1.3: Lipid tails and satiety (2023)

fatty acids

Most of the membrane's lipid tails are composed of esterified fatty acids. This means that the carboxylic acid group forms a bond with the glycerol backbone, resulting in an ester group. The hydrophobic tail consists of fatty acids. Fatty acids vary in both chain length and degree of saturation. The chain length is the number of carbons in the fatty acid. Fatty acids can be fully saturated, meaning the carbons are all connected by single bonds, or unsaturated, meaning the chain contains one or more double bonds. A given fatty acid can be named one of three ways based on its chain length and degree of saturation. First, many have common or trivial names. For example, a fully saturated 16-carbon fatty acid is palmitic acid. An 18-carbon fatty acid with a cis double bond at the 9th position is oleic acid. Another naming system is the shorthand structure. It consists of 2 numbers separated by a colon. The first number is the number of carbons in the chain, while the second is the number of double bonds. Additional numbers are added after a delta symbol (Δ) to indicate double bond positions. Since most biologically occurring unsaturated fatty acids have cis double bonds, nothing is added after the number to denote cis. However, when the double bond is trans, the number is followed by a "t". Palmitic acid and oleic acid are therefore 16:0 and 18:1^D9(Sitwell 2013). Finally, there is the naming system supported by IUPAC and IUBMB. In this system, the fatty acid assumes its name of carboxylic acid. A saturated fatty acid with 16 carbon atoms is hexadecanoic acid. A 9-position monounsaturated fatty acid containing 18 carbon atoms is 9Z-octadecenoic acid. The "9Z" at the beginning indicates where the double bond is and that it is cis. When naming a phospholipid or glycerolipid containing fatty acid tails, the tail name precedes the head group name. As fatty acids are esterified, the suffix is ​​changed from -oic to -oyl. The systematic name for a common phospholipid is 1-hexadecanoyl-2-(9Z-octadecenoyl)-legal-Glycero-3-phosphocholine (Fahy et al 2011). This has hexadecanoic acid at position 1 and 9Z-octadecenoic acid at position 2 of the glycerol backbone (Figure \(\PageIndex{2}\)A). Fatty acids can also be given an omega designation, determined by the position of the carbon double bond closest to the methyl terminus. An omega-3 fatty acid is one in which the 3rd and 4th methyl carbon atoms are linked by a double bond (Uttaro 2008).

Glycerophospholipid

The most common type of membrane lipids are glycerophospholipids or glycerolipids. Phosphoglycerides are prominent in higher animals (such as vertebrates), while glycosylglycerides are important in plants. There are three main classes of membrane lipids that are widely distributed - glycerolipids, sphingolipids and steroids. In terms of abundance, glycerolipids are the most important group. They can be divided into two main groups - Phosphoglycerides (with phosphorus) and Glycosylglycerides (without phosphorus but with a sugar portion). Phosphoglycerides are the main lipid components of most biological membranes. Phosphoglycerides consist of a wide and diverse range of structures. They are the primary lipid components in most membranes, except for photosynthetic membranes of plants, algae and cyanobacteria, and membranes of archaea. Normally, phosphoglycerides contain fatty acids esterified at the 1 and 2 positions of glycerol, making them diacylphosphoglycerides. The names of these lipids derive from the phosphate-linked moiety esterified at position 3 of glycerol. Therefore, the compounds are essentially derivatives of diacylglycerols in which the hydroxyl on carbon 3 is esterified with phosphoric acid. Furthermore, the simplest phosphoglyceride contains only phosphoric acid bound to diacylglycerol, called phosphatidic acid. (Gurr, 2005)

Glycosylglycerides are predominant components of photosynthetic membranes and play an important role in some microorganisms. Its structure is comparable to that of glycerophospholipids, with the sugars being glycosidically linked at position 3 of glycerol and the fatty acids esterified at the other two positions. Approximately 40% of the dry weight of the photosynthetic membranes of higher plants consists of two galactose-containing lipids - monogalactosyldiacylglycerol and digalactosyldiacylglycerol. There is a β bond in the first position of the galactose ring to glycerol, while there is an α,1,6 bond between the sugars in digalactosyldiacylglycerol. Many combinations of residues in diglycosyldiacylglycerols can be found in bacteria - the most common are two glucose, two galactose or two mannose residues linked α,1-2 or β,1-6.

In addition to galactose-containing lipids, a third glycosylglyceride, the plant sulfolipid, is found in chloroplasts. It is more formally referred to as sulfoquinovosylglycerol and contains a sulfonate moiety at carbon 6 of a deoxyglucose residue. This is a negatively charged naturally occurring molecule with a very stable sulfonic acid group. This sulfolipid is very characteristic of the photosynthetic membranes of chloroplasts and cyanobacteria. All chloroplast glycolipids are normally made up of large amounts of α-linolenic acid. Furthermore, in some plants, monogalactosyldiacylglycerol can have up to 97% of its total acyl groups as this single component. (Gurr, 2005)

sterols

Since the hydrophobic tail of a sterol is essentially a steroid, it is appropriately named. The -ol suffix is ​​added to indicate that it is a sterol (Figure \(\PageIndex{2}\)B).

