Lipids are organic molecules that are composed of the elements carbon, hydrogen and oxygen, with the hydrogen to oxygen ratio greater than 2:1. They are highly hydrophobic because of having a long hydrocarbon chain. This family of compound includes fats, waxes, steroids and phospholipids. This article focuses more on the metabolic processes involved in processing fats and fatty acids.
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Fries cooking

Image: “Fries or chips cooking in oil in a chip-pan.” by Hayford Peirce – Own work. License: CC BY-SA 3.0


Fats and Oils

Fats and oils are the most common forms of lipids. Fat is a natural lipid material that is normally solid at room temperature. Oils are quite similar to fats except that they are generally liquid at room temperature.

These two materials are integral parts of the human diet. They are normally used as a shortening agent in food processing. It normally provides lubrication in some food products, and may also give structure to baked materials such as cakes and doughnuts. Oils are also used for frying different food products. Because of the high heat transfer properties of oil, it is normally used to produce foods that are crisp and firm.

Aside from its function as a shortening agent, oil also contributes a great amount in terms of the taste of the food. When food products are fried, certain oxidation reactions occur producing a complex set of possible flavors. Olive oil, for example, is a good oil to use for gourmet food products and in pastas and salad dressing. Coconut and palm oil, on the other hand, are very good for frying.

The last main important purpose of fats and oil is that because of their dense nature, they are easily stored sources of calories. Also, because of their non-polar nature, they can be used to transport various fat-soluble vitamins such as vitamins A, D and E. In the human body, lipids also play a big role in the synthesis of lipid hormones. Different polyunsaturated fatty acids are needed by the body to produce prostaglandins.

Fats and oils are generally called triacylglycerides. They mainly function for energy storage. The general structure of a triacylglyceride is a fatty acid trimester of glycerol.

Lipid Metabolism

Lipid metabolism can be divided into two major types. One if these is lipogenesis, which involves the synthesis of lipids and fatty acid oxidation in which energy is produced when fatty acids are metabolized. The metabolism of lipids is closely related to the metabolic processes for carbohydrates. This is because there are times when carbohydrates are metabolized, sometimes forming fats.

Triacylglycerols are transported in body fluids by molecules called lipoproteins. The general configuration of a lipoprotein involves having the hydrophobic region inside the core of the sphere, while the superficial layer is composed of the hydrophilic regions.

At different parts of the digestive tract, metabolism of triacylglycerols may occur. The mechanism of metabolism is facilitated by a group of enzymes called lipases. Lipases catalyze the hydrolysis of the ester bond of the triacylglycerol.

Lipases are generally classified as intracellular or extracellular. Intracellular lipases include hormone sensitive lipase in adipocytes and lysosomal lipases. Extracellular lipases, on the other hand, include pancreatic lipase and lipoprotein lipase.

Hormones control the activity of a hormone sensitive lipase. An example of this is the increase in lipase activity when an individual suffers from stress. The stress triggers a release of the hormones adrenaline/noradrenaline.

The hormone attaches itself to a receptor which eventually leads to processing of inactive lipases to convert them to active ones. Active lipases separate the triacylglycerol into glycerol and free fatty acids. The fatty acids produced are taken up from the blood plasma by tissues that require nutrients.

On the other hand, glycerol, since it cannot be utilized in adipocytes, serves as a substrate for gluconeogenesis in the liver.

Glycerol is acted upon by glycerol kinase, consuming one ATP in the process, to produce L-glycerol-3-phosphate (L-G3P). L-G3P will be converted to a dihydryacetone phosphate using glycerol phosphate dehydrogenase. An NAD+ is also converted to NADH in the process.

Finally, the dihydroxyacetone phosphate readily isomerizes to form the D-glyceraldehyde-3-phosphate.

To be able to synthesize triacylglycerol back, a glycerol-3-phosphate can be processed by first reacting it with a fatty acid in the presence of an acytransferase and in the expense of an FAD molecule. Addition of one fatty acid leads to the production of a lysophosphatidic molecule. The process can be repeated to add another fatty acid to glycerol, forming a phosphatidic acid.

The next step is to remove the phosphate group using phosphatase. The product for this step is a diacylglycerol. Another fatty acid may then esterify with the last OH group of the diacylglycerol to produce the triacylglycerol.

Fatty Acid Metabolism

Fatty acids are one of the major forms of storage of metabolic energy. They are good sources of energy because they are mainly composed of –CH2 groups which can be readily reduced. Because of their hydrophobic nature, they need not to be solvated, compared to carbohydrates. The long hydrophobic tail of fatty acids can be packed tightly in storage tissues. This allows for concentrated storage.

Beta Oxidation

Image: “1st step of beta oxidation.” by NEUROtiker – Own work. License: Public Domain

One of the important metabolic processes that occur in fatty acids is β-oxidation. Here, short fatty acid chains are transported into the mitochondrial matrix as free acids. As they enter the mitochondrial matrix, they are activated by the enzyme acyl-CoA synthetase to convert it into acyl-CoA. Long chains of fatty acids move to inner parts of the mitochondrial membrane and are converted to acyl-carnitine. The acyl group of the molecule is then transferred to CoA to form acyl-CoA.

Beta Oxidation 2nd step

Image: “2nd step of beta oxidation.” by NEUROtiker – Own work. License: Public Domain

The saturated fatty acyl-CoA molecule will be degraded in a series of four reactions that are part of the β-oxidation. The first reaction involves oxidizing the molecule through the α and β-carbon forming a double bond. Acyl-CoA dehydrogenase catalyzes this process in the expense of a FAD molecule. The product of this process is a trans-Δ2-enoyl CoA.

Beta Oxidation 3rd step

Image: “3rd step of beta oxidation.” by NEUROtiker – Own work. License: Public Domain

In the second step, the trans-Δ2-enoyl CoA will be hydrated in presence of an enoyl-CoA hydratase forming an L-3-hydroxyacyl CoA. It is then oxidized further to 3-ketoacyl CoA using L-3-hydroxyacyl CoA dehydrogenase as a catalyst. This process proceeds using up one NAD+ to form NADH and H+.

Beta Oxidation 4th step

Image: “4th step of beta oxidation.” by NEUROtiker – Own work. License: Public Domain

The final step is catalyzing by thiolase. A cysteine residue in the enzyme attacks the β-keto carbon atom. Bond cleavage occurs producing the enolate of acetyl CoA and a thioester intermediate. The final product of β-oxidation is a 2-carbon shorter acyl-CoA. These β-oxidation steps are successively repeated until enough acetyl CoA units are produced.

Fatty Acid Biosynthesis

Fatty acid synthesis follows the same sequence as β-oxidation, except it is in the reverse order. Here, NADPH acts as a reductant to reduce the ketoacyl and enoyl intermediate of the process. The elongation step is driven by decarboxylation. This involves a reverse of the thiolase reaction in β-oxidation. For the chain to elongate, a malonyl CoA, a dicarboxylate molecule, condenses with the growing acyl chain in the process, losing a free carboxylate molecule.

The whole synthesis process obtains enough energy from the decarboxylation step for it to occur.

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