number 22 Done by Baraa Ayed Corrected by Yaseen Fatayer Doctor Faisal Al-Khatib 1 P a g e
Today we are going to cover these concepts: Oxidation of odd number fatty acids Oxidation of very long fatty acids Oxidation of fatty acid at α carbon Ketone bodies Oxidation of odd number fatty acids: Usually these fatty acids are found in the animal fat. Only 10% of animal fat is odd number fatty acids. So, how we metabolize these fat? for example we want to metabolize a 15 carbons fatty acid (odd number): After 6 rounds of β oxidation (note that we remove two carbon in each round).so, we will end up with 3 carbons fatty acid that joined to CoA which called propionyl CoA. So, propionyl CoA is the final product of oxidation of odd number fatty acid. The first step of metabolizing propionyl CoA is addition a carboxyl group (CO2) by the carboxylation reaction. This carboxylation reaction requires biotin as a co enzyme to carry a carboxyl group to propionyl CoA. This reaction is catalized by the enzyme propionyl CoA carboxylase. This carboxylation reaction needs ATP which will be converted to ADP in order to add the carboxyl group to propionyl CoA. Remember: Carboxylation reactions are always endergonic (need ATP) while Decarboxylation reaction where CO2 is released always exergonic(release ATP) and it happens spontaneously because it has large negative delta G so the reaction is highly irreversible toward releasing the carboxyl group. 2 P a g e
Note that the addition of CO2 is on the middle carbon (the second carbon) of propionyl CoA. The result compound is called D- Methylmalonyl CoA Now, D-Methylmalonyl CoA is converted by the enzyme Methylmalonly CoA racemase to L-Methylmalonyl coa ( switch from "D" form to "L" form ) Then the carboxyl group that we added on the middle carbon will be shifted to the carbon number 3 (the methyl group of methylmalonyl CoA) by the enzyme methymalonyl CoA mutase to produce succinyl CoA. Note that succinyl CoA which is an intermediate of TCA cycle will be converted to oxaloacetate at the end of TCA cycle and the oxaloacetate will be converted to glucose by gluconeogenesis pathway. Note that this is the only part of fatty acid which can be converted to glucose (the last three carbons of the odd number fatty acid) which is very little amount that is insignificant. This is the only exception of fatty acid that produce glucose which is minor percentage that is not important Methylmalonyl CoA mutase requires vitamin B12 as a coenzyme. So, if there is a deficiency of Vitamin B12, the enzyme will not function and that lead to accumulation of methylmalonylic acid. So, methylmalonylic acid will appear and excrete in the urine which cause methylmelanouria. 30 years ago, the measure of the status of vitamin B12 is done by measuring 3 P a g e
the amount of methylmalonyl CoA in the urine. So, if the concentration of methylmalonyl CoA is high in the urine which means that the patient has vitamin B12 deficiency. So, this measurement is called indirect measurement of the vitamin B12 status. Nowadays, we can measure the level of vitamin B12 in the plasma; we take a blood sample and give it to the laboratory but this technique was not available like 30 years ago. So, we called this measurement direct measurement of vitamin B12 status Oxidation of very long fatty acid: These fatty acids contain 22 carbon atoms or more. They are found in the brain They are oxidised in the peroxisome not in the mitochondria The steps of oxidation of those fatty acids: They are oxidised in the same manner of oxidation saturated fatty acid but the enzyme that produce the double bond between carbon number 2 and carbon number 3 is not Acyl CoA dehydrogenase and it is known as FAD containing Oxidase in which FAD is converted to FADH2. Note that we call this enzyme oxidase because it transfers the electrons of FADH2 to O2 to produce H2O2 because here we cannot transfer the electrons to the electron transport chain in the mitochondria. Peroxisome is the place where H2O2 are found and they are reactive oxygen species ( ROS) Note that the difference between oxidase and dehydrogenase is : Oxidases transfer the electrons to oxygen to produce H2O or H2O2 while Dehydrogenases transfer the electrons to NAD or FAD or NADP. Remember: the produced FADH2 doesn t re-oxidize in the electron transport chain, because the reaction is not in the mitochondria. The reaction continues until the FA becomes medium or short chain for β-oxidation so that it can be transported and oxidized in the mitochondria. 4 P a g e
