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 may not be used immediately, but converted into reserves. The role of such a reservoir is played by fats (lipids), consisting of glycerin and fatty acids. The latter are used by the cell as fuel. In this case, fatty acids are oxidized to CO 2 and H 2 O.
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 amounts of double bonds. If the latter are completely absent, fatty acids are called saturated (typical of 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 compounds with an odd number of links. The oxidation of these two types of compounds is slightly different.
general characteristics
The process of oxidation of fatty acids is complex and multi-stage. It begins with their penetration into the cell and ends in the respiratory chain. In this case, the final stages actually repeat the catabolism of carbohydrates (Krebs cycle, the transformation of the energy of the transmembrane gradient into a macroergic bond). The end products of the process are ATP, CO 2 and water.
The oxidation of fatty acids in a eukaryotic cell occurs in mitochondria (the most characteristic site of localization), peroxisomes, or the endoplasmic reticulum.
Varieties (types) of oxidation
There are three types of fatty acid oxidation: α, β, and ω. Most often, this process proceeds according to the β-mechanism and is localized in mitochondria. The omega pathway is a minor alternative to the β mechanism and occurs in the endoplasmic reticulum, and the alpha mechanism is characteristic of only one type of fatty acid (phytanic acid).
Biochemistry of the oxidation of fatty acids in mitochondria
For convenience, the process of mitochondrial catabolism is conditionally divided into 3 stages:
- activation and transportation to mitochondria;
- oxidation;
- oxidation of the formed acetyl coenzyme A through the Krebs cycle and electric transport chain.
Activation is a preparatory process that converts fatty acids into a form that is accessible for biochemical transformations, since these molecules themselves are inert. In addition, without activation, they cannot penetrate the membranes of mitochondria. This stage proceeds at the outer mitochondrial membrane.
Actually, oxidation is a key step in the process. It includes four stages, at the end of which the fatty acid is converted into acetyl-CoA molecules. The same product is formed during the utilization of carbohydrates, so that the subsequent steps are similar to the last stages of aerobic glycolysis. The formation of ATP occurs in the electron transfer 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 2 molecules are also formed, which also enter the respiratory chain as electron donors. As a result, the total energy yield 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 are inert in themselves and cannot be oxidized. Activation brings them into a form available for biochemical transformations. In addition, unchanged, these molecules cannot penetrate the mitochondria.
The essence of activation is the conversion of a fatty acid into its Acyl-CoA-thioether, which subsequently undergoes oxidation. This process is carried out by special enzymes - thiokinases (Acyl-CoA synthetases) attached to the outer mitochondrial membrane. The reaction proceeds in 2 stages, associated with the energy expenditure of two ATP.
Three components are required for activation:
First, the fatty acid interacts with ATP to form acyladenylate (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 derivative of a fatty acid, which is transported to mitochondria.
Transport to mitochondria
This stage is called transesterification with carnitine. The transfer of acyl-CoA to the mitichondrial matrix is through the pores with the participation of carnitine and special enzymes - carnitine acyltransferases.
For transport across membranes, CoA is replaced with carnitine to form acyl-carnitine. This substance is transferred to the matrix by facilitated diffusion with the participation of an acyl-carnitine / carnitine carrier.
Inside the mitochondria, a reverse reaction is carried out, consisting in the detachment of the retinal, again entering the membranes, and the restoration of acyl-CoA (in this case, "local" coenzyme A is used, and not the one with which the connection was formed at the activation stage).
The main reactions of fatty acid oxidation according to the β-mechanism
The simplest type of energy utilization of fatty acids is β-oxidation of double-bonded chains in which the number of carbon units is even. As a substrate for this process, as noted above, acyl coenzyme A.
The process of β-oxidation of fatty acids consists of 4 reactions:
- Dehydrogenation is the removal of hydrogen from a β-carbon atom with the appearance of a double bond between chain links located in the α and β positions (first and second atoms). The result is enoyl-CoA. The reaction enzyme is acyl-CoA dehydrogenase, which acts in conjunction with the coenzyme FAD (the latter is reduced to FADH2).
- Hydration is the attachment of a water molecule to enoyl-CoA, resulting in the formation of L-β-hydroxyacyl-CoA. It is carried out by enoyl-CoA hydratase.
- Dehydrogenation is the oxidation of the product of the previous reaction with a NAD-dependent dehydrogenase to form β-ketoacyl-coenzyme A. In this case, NAD is restored to NADH.
- Cleavage of β-ketoacyl-CoA to acetyl-CoA and acyl-CoA shortened by 2 carbon atoms. The reaction is carried out under the action of thiolase. A prerequisite is the presence of free HS-CoA.
Then it all starts again with the first reaction.
A cyclic repetition of all stages is carried out until the entire carbon chain of the fatty acid is converted into acetyl coenzyme A molecules.
The formation of Acetyl-CoA and ATP on the example of the oxidation of palmitoyl-CoA
At the end of each cycle, acyl-CoA, NADH and FADH2 molecules are formed in a single quantity, and the chain of the acyl-CoA-thioether becomes shorter by two atoms. By transferring electrons to the electric transport circuit, FADN2 gives one and a half molecules of ATP, and NADH - two. As a result, 4 ATP molecules are obtained from one cycle, not counting the energy output of acetyl-CoA.
The chain of palmitic acid includes 16 carbon atoms. This means that at the oxidation stage 7 cycles should take place with the formation of eight acetyl-CoA, and the energy output from NADH and FADN 2 in this case will be 28 ATP molecules (4 × 7). The oxidation of acetyl-CoA also goes to the generation of energy, which is stored as a result of the Krebs cycle products entering the electric transport circuit.
The total yield of the oxidation stages and the Krebs cycle
The oxidation of acetyl-CoA yields 10 ATP molecules. Since the catabolism of palmitoyl-CoA gives 8 acetyl-CoA, the energy yield will be 80 ATP (10 × 8). If you add this to the oxidation result of NADH and FADN 2 , you get 108 molecules (80 + 28). From this amount, subtract 2 ATP, which went into the activation of fatty acids.
The final equation for the oxidation of palmitic acid will be: palmitoyl-CoA + 16 O 2 + 108 Pi + 80 ADP = CoA + 108 ATP + 16 CO 2 + 16 H 2 O.
Energy Calculation
The energy exhaust from 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 number of ATP generated during each cycle due to NADH and FADN2, (n / 2 - 1) is the number of cycles, n / 2 × 10 is the energy output from the oxidation of acetyl-CoA, and 2 is the activation cost.
Reaction features
The oxidation of unsaturated fatty acids has some features. Thus, the complexity of the oxidation of double-bond chains lies in the fact that the latter cannot be exposed to enoyl-CoA hydratase due to their cis position. This problem is eliminated by enoyl-CoA isomerase, due to which the bond takes on 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 are oxidized in the same way as saturated acids.
Enoyl CoA isomerase is sometimes insufficient to continue the process. This applies to chains in which the configuration of cis9-cis12 is present (double bonds at the 9th and 12th carbon atoms). Here, the interference is not only the configuration, but also the position of the double bonds in the chain. The latter is corrected by the enzyme 2,4-dienoyl-CoA reductase.
Odd Atomic Fatty Acid Catabolism
This type of acid is characteristic of most lipids of natural (natural) origin. This creates a certain complexity, since each cycle involves shortening by an even number of links. For this reason, cyclic oxidation of higher fatty acids of this group continues until a 5-carbon compound appears as a product, which is split into acetyl-CoA and propionyl-coenzyme A. Both compounds enter another cycle of three reactions, resulting in succinyl-CoA . 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, ending with the formation of the product in the form of acetyl-CoA, but it has several key differences. So, the hydrogen cleaved at the dehydrogenation stage does not restore FAD, but switches to oxygen with the formation of hydrogen peroxide. The latter is immediately cleaved by catalase. As a result, the energy that could be used to synthesize ATP in the respiratory chain is dissipated as heat.
The second important difference is that some peroxisome enzymes are specific for certain rare fatty acids and are absent in the mitochondrial matrix.
The peculiarity of peroxisome of liver cells is that there is no enzyme apparatus of the Krebs cycle there. Therefore, as a result of beta oxidation, short-chain products are formed, which are transported to mitochondria for oxidation.