain (6-12C) Acyl-CoA dehydrogenase Carriers of this mutation: 1 in 40 Disease: 1 in 10 000 Song Recurring episodes of syndrome include fat accumulation in liver, high blood levels of octanoic acid, low blood glucose levels, sleepiness, vomiting and coma Episodes are very serious, despite no symptoms in between Mortality: 25-60% in early childhood. With early detection and careful management of the diet the prognosis is good Since most of the fatty acids that we have in our body is long chained fatty acids, the symptom of this disease is rare. We get the medium-chained fatty acids from plant nature, whereas the small-chained fatty acids will come from “orgat- bacteria”. This bacterium produces fatty acids from complex - gut carbohydrates that we are not able to digest. There is an important function of these short-chained fatty acids in the cardiovascular system, and they are necessary. β-oxidation in mitochondria vs peroxisomes The β-oxidation occurs differently in the peroxisomes. The major difference is that peroxisomal acyl-CoA dehydrogenase passes electrons directly to molecular oxygen Energy is released as heat and there is the creation of hydrogen peroxide because the final acceptor of electrons from the FADH2 is oxygen. This creates hydrogen peroxide which is a free radical. Peroxisomes contains a lot of catalases that can eliminate these free radicals. Peroxisomes will act a lot more actively on very long-chained fatty acids (22 and more carbon atoms). 6  Aurora Killi 31.08.21 There is a small amount of very-long fatty acids in comparison to long-chained fatty acids. Those few very-long fatty acids can be synthesized by ruminant animals and is found in some fish types. Another thing that is important is that peroxisomes will act more actively on very long chain fatty acids (≥22C) such as hexacosanoic acid (26C). X-linked adrenoleukodystrophy Gender related pathology that affects young boys before the age of 10. If leads to loss of vision, behavioral disturbances, and death within a few years. There is a mutation in the gene that codes for the transporter of the very long fatty acids that ensures their transport into peroxisomes. Once the very long fatty acids cannot be oxidized, they accumulate and start to generate inflammation which causes more free radical formation which leads to cell death. Since the brain is mostly dependent on the energy from oxidation of very-long chained fatty acid, this is the organ that is mostly affected by this mutation. Oxidation of unsaturated fatty acids Naturally occurring unsaturated fatty acids contain cis double bonds. They Monounsaturated fatty acids are not a substrate for enoyl-CoA hydratase, and therefore we need two require isomerase additional enzymes: Polyunsaturated fatty acids Isomerase: converts cis double bonds starting at carbon 3 to trans double requires both enzymes bonds Reductase: reduces cis double bonds not at carbon 3 Oxidation of monounsaturated fatty acids: This process starts with regular β-oxidation: 1. We cleave of the acetyl-CoA´s one by one, until we get to the double bond on the monounsaturated fatty acid. 2. In this case we can see that the cis double bond, is shifted to a trans double bond with the help of the enzyme enoyl-CoA isomerase. The cis double bond is between C3 and C4 while in the trans acid it is C2 and C3. The double bond is changed to the position which further on can be used normally in the β- oxidation cycle. With this kind of isomerization, we ensure further β-oxidation but without one FADH2 creation (one FADH2 less). IN comparison to the oxidation of saturated fatty acid, we will therefore loose 1.5 ATP. 7  Aurora Killi 31.08.21 Oxidation of polyunsaturated fatty acids: Initially we start with regular β-oxidation: 1. We remove the acetyl-CoA until the point where we reach the first double bond. 2. The first double bond is isomerized from the 3rd and At this point, acyl-CoA 4th carbon atom to the 2nd and 3rd by the isomerase dehydrogenase step is 3. One more 𝛽-oxidation cycle and release one more skipped, which results acetyl-CoA in the loss of FADH2. 4. In the next 𝛽-oxidation cycle we form one more Because NADPH + H+ double bond by the acyl-CoA dehydrogenase, but reduces the remaining now we are introducing one more double bond by unsaturated bond, there regular 𝛽-oxidation is another loss of FADH2. 5. The enzyme reductase is saturating the next double bond and forming a new double bond between the 3rd and 4th carbon atom. NADPH will donate hydrogen to the 5th carbon atom and to the 2nd carbon atom so we can translocate the double bond between the 3rd and 4th carbon atom 6. Another isomerase where we again are shifting this double bond between the 3rd and 4th to the 2nd and 3rd carbon atom. Summary of mono- and polyunsaturated fatty acid oxidation: Both oxidation of mono- and polyunsaturated fatty acids will result in the loss of one FADH2 molecule because of the already present double bond. We are further on performing regular 𝛽-oxidation. For colloquium you do not need to know the specific order of this reactions. However, we do need to know that we are using isomerase and reductase as enzymes to saturate the double bond, to further on perform normal 𝛽-oxidation. Oxidation of odd-numbered fatty acids Most dietary fatty acids are even numbered, but in nature, there are also unsaturated odd-numbered fatty acids. Many plants and some marine organisms also synthesize odd-numbered fatty acids. The only nuance from the saturated oxidation is in the fact that once we come to the fatty acyl-CoA that has 5 carbon atoms, the last beta oxidation round takes place to create one acetyl-CoA and one propionyl-CoA. Propionyl-CoA has three carbons, and further oxidation is not possible because there is no single carbon atom molecule presented in the human body. Therefore, propionyl-CoA is further carboxylated by propionyl-CoA carboxylase. Once we get a four-carbon molecule, it is just about the two consecutive reactions that catalyze isomerization from the D- methylmalonyl-CoA to succinyl-CoA. Succinyl-CoA fuels the number of intermediates in the citric acid cycle and can be used in further energy production. 8  Aurora Killi 31.08.21 ω-oxidation The oxidation of fatty acids can not only occur in the mitochondria and peroxisomes, but also in the endoplasmic reticulum Place of action: ER, rrimarily in liver and kidney The ω oxidation of fatty acids in the ER The omega oxidation of fatty acid, is an exceptional case and primarily occurs in the liver and kidney Preferred substrates for this type of oxidation are fatty acids with 10 or 12 carbons This type of oxidation is important when beta oxidation is defective (e.g., in case of - mutation or carnitine deficiency) Product: fatty acid with carboxyl group at each end. Either end can be attached to CoASH The molecule enters mitochondria for further beta oxidation The final 4C molecule (succinate) can enter the citric acid cycle The reaction is catalyzed by the enzyme mixed-function oxygenase which is responsible for oxygen on the omega side of chain. Further on, there is occurring dehydrogenase reaction and finally there is introduced a new carboxyl-group. This is interesting since both ends of the fatty acid contain carboxyl-groups and either of these can be attached to CoA. Once this happens the molecule can be transferred to the matrix of mitochondria where it can undergo the 𝛽-oxidation which results in the formation of succinate α-oxidation Oxidation of branched chained fatty acids (phytanic acid) in peroxisomes Place of action: Peroxisomes, pronounced in liver and brain α oxidation of phytanic acid Special type of oxidation of one type of branched chain fatty acid called phytanic acid. This oxidation happens in peroxisomes (similarly to beta oxidation). More pronounced in the liver and brain Phytanic acid is a long chain fatty acid with methyl branches. The methyl group is at the beta carbon atom and thus we are not able to perform regular beta oxidation The primary idea is that one carbon atom is removed from carboxyl group side of the phytanic acid. Once this carbon atom is removed and subsequent further reactions have happened, we are forming pristanic acid that does not have any beta carbon atom that is blocked and can go through beta oxidation. Western diet typically includes 50-100 mg of phytanic acid per day What happens? There is a removal of one carbon atom from this phytanic acid from the carboxyl-group side of the molecule. Once this carbon atom is removed and a few more reaction we are forming pristanic acid that do not longer have the 𝛽-carbon atom blocked. The 𝛽-carbon is normal and can proceed and go through the regular reaction of the 𝛽-oxidation. Through this regular reaction of 𝛽-oxidation this fatty acid is further on broken down and one of the products released is Propionyl-CoA. 9  Aurora Killi 31.08.21 Ketogenesis How can the acetyl-CoA be Place of action: exclusively in the mitochondria of the liver used further on if there is a Substrate: Acetyl-CoA deficiency of carbohydrates? Ans: ketogenesis Product: Ketone bodies Ketogenesis is the process during which ketone bodies are formed. The reverse reaction is called ketone utilization and it takes place mainly in the brain. Ketone bodies is a term that is used to represent three molecules that are created during the ketogenesis: Acetone Acetoacetate D-β-Hydroxybutyrate (not actually a ketone) Ketone bodies are formed in case there is no oxaloacetate present, because in that case the acetyl-CoA cannot enter the citric acid cycle and there is a substrate for the ketone body formation. The formation of the ketone bodies released the Co-A group from the acetyl-CoA, and therefore it can proceed for the beta oxidation. The ketone bodes are water soluble. They can be transported from liver, with the blood without specific transport-system, to the periphery and reconverted back to acyl-CoA. This energy is vital for skeletal, heart muscle, brain, and renal cortex. The most prominent of all the ketone bodies is D- 𝛽 - hydroxybutyrate, and its concentration in the blood is the highest one. Ketone bodies are: Evaporated through the lungs Water soluble Energy for skeletal and heart muscle, brain, and renal cortex Most prominent of the ketone bodies is in the blood is D-β-Hydroxybutyrate Step 1: generating free CoA-SH 1. The first step of ketogenesis is reverse of the last step in the β-oxidation in which thiolase reaction joins to acetates from acetyl-CoA forming Acetoacetyl-CoA. 2. A third acetyl-CoA molecule is incorporated by the enzyme HMG-CoA synthase to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) which is a 6C molecule. These steps are identical to the ones in the cholesterol biosynthesis, the only difference is the place of action. In the cholesterol synthesis happens in the cytosol, while the ketogenesis happen in mitochondria. Altogether, two CoA-SH are freed from the three acetyl-CoA involved in this step. Step 2: degradation of HMG-CoA 1. To travel to other tissues, the previously added CoA-SH must be removed from the HMG-CoA molecule. The enzyme HMG-CoA lyase removes one acetyl-CoA from 𝛽-Hydroxy- 𝛽-methylglutaryl-CoA (HMG- CoA) and acetoacetate is created. As a results, acetoacetate is the first ketone body that is produced. Acetoacetate is the substrate for the two other ketone bodies: acetone and D-β-hydroxybutyrate. 2. D-β-Hydroxybutyrate dehydrogenase performs a dehydrogenation reaction and with help of NADH + H+ it converts acetoacetate into D-β- hydroxybutyrate. This means that there is a loss of energy, however, once this molecule goes to the extrahepatic tissue, the reverse reaction occurs, and we regain energy. 10  Aurora Killi 31.08.21 3. Acetoacetate decarboxylase can convert acetoacetate into acetone. This reaction can happen non- enzymatically and spontaneously. Acetone is a metabolite that cannot be further oxidized or used in the human metabolism. Acetone is later removed as a gas through the lungs, while acetoacetate and D-β-hydroxybutyrate can traffic to the brain for use in energy production there. Utilization of ketone bodies Liver produces ketone bodies while extrahepatic tissues use ketone bodies. Even though some of these reactions are similar they are NOT reverse reactions. 1. D- β-Hydroxybutyrate dehydrogenase by a reverse reaction is formed back into acetoacetate. 2. Acetoacetate is now through a transferring reaction receiving a CoA from the Succinyl-CoA that we find in CAC. Succinate can proceed in the CAC. 3. By addition of another CoA can Acetoacetyl-CoA transform into 2 Acetyl-CoA The liver lacks the enzyme β-ketoacyl-CoA transferase, which restricts the ketogenesis When does the ketogenesis occur? It is typical when we are low on carbohydrates and there is not enough energy for the brain and some other tissues. This is typical in case of diabetes. When there is lack in insulin in diabetic patients, glucagon is released in the blood. Glucagon signals for fat cells to release TAGs and breaks them down into fatty acids and glycerol. Glycerol can be used for gluconeogenesis in the liver, once transported there. While TAGs can be used for the beta oxidation and then the acetyl-CoA can be used for ketogenesis. Insulin insufficiency also affects the muscle cells that do release amino acids. These amino acids can be glucogenic and facilitate the synthesis of glucose or ketogenic which is used for ketone bodies synthesis. Fatty acids in case of lack of energy, are broken down and create the Acetyl-CoA. If we then lack oxaloacetate is empty and used up for the gluconeogenesis the Acetyl-CoA cannot be used for CAC. Thus, there is a shifted where Acetyl-CoA is used in ketogenesis which allows us to free more CoA that can be reused for fatty acid oxidation. This also support the tissues that lack energy. The major problem in diabetic patients is that even though there exist carbohydrates in their body they are not able to enter the peripheral tissues (muscle tissue and fat cell). The reason for this is because is that carbohydrates need insulin as a signal key to enter these tissues (GLUT4 receptor). If this is happening these cells are starving. Ketogenesis in diabetic patients In case of lack of energy, fatty acids are broken down in the liver and creates Acetyl-CoA. Normally, Acetyl-CoA would proceed in the citric acid cycle, but as oxaloacetate as an intermediate has been used up for the synthesis of glucose through the gluconeogenesis (glycerol) as fuel for the brain, the citric acid cycle cannot take place. Therefore, acetyl-CoA is instead used for the synthesis of ketone bodies, which allows us to open more of the Co-A that can be reduced for fatty acid oxidation and continue its breakdown. We go into a cycle of extensive fatty acid breakdown and formation of ketone bodies that similarly to glucose also support the tissues that need energy. Diabetic ketoacidosis and hyperosmolar hyperglycemic state 11  Aurora Killi 31.08.21 V V ~ L ~ If the ketone bodies concentration is too high in the body, we can experience a state of ketoacidosis and hyperosmolar hyperglycemic state, which is characteristics for diabetic patients. The difference between the two states is the insulin concentration Ketoacidosis (DKA): insulin deficiency at extreme levels. Hyperosmolar hyperglycemic state (HHS): insulin is present in an amount that still can ensure systemic activity and thus minimizes the production of ketone bodies, but it is not as a high concentration a needed to fuel glucose fully in the cells. The concentration of blood glucose levels is high, however, the ketogenesis is not yet occurring to such extent that it has become a threat for the patient’s life. We should notice in case of HHS the glucose concentration is greater compared to the DKA and the arterial pH is normal. Whereas in the case of DKA the ketone bodies accumulation influences the pH levels since the peripheral tissues cannot utilize them fast enough. This can lead to death. The ketone bodies are excrete through the urine, so we can observe the acidity in the urine. Summary of lipid catabolism Unlike carbohydrate fuels, which enter the body primarily as glucose or sugars that are converted into glucose, lipid fuels are heterogenous with respect to chain length, branching, and unsaturation The catabolism of fats is primarily a mitochondrial process, but it also occurs in peroxisomes (both beta and alpha oxidation) and omega oxidation in the endoplasmic reticulum During peroxisomal oxidation, fats can be oxidized to generate heat, however, also produces H 2O2 Using a variety of chain length-specific transport processes and catabolic enzymes, the primary pathways of catabolism of fatty acids involve their oxidative degradation in two-carbon units, a process known as beta oxidation, which produces acetyl-CoA In the proves, a lot of NADH + H+ and FADH2 forms. These can yield a lot of ATP´s in the electron transport chain In most tissues, the acetyl-CoA units are oxidized further and used for ATP production in the mitochondrion In the liver, acetyl-CoA is catabolized to ketone bodies, primarily acetoacetate and beta- hydroxybutyrate, by a mitochondrial pathway termed ketogenesis The ketone bodies are exported from the liver for energy metabolism in peripheral tissue 12  Aurora Killi 31.08.21 Ketonemia and ketonuria develop gradually during fasting, whereas ketoacidosis may develop during poorly controlled diabetes, when fat metabolism is increased to high levels for support of gluconeogenesis 13