The main condition for the life of any organism is the continuous supply of energy, which is spent on various cellular processes. At the same time, a certain part of the nutrient compounds can not be used immediately, but can be converted into reserves. The role of such a reservoir is performed by fats (lipids), consisting of glycerol and fatty acids. The latter are used by the cell as fuel. In this case, fatty acids are oxidized to CO2 and H2O.
Fatty acid basics
Fatty acids are carbon chains of various lengths (from 4 to 36 atoms), which are chemically classified as carboxylic acids. These chains can be either branched or unbranched and contain different numbers of double bonds. If the latter are completely absent, fatty acids are called saturated (typical for many lipids of animal origin), and otherwise -unsaturated. According to the arrangement of double bonds, fatty acids are divided into monounsaturated and polyunsaturated.
Most chains contain an even number of carbon atoms, which is due to the peculiarity of their synthesis. However, there are connections with an odd number of links. The oxidation of these two types of compounds is slightly different.
General characteristics
The process of fatty acid oxidation is complex and multi-stage. It begins with their penetration into the cell and ends in the respiratory chain. At the same time, the final stages actually repeat the catabolism of carbohydrates (the Krebs cycle, the transformation of the energy of the transmembrane gradient into a macroergic bond). The final products of the process are ATP, CO2 and water.
Oxidation of fatty acids in a eukaryotic cell is carried out in mitochondria (the most characteristic localization site), peroxisomes or endoplasmic reticulum.
Varieties (types) of oxidation
There are three types of fatty acid oxidation: α, β and ω. Most often, this process proceeds by the β-mechanism and is localized in mitochondria. The omega pathway is a minor alternative to the β-mechanism and is carried out in the endoplasmic reticulum, while the alpha mechanism is characteristic of only one type of fatty acid (phytanic).
Biochemistry of fatty acid oxidation in mitochondria
For convenience, the process of mitochondrial catabolism is conventionally divided into 3 stages:
- activation and transport to mitochondria;
- oxidation;
- oxidation of the formed acetyl-coenzyme A through the Krebs cycle and the electric transport chain.
Activation is a preparatory process that transforms fatty acids into a form available for biochemical transformations, since these molecules themselves are inert. In addition, without activation, they cannot penetrate the mitochondrial membranes. This stage takes place at the outer membrane of the mitochondria.
Actually, oxidation is a key step in the process. It includes four stages, after which the fatty acid is converted into Acetyl-CoA molecules. The same product is formed during the utilization of carbohydrates, so that the further steps are similar to the last steps of aerobic glycolysis. The formation of ATP occurs in the electron transport chain, where the energy of the electrochemical potential is used to form a macroergic bond.
In the process of fatty acid oxidation, in addition to Acetyl-CoA, NADH and FADH molecules are also formed2, which also enter the respiratory chain as electron donors. As a result, the total energy output of lipid catabolism is quite high. So, for example, the oxidation of palmitic acid by the β-mechanism gives 106 ATP molecules.
Activation and transfer to the mitochondrial matrix
Fatty acids themselves are inert and cannot be oxidized. Activation brings them into a form available for biochemical transformations. In addition, these molecules cannot enter the mitochondria unchanged.
The essence of activation isthe conversion of a fatty acid into its Acyl-CoA-thioester, which subsequently undergoes oxidation. This process is carried out by special enzymes - thiokinases (Acyl-CoA synthetases) attached to the outer membrane of mitochondria. The reaction proceeds in 2 stages, associated with the expenditure of energy of two ATP.
Three components are required for activation:
- ATF;
- HS-CoA;
- Mg2+.
First, the fatty acid reacts with ATP to form acyladenylate (an intermediate). That, in turn, reacts with HS-CoA, the thiol group of which displaces AMP, forming a thioether bond with the carboxyl group. As a result, the substance acyl-CoA is formed - a fatty acid derivative, which is transported to the mitochondria.
Transportation to mitochondria
This step is called transesterification with carnitine. The transfer of acyl-CoA to the mitochondrial matrix is carried out through the pores with the participation of carnitine and special enzymes - carnitine acyltransferases.
For transport across membranes, CoA is replaced by carnitine to form acyl-carnitine. This substance is transported into the matrix by acyl-carnitine/carnitine transporter facilitated diffusion.
Inside the mitochondria, an inverse reaction takes place, consisting in the detachment of retinal, which again enters the membranes, and the restoration of acyl-CoA (in this case, the "local" coenzyme A is used, and not the one with which the bond was formedat the activation stage).
Main reactions of fatty acid oxidation by β-mechanism
The simplest type of energy utilization of fatty acids is β-oxidation of chains that do not have double bonds, in which the number of carbon units is even. The substrate for this process, as noted above, is acyl coenzyme A.
The process of β-oxidation of fatty acids consists of 4 reactions:
- Dehydrogenation is the splitting off of hydrogen from a β-carbon atom with the formation of a double bond between chain links located in α and β-positions (first and second atoms). As a result, enoyl-CoA is formed. The reaction enzyme is acyl-CoA dehydrogenase, which acts in combination with the coenzyme FAD (the latter is reduced to FADH2).
- Hydration is the addition of a water molecule to enoyl-CoA, resulting in the formation of L-β-hydroxyacyl-CoA. Carried out by enoyl-CoA-hydratase.
- Dehydrogenation - oxidation of the product of the previous reaction by NAD-dependent dehydrogenase with the formation of β-ketoacyl-coenzyme A. In this case, NAD is reduced to NADH.
- Cleavage of β-ketoacyl-CoA to acetyl-CoA and a 2-carbon shortened acyl-CoA. The reaction is carried out under the action of thiolase. A prerequisite is the presence of free HS-CoA.
Then everything starts again with the first reaction.
The cyclic repetition of all stages is carried out until the entire carbon chain of the fatty acid is converted into molecules of acetyl-coenzyme A.
Formation of Acetyl-CoA and ATP on the example of palmitoyl-CoA oxidation
At the end of each cycle, acyl-CoA, NADH and FADH2 molecules are formed in a single amount, and the acyl-CoA-thioether chain becomes shorter by two atoms. By transferring electrons to the electrotransport chain, FADH2 gives one and a half ATP molecules, and NADH two. As a result, 4 ATP molecules are obtained from one cycle, not counting the energy yield of acetyl-CoA.
The palmitic acid chain has 16 carbon atoms. This means that at the stage of oxidation 7 cycles should be carried out with the formation of eight acetyl-CoA, and the energy yield from NADH and FADH2 in this case will be 28 ATP molecules (4×7). The oxidation of acetyl-CoA also goes to the formation of energy, which is stored as a result of the products of the Krebs cycle entering the electric transport chain.
Total yield of oxidation steps and Krebs cycle
As a result of the oxidation of acetyl-CoA, 10 ATP molecules are obtained. Since the catabolism of palmitoyl-CoA yields 8 acetyl-CoA, the energy yield will be 80 ATP (10×8). If you add this to the result of the oxidation of NADH and FADH2, you get 108 molecules (80+28). From this amount, 2 ATP should be subtracted, which went to activate the fatty acid.
The final equation for the oxidation of palmitic acid will be: palmitoyl-CoA + 16 O2 + 108 Pi + 80 ADP=CoA + 108 ATP + 16 CO 2 + 16 H2O.
Calculation of energy release
Energy exhauston the catabolism of a particular fatty acid depends on the number of carbon units in its chain. The number of ATP molecules is calculated by the formula:
[4(n/2 - 1) + n/2×10] - 2, where 4 is the amount of ATP generated during each cycle due to NADH and FADH2, (n/2 - 1) is the number of cycles, n/2×10 is the energy yield from the oxidation of acetyl-CoA, and 2 is the cost of activation.
Features of reactions
Oxidation of unsaturated fatty acids has some peculiarities. Thus, the complexity of the oxidation of chains with double bonds lies in the fact that the latter cannot be exposed to enoyl-CoA-hydratase due to the fact that they are in the cis position. This problem is eliminated by enoyl-CoA isomerase, due to which the bond acquires a trans configuration. As a result, the molecule becomes completely identical to the product of the first stage of beta-oxidation and can undergo hydration. Sites containing only single bonds oxidize in the same way as saturated acids.
Sometimes enoyl-CoA-isomerase is not enough to continue the process. This applies to chains in which the cis9-cis12 configuration is present (double bonds at the 9th and 12th carbon atoms). Here, not only the configuration is a hindrance, but also the position of the double bonds in the chain. The latter is corrected by the enzyme 2,4-dienoyl-CoA reductase.
Catabolism of odd fatty acids
This type of acid is typical for most lipids of natural (natural) origin. This creates a certain complexity, since each cycleimplies shortening by an even number of links. For this reason, the cyclic oxidation of the higher fatty acids of this group continues until the appearance of a 5-carbon compound as a product, which is cleaved into acetyl-CoA and propionyl-coenzyme A. Both compounds enter another cycle of three reactions, as a result of which succinyl-CoA is formed. It is he who enters the Krebs cycle.
Features of oxidation in peroxisomes
In peroxisomes, fatty acid oxidation occurs via a beta mechanism that is similar, but not identical, to mitochondrial. It also consists of 4 stages, culminating in the formation of the product in the form of acetyl-CoA, but it has several key differences. Thus, the hydrogen split off at the dehydrogenation stage does not restore FAD, but passes to oxygen with the formation of hydrogen peroxide. The latter immediately undergoes cleavage under the action of catalase. As a result, energy that could have been used to synthesize ATP in the respiratory chain is dissipated as heat.
The second important difference is that some peroxisome enzymes are specific to certain less abundant fatty acids and are not present in the mitochondrial matrix.
Feature of peroxisomes of liver cells is that there is no enzymatic apparatus of the Krebs cycle. Therefore, as a result of beta-oxidation, short-chain products are formed, which are transported to the mitochondria for oxidation.
