Overview of Lipids
Lipids are a class of molecules that aren’t defined by a chemical structure, but rather by a common property: they’re all extremely hydrophobic, i.e., insoluble in water. And though they are chemically diverse, they share common building blocks.
Three representative building blocks for lipids: a fatty acid molecule, glycerol and a sphingosine.
Fatty acids are the lipid tails made up of a polar carboxyl group and a hydrophobic hydrocarbon chain or tail. They have variable length tails, though that number is usually even (due to the mechanism of fatty acid synthesis) with lengths between 12 and 24 carbons. They come in two types: saturated and unsaturated.
Saturated Fatty Acids have only single bonds between the carbons, meaning all four valence electrons are bonded to separate atoms (C, C, H, H). The chain is linear. Saturated fatty acids are found in animal fats.
Unsaturated Fatty Acids have at least one carbon-carbon double bond. Their chains can become bent, usually in the cis conformation. Trans is possible too, which as we know leads to a very unhealthy fat: “trans-fat”. Trans fats do not bend as much and tend to resemble straight chains. Unsaturated fatty acids are found in vegetable oils.
(Adding a few extra tidbits from the Nelson/Cox biochemistry book here since this lecture was rather frantic and cartoonish)
In fatty acids, the double bond is almost always between C9 and C10. For polyunsaturated fats, C12 and C15 are usually double. The double bonds are never adjacent.
Also of interest in the polyunsaturated fatty acids (PUFAs) are the double bonds near the methyl end of the tail, or the omega (ω) end. The convention with PUFAs is to number the carbons from the tail to the head. From this, those tails with a double bond between C-3 and C-4 are called the omega-3 (ω-3) fatty acids. Those with a bond between C-6 and C-7 are the omega-6 fatty acids.
(back to the lecture “minutes’)
Fatty acids interact via weak van der Waals interactions along their hydrocarbon chain. As the number of van der Waals interactions increase, so increases the energy needed for the molecules to break free. This means a higher melting point.
The van der Waals interactions arise from tail length (more surface to interact with) and increased saturation (because they pack tighter: linear vs. bent). Thus, saturated carbons have a higher melting point than unsaturated.
Example: butter (saturated fat) is solid at room temperature. Olive oil is liquid. Butter has the higher melting point.
When a lipid is formed, fatty acids tails react with different backbone molecules to create various linkages.
For example, the carboxyl groups in fatty acids can react with the hydroxyl groups in glycerol to form ester linkages.
Ester and phosphate groups:
They can also react with phosphate group and link to the hydroxyl group on either backbone molecule to form a phosphodiester linkage.
A fatty acid carboxyl group can also link to the amino group of a sphingosine forming an amide bond.
The most common lipid found in cells is the triacylglycerol (TAG), a.k.a., triglyceride: tri-three, acyl-an acyl group, and glycerol-the backbone molecule with three alcohol groups.
They needn’t be the same fatty acids, or even line up in parallel. TAGs are simple when the fatty acids are the same. Mixed otherwise.
In a TAG, each carboxyl head of each fatty acid reacts with one of the alcohol groups of the glycerol to form an acyl functional group as part of a new ester linkage.
Because the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages, TAGs are nonpolar, hydrophobic, and insoluble in water. This hydrophobic nature of TAGs has many consequences such as the amount of energy they yield (by oxidization) and the way they are stored in cells.
Membranes separate the contents of cells from the extracellular environment formed by structural lipids. These membranes also embed membrane proteins. Inside a cell, a different type of lipid-based membrane provides storage compartments essential for cellular function.
Two major types of structural lipids are glycerophospholipids and sphingolipids.
So at the C3 of the glycerol there’s a phosphate group. These connect via a phosphodiester linkage to a variety of head groups (X above). When that head group is just hydrogen, it’s called phosphatidic acid. Other glycerophospholipids are derivatives of this. Glycerophospholipids are the main structural component in cell membranes (note that the head group is polar).
In sphingolipids, one of the hydrophobic tails is the sphingosine itself. The other is a fatty acid connected at C2 in an amide linkage. A variable group X forms at C1.
“Sphingolipids were discovered in the 1870s in brain extracts and were named after the Sphinx because of their enigmatic nature” – Wikipedia.
The variable group in sphingolipids is often a sugar molecule, which creates structural and functional diversity.
As shown above, both of these lipids are amphipathic with a hydrophobic side and a polar side. This is key to membrane lipids. A pair of such lipids face one another to form a hydrophobic core and while each of the hydrophilic heads face the aqueous environment.
These membranes can be spherical too, which means the lipids need to bend. While many lipids are cylindrical, some are conical (phosphatidylethanolamine, e.g.), which allows for the curves in the membrane surface. These conical lipids will have a smaller head group and unsaturated fatty acids that bend. They also usually appear on the inner membrane.
FYI: there are over 1000 lipid species in Eukaryotic cells.
The quick tour of lipids ends with cholesterol, which is rather different than the others mentioned. It’s flat with four rings and only has one polar group, making it very hydrophobic. Cholesterol often embeds itself between other membrane lipids, which affects the mobility of the membrane.