Many sterols are important building blocks of many organisms, particularly their outer cell membranes. However, sterols are not necessary or critical components of all organisms, even those with significant sterol content. Sterols are common in eukaryotes but rare in prokaryotes. Yeasts and fungi have side-chain alkyl compounds, while plants and algae contain sitosterol and stigmasterol as their most common sterols. The primary function of the membrane sterol is the modulation of fluidity, which is resolved by the interaction of the sterol with the glycerolipid components.

sphingolipid

The hydrophobic tails ofsphingolipidconsist of a fatty acid and a long carbon chain at the base of the sphingoid. The name of the lipid is therefore a combination of the name of the sphingoid base and the fatty acid (Figure \(\PageIndex{2}\)C) (Fahy et al 2011).

fatty acid biosynthesis

fatty acid synthesis pathwaysare placed into two classes, Type I and II. The steps in the Type II pathway are catalyzed by separate enzymes. In type I, multienzymes contain multiple domains that catalyze various steps in the synthetic pathway. Type II occurs in eubacteria and organelles of prokaryotic origin (mitochondria and plastids), type I in eukaryotes (Schweizer and Hofmann 2004). Although the enzymatic organization is different, the basic steps of fatty acid synthesis are conserved between bacteria and eukaryotes and involve the sequential addition of acetyl groups from acetyl-CoA to a growing chain of fatty acids. To make saturated fatty acids, the addition of acetyl groups results in the formation of a chain of typically 16 or 18 carbon atoms (Figure \(\PageIndex{3}\)).

\[ 8 \text{Acetil-coA} + 7\text{ATP} + 14\text{NADPH} \rightarrow \text{Palmitinsäure} + 7\text{ADP} + 7\ce{P_i} + 14\text { NADP} + 6 \ce{H2O} + 8 \text{coA}\]

The synthesis of an unsaturated fatty acid in bacteria proceeds similarly to the synthesis of a saturated fatty acid down to the 10-carbon intermediate. At this point, a double bond is formed between the 3rd and 4th carbons of the chain (addition side numbering). After the cis double bond is added, addition of acetyl moieties continues up to 16 or 18 carbons, meaning the double bond is at position 9 or 11, respectively (Chan and Vogel 2010). In eukaryotes, double bonds are added after fatty acid synthesis. Enzymes that produce monounsaturated fatty acids preferentially introduce a double bond between carbons 9 and 10. This means 16:1^D9e 18:1^D9are common in lipid tails. Another group of enzymes adds double bonds near the methyl end of already unsaturated fatty acids. These preferentially create bonds between the 3rd and 4th carbon and between the 6th and 7th carbon of the methyl end (omega 3 and 6). Mammals lack the enzymes needed to add double bonds near the methyl end. Additional double bonds are added near the carboxyl end of fatty acids that already contain double bonds added by the other two classes of enzymes (Uttaro 2008). Thus, the degree of unsaturation and the placement of double bonds are determined by these enzymes.

Two fatty acid chains are joined at positions 1 and 2 of glycerol-3-phosphate to form phosphate acid (Figure \(\PageIndex{4}\)). This is further converted into various other lipids by the addition of various headgroups. Some bacteria are able to introduce double bonds into the fatty acid chain after lipid formation (Zhang and Rock 2008).

Membrane effects

Fluidity and Phase of the Membrane:The fluidity of a biological membrane is related to the melting temperature of the lipid tails it contains. Melting temperature is affected by both chain length and tail saturation. Longer chains result in a higher melting temperature because they can form stronger van der Waals forces between molecules. Higher levels of unsaturation result in a lower melting temperature. This is because the double bonds add kinks to the fatty acid chains that prevent the lipid molecules from sticking together tightly. In general, the degree of saturation has a greater effect on membrane fluidity than chain length (Sitwell 2013). Membranes with a single lipid composition exist in two phases. At high temperatures they are in a thin liquid crystalline phase. At low temperatures they are in a hard gel phase. In cholesterol-containing membranes, the gel phase is replaced by an ordered liquid phase that retains some fluidity (Komura and Andelman 2014). Lipids with saturated tails tend towards the ordered liquid phase, while lipids with unsaturated tails tend towards the liquid crystalline or disordered liquid phase. In the ordered liquid phase, the carbon-carbon bonds retain the transrotomers, which means that the tails straighten out as much as possible. The width of the hydrophobic portion of lipids with long saturated tails is greater than that of lipids with shorter or unsaturated tails because double bonds can prevent the tails from straightening fully and clumping together tightly. The difference in hydrophobic width can lead to hydrophobic incompatibility and phase separation (Komura and Andelman 2014).

Interactions with membrane proteins:Hydrophobic lipid tails are responsible for solubilizing the transmembrane portions of a membrane protein within the membrane, illustrating one of their many roles. A ring of lipids, called a lipid ring, surrounds the protein, and relatively flexible tails cover the surface of the protein (Lee 2011). Membrane properties conferred by the lipid tail, such as hydrophobic phase thickness, compressibility, and intrinsic curvature, can significantly affect protein folding and function within the membrane (Andersen et al. 2007).

quotes

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11. GChemPaint editor for chemical structures.http://savannah.nongnu.org/projects/gchempaint/
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Credits and Attributions

• Philip Day (UC Davis) e William Scott (UC Davis)