Chemistry of Fatty Acids and Lipids
Okay, well, that's quite broad. Maybe just using slide titles wasn't such a great idea :P I guess I'll have to look at the actual content of the slides and deduce the main points? I feel like I'm plagiarising here but I guess this is the best option that I have at the moment.
Apparently a lipid is "all material extractable from living systems by organic solvents." These include fats and other molecules such as steroid hormones etc.
Fatty acids are long hydrocarbon chains with a -COOH group at one end. The other end is known as the omega terminus.
Fatty acids can be saturated or unsaturated. Saturated fatty acids have only single bonds between carbon atoms and have as much hydrogen as they can hold (hence they are saturated). Unsaturated fatty acids have double bonds in their chains. Naturally occurring fatty acids are in the cis form, though some bacteria (I think) and other processes can convert fatty acids to the trans form. A monounsaturated fatty acid has only one double bond whereas a polyunsaturated fatty acid has multiple double bonds. Now you know what those labels mean when you buy food :D
Another way that unsaturated fatty acids can be classified is by looking at how far away the first double bond is from the omega terminus. For example, if the first double bond is located three carbons away from the omega terminus, the fatty acid is an omega-3 fatty acid. If the first double bond is located six carbons away from the omega terminus, you have an omega-6 fatty acid, and so on.
There are shorthand ways of writing the structure of fatty acids. The structure of a fatty acid can be written x:y, where x is the number of carbon atoms in the chain (including the carbonyl carbon) and y is the number of double bonds. For example, 18:1 would be an 18-carbon fatty acid with one double bond. The location of double bonds can be given in brackets. The location is given by the number of the first carbon involved in the double bond. When counting, you need to start at the carbonyl carbon- not at the omega terminus. Most double bonds in fatty acids are located 3 carbons away from each other.
Here are some common fatty acids you should know:
- Myristic acid- 14:0
- Palmitic acid- 16:0
- Stearic acid- 18:0
- Oleic acid- 18:1 (9)
- Linoleic acid- 18:2 (9, 12)
- Linolenic acid- 18:3 (9, 12, 15)
- Arachidonic acid- 20:4 (5, 8, 11, 14)
Properties of Fatty Acids
#1: Amphiphilic
Fatty acids are amphiphilic- that is, they have one end that is soluble in water (hydrophilic) and one end that is not (hydrophobic). The hydrophilic end of the fatty acid is the -COOH group as its =O and -OH groups can form hydrogen bonds with water. Additionally, this end is charged at physiological pH, as I'll talk more about shortly. The hydrophobic end of the fatty acid is the long carbon chain. When the carbon chain is short, the fatty acid as a whole tends to be soluble in water, but as the carbon chain grows, it begins to "overpower" the -COOH group at the end and thus longer-chain fatty acids are insoluble in water.
#2: pKa of roughly 5
Fatty acids have a pKa of around about 5. As the physiological pH of roughly 7 is higher than 5, fatty acids tend to be deprotonated. This means that ionic -COO- groups are found rather than -COOH groups. As mentioned above, this increases solubility as ionic groups interact better with water.
#3: Chain length and double bonds affect melting points
The melting points of fatty acids depend on several factors. One of these factors is the chain length. As the chains become longer, the van der Waals forces between fatty acids become stronger (see my earlier post on intermolecular bonding for a bit more information on why this is so). As such, melting point increases with an increase in chain length. Another factor is the number of cis double bonds in the molecules. Cis double bonds create a "kink" in the chain, which prevents fatty acid molecules from packing as closely together as saturated fatty acids. It thus becomes easier to separate molecules with more cis double bonds. Hence, melting point decreases with an increase in the number of cis double bonds.
4: Even number of carbons
Fatty acids usually have an even number of carbons, as they are synthesised and degraded two carbons at a time. I will cover these processes in a later post.
Behaviour of Fatty Acids in Aqueous Solution (at neutral pH) and the Hydrophobic Effect
As mentioned before, fatty acids are amphiphilic: they have a water-loving end and a water-fearing end. This affects how they behave in water.
If fatty acids are found at the water surface, generally what you'll find is that the fatty acids will "stand up" with the hydrophilic part in the water and the hydrophobic part poking out of the water.
If fatty acids are found in bulk solution past a particular concentration (the "critical micellar concentration"), they begin to form spheres, or micelles. In these spherical micelles, the hydrophobic parts cluster together on the inside, while the hydrophilic parts remain on the outside. This mainly occurs due to the hydrophobic effect, which I mentioned in my post about protein folding. Essentially water doesn't like being interrupted by a big, bulky, hydrophