Fatty acids

Fatty acids are composed of hydrocarbon chains ending with carboxylic acid groups. Fatty acids and their associated derivatives are the major constituents of lipids. The length and degree of saturation of the hydrocarbon chain is highly variable between each fatty acid and dictates the associated physical properties (e.g. melting point and fluidity). In addition, fatty acids are responsible for the hydrophobic (water-insoluble) properties of lipids.

Function of fatty acids

Fatty acids have important roles in: 1) signal transduction pathways; 2) cellular fuel sources; 3) hormone and lipid composition; 4) protein modification; and 5) energy storage within the body. tissue adipose tissue (specialised fat cells) in the form of triacylglycerols.

Biological signalling

Fatty acids are involved in a wide range of biological signalling pathways. Following dietary intake of polyunsaturated lipids, the products of lipid peroxidation can function as precursors of powerful signalling mediators. Examples of such signalling include eicosanoid production, LDL peroxidation and modulation of metabolic and neurological pathways.

Of particular importance is the role of fatty acids in the formation of eicosanoids, which are a group of signalling molecules involved in the immune response. Eicosanoids consist of 20-carbon polyunsaturated fatty acids that form the precursors of several molecules responsible for platelet aggregation, chemotaxis and growth factors. Dietary intake of polyunsaturated fatty acids can also result in LDL peroxidation. When peroxidised LDL is engulfed by macrophages, the resulting immune activation can lead to the development of atherosclerosis. In addition, increased intake of cholesterol, saturated and trans fatty acids has been linked to the development of several cardiovascular diseases.

In contrast to the negative effects of LDL-cholesterol, saturated and trans fatty acids, the intake of monounsaturated and polyunsaturated ω-3 and ω-6 fatty acids is associated with anti-inflammatory effects. In particular, these fatty acids increase the uptake of circulating LDL by the liver and reduce leukocyte activation and platelet reactivity, lymphocyte proliferation and blood pressure. In addition, polyunsaturated fats are also necessary for normal growth and development, as well as for the regulation of visual acuity and cognition in the nervous system central. Other beneficial effects of polyunsaturated fatty acids have been observed with respect to cancer cell inhibition, proliferation and anti-tumour effects in animal models.

Fatty acid metabolism as a fuel source.

Fatty acid metabolism involves the uptake of free fatty acids by cells via fatty acid-binding proteins that transport fatty acids intracellularly from the plasma membrane. The free fatty acids are then activated by acyl-CoA and transported to: 1) mitochondria or peroxisomes to be converted to ATP and heat as a form of energy; 2) facilitate gene expression by binding to transcription factors3) the endoplasmic reticulum for esterification into various kinds of lipids that can be used for energy storage.

When used as an energy source, fatty acids are released from triacylglycerol and processed into two-carbon molecules identical to those formed during the breakdown of glucose; in addition, two-carbon molecules generated from the breakdown of fatty acids and glucose are used to generate energy through the same pathways. Glucose can also be converted into fatty acids under conditions of excess glucose or energy within a cell.

Energy storage

Fatty acids are also used as a form of energy storage, as droplets of fat triacylglycerol compounds hydrophobic within specialised fat cells called adipocytes. When stored in this form, fatty acids are important sources of thermal and electrical insulation, as well as protection against mechanical compression. Fatty acids are the preferred form of energy storage over glucose because they produce approximately six times the amount of usable energy. Storage in the form of triacylglycerol molecules consists of three fatty acid chains attached to a molecule from glycerol.

Formation of cell membranes

One of the most critical functions of fatty acids is the formation of cell membranes. cell membranewhich envelops all cells and associated intracellular organelles. In particular, cell membranes are composed of a phospholipid bilayer made up of two fatty acid chains linked to glycerol and a phosphate group hydrophilic attached to a smaller hydrophilic compound (e.g. choline). Therefore, each molecule of phospholipid has a hydrophobic tail made up of two fatty acid chains and a head hydrophilic head composed of the phosphate group. Cell membranes are formed when two phospholipid monolayers associate with tails that are bound together in an aqueous solution to create a phospholipid bilayer.

An important characteristic of cell membranes is membrane fluidity, which refers to the viscosity of the lipid membrane. Membrane fluidity is influenced by the diversity of lipid chains that make up the cell membrane, including chain length and level of saturation. When membrane fluidity changes, the function and physical characteristics of the membrane are also altered. For example, increased consumption of ω-3 fatty acids can increase the level of EPA and DHA in cell membranes. When such incorporation occurs in the cell membranes of retinal cells, light transduction is enhanced. In addition, increased accumulation of ω-3 fatty acids in red blood cell membranes results in increased membrane flexibility, potentially resulting in improved microcirculation.

Protein modification

Fatty acids play several critical roles through their interaction with various proteins. Protein acylation is an important function of polyunsaturated fatty acids, as it is essential for the anchoring, folding and function of multiple proteins. In addition, fatty acids can also interact with various nuclear receptor proteins and promote gene expression, as several fatty acid-protein complexes function as transcription factors. In this way, fatty acids have been found to regulate the transcription of genes related to metabolism, cell proliferation and apoptosis.

Types of fatty acids

Unsaturated fatty acids (polyunsaturated and monounsaturated)

Monounsaturated fatty acids contain a carbon-carbon double bond, which can be found at different positions along the fatty acid chain. Most monounsaturated fatty acids are between 16 and 22 carbons in length and contain a cis-double bond; this means that the atoms of hydrogen atoms are oriented in the same direction, introducing a bend in the molecule. In addition, the cis-configuration is associated with thermodynamic instability and therefore a lower melting point compared to trans and saturated fatty acids.

Polyunsaturated fatty acids contain more than one double bond. When the first double bond is located between the third and fourth or the sixth and seventh atoms of carbon of the carbon-oxygen bond; they are called ω-3 and ω-6 fatty acids, respectively. Polyunsaturated fatty acids are produced only by plants and the phytoplanktonand are essential for all higher organisms.

Saturated

Saturated fatty acids are hydrogen saturated and most are linear hydrocarbon chains with an even number of carbon atoms. The most common fatty acids contain 12 to 22 carbon atoms.

Long chain

Long-chain fatty acids (C16 and above) can be saturated or mono/polyunsaturated depending on the presence of one or more double bonds in the carbon chain. Oleate is the most abundant long-chain monounsaturated fatty acid; with a chain length of 18 carbons and a double bond located between C9 and C10 from the methyl end (C18: 1n-9). In addition, long-chain fatty acids are insoluble in water and circulate through the plasma as an esterified complex; triacylglycerols or non-esterified forms weakly bound to albumin.

Short chain

Short-chain fatty acids are the main end products of bacterial metabolism in the human large intestine. Furthermore, while short-chain fatty acids are formed from various precursors by anaerobic microorganisms, carbohydrates are the most common progenitors of short-chain fatty acids.

Structure of fatty acids

Fatty acids are made up of carbon chains containing a methyl group at one end and a carboxyl group on the other. The methyl group is called the omega (ω) group and the carbon atom next to the carboxyl group is called the “α” carbon, followed by the “β” carbon, and so on. Fatty acid molecules also have two chemically distinct regions: 1) a long hydrophobic hydrocarbon chain, which is not very reactive; and 2) a carboxyl group (-COOH), which is hydrophilic and highly reactive. In the cell membrane, virtually all fatty acids form covalent bonds with other molecules via the carboxylic acid groups.

As described above, fatty acids may contain double bonds (unsaturated fatty acids) or no double bonds (saturated fatty acids) in the hydrocarbon chains. The presence of double bonds results in the formation of bends or kinks in the molecules and impacts the ability of fatty acid chains to stack. Other differences between fatty acids include the length of the hydrocarbon chains, as well as the number and position of double bonds. The presence of the double bond will also influence the melting point; unsaturated fatty acids have a lower melting point than saturated fatty acids. The melting point is also influenced by whether there is an odd or even number of carbon atoms; an odd number of carbons is associated with a higher melting point. In addition, saturated fatty acids are very stable.

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