SO, oxidation of very long fatty acids start in the peroxisome until the fatty acid becomes shorter then it will transferred to mitochondria where the oxidation continues This picture shows the oxidation of long chain fatty acids Oxidation of fatty acids at α carbon: It happens in branched-chain fatty acids that have several methyl groups (every forth carbon) and the first methyl group on carbon β. These fatty acids are found as a product of degradation of chlorophyll which found in the green leaves of the plants. So, when we eat green leaves, we will metabolize chlorophyll and this kind of fatty acid will be produced. How this kind of fatty acids can be oxidised? Note that this fatty acid cannot be oxidised by β oxidation because there is a methyl group on the β carbon. So, α oxidation starts: The first step of α oxidation is hydroxylation of α carbon by oxidative hydroxylase Then, the hydroxyl group on α carbon is oxidised to carboxyl group and this step is accompanied by decarboxylation of the carboxyl group of the fatty acid to be released as CO2( oxidativedecarboxylation reaction) ( look to the picture) 5 P a g e
Because we shorten the fatty acid by one carbon, the previous α carbon becomes the carboxyl group and the β carbon (that has branched) becomes α carbon. So, the new β carbon is free from the methyl group and it can undergo the β oxidation. What the significance of this oxidation? Because we as human beings consume green leaves of plant, so we take some chlorophyll which is the source of these fatty acid and we can metabolize them by this oxidation. Note that there are some people have deficiency of the hydroxylation enzyme. So if the hydroxylation enzyme is missing, the whole pathway cannot occur. There will be accumulation of these fatty acids in the cells and that leads to destruction of the cells and the most tissue that affected by destruction of the cells is the nervous tissue because these cells cannot regenerate and that leads to neurological disorders in the CNS. This disease doesn t appear at birth, it appears when the children start eating the plants. If we know that someone has this disease, we can avoid the damage simply by avoiding the food that contains chlorophyll. Also, we can know that someone might have a disease by knowing the family history of the disease. Ketone bodies: They are 3 compounds known as ketone bodies; the first ketone body acetoacetate which is a derivative of butyric acid (Ketobutyric acid) in which the ketone group at the carbon 3. If we divide the word (actoacetate) into two halves, the left half is the acetic acid (CH3CO-), 6 P a g e
and the right half is acetate, acetic group and acetic group linked together, so it s named acetoacetate. Acetoacetate can be reduced by NADH (which is oxidized to NAD+) so the ketone group is reduced to a hydroxyl group to give 3-Hydroxybutyrate, which is considered the second ketone body although it is not actually ketone. Acetoacetate can spontaneously undergo decarboxylation (without any enzyme) to give the third ketone body which is called acetone These ketone bodies are synthesized in the liver and Acetyl CoA is considered as a raw material (the precursor) to synthesize these compounds. Ketone bodies are produced at high rate during fasting and uncontrolled diabetes. In order to know why they synthesized at high rates during these two conditions, we should know the pathway of synthesizing the ketone bodies: The first step is a condensation of 2 acetyl CoA to produce Acetoacetyl CoA by the enzyme thiolase. Note that this reaction is reversal to the last reaction in β oxidation, the last step in β oxidation is the cleavage of Acetoacetyl CoA to 2 acetyl CoA. Therefore, high amounts of acetyl CoA whether it s coming from β oxidation or from any other source will increase the rate of this condensation reaction. The condensation reaction happens when the concentration of acetyl CoA is very high The second step involves the addition of one more Acetyl CoA to Acetoacetyl CoA, producing a product called HMG CoA. It is named (HMG CoA) because if you look at its structure you ll notice that it is a 5-carbon chain with 2 carboxyl groups at each end, it s similar to glutaric acid which is a 5-carbon dicarboxylic acid. So, HMG CoA is derivative of glutaric acid, but it is modified by adding methyl and 7 P a g e
hydroxyl groups at the third carbon, so the name becomes 3-hydroxy-3- methylglutaryl CoA (HMG CoA.) The enzyme responsible for this step is HMG CoA synthase The third step is cleavage of Acetyl CoA from HMG CoA to form acetoacetate by the enzyme HMG CoA lyase. Notice that the removed acetyl CoA is not the same that is added in the second step. What is the net reaction of this pathway? First, any compound that is produced in one step and used in another step is not counted in the net reaction such as acetoacetyl CoA ( used in one step and utilized in the another step) and HMG, CoA which are intermediates. The net reaction tills you what the purpose and the function of the pathway. So,the net reaction of ketone bodies synthesis pathway is : 2 Acetyl CoA Acetoacetate + 2 CoA So, From the net reaction you can tell the purpose of the pathway for these ketone bodies, there are three purposes, production of CoA (regeneration), Production of acetoacetate, Removal of Acetyl CoA. For the liver, the production of acetoacetate is considered as a waste product, it's not important. However, the importance one is the production CoA. In other tissue like the muscle, the production of acetoacetate is important in order to be utilized in some purposes. For example, in β oxidation of palmitic acid (16-carbon fatty acid) is oxidized to give 8 acetyl CoA, 7 NADH and 7 FADH2.NADH and FADH2 are reoxidized in electron transport chain to produce energy. Acetyl CoA will enter TCA cycle and get oxidized (combine with oxaloacetate producing citrate). So, oxaloacetate should be available for the acetyl CoA to be oxidized. So, CoA is produced Note that oxaloacetate is not considered a product of citric acid cycle and it is not consumed by the cycle because every time oxaloacetate is used, it is regenerated by the cycle. 8 P a g e
Note that CoA that is produced in the first step of citric acid cycle is the same that we used in β oxidation to activate the fatty acid. So, regeneration of CoA is important for the continuation of β oxidation During fasting, concentration of blood glucose is low. So, gluconeogenesis happens by consuming oxaloacetate to form glucose. The level of oxaloacetate is greatly decreasing so, the TCA cycle will stop. The Acetyl CoA that is produced by β oxidation will trapped in the cells and it will lock the all CoA so, free CoA is not available to keep the β oxidation going on. How can we regenerate CoA when TCA is not running?? By production of ketone bodies! During prolonged fasting, glucagon activates hormone-sensitive lipase, which results in high amounts of FA reaching the liver. Ketone bodies production is increased. So the aim of ketone bodies production is to regenerate CoA that is essential for β oxidation to keep running and producing energy (in the form of NADH and FADH2). So, we notice that in the liver, the acetoacetate that result from ketone bodies synthesis is not important (waste product) and the important one is CoA. During uncontrolled diabetic, insulin level is low and glucagon level is high. So when the blood glucose is high, the liver still do gluconeogenesis because the liver cannot sense the concentration of glucose, it only sense the hormones (insulin and glucagon) and as we said that in uncontrolled diabetic, the insulin hormone level is low which will give indication to liver that the blood glucose is low however, it is actually very high and the liver keep doing gluconeogenesis!. Also, lipolysis is active and that will increase the free fatty acids in the plasma. In the liver, these fatty acids will convert to ketone bodies ( note that two of them are acids, acetoacetate and 3 hydroxybutryic acid) and if those acids are produced in large amount, this will lead to decrease the ph of blood. A state called ketoacidosis. and because this happens in diabetes, we called this state diabetic ketoacidosis (DKA) The diabetic ketoacidosis (DKA) is very common in uncontrolled diabetes, and high ketone bodies concentration in the plasma that will excreted with urine with sodium salt. Loss of sodium > Loss of water > Dehydration, that lead to bad general condition. Also, it leads to coma (loss of conscious) and that may lead to death. 9 P a g e
.. The ketone bodies produced during prolonged fasting and in uncontrolled diabetes are released to the plasma and taken up by other tissues, like muscle tissue. Hydroxybutyrate can be oxidized to acetoacetate, then acetoacetate can be consumed but it requires CoA, it should be linked to CoA to produce Acetoacetyl CoA. Succinyl CoA (an intermediate in TCA) is the donor of CoA; it provides CoA for acetoacetate to be converted into Acetoacetyl CoA which can be cleaved into 2 acetyl CoA to be used in the citric acid cycle in the muscles. (So muscle use these acetyl CoA as a source of energy Note that skeletal muscle use Ketone bodies as a source of energy 10 P a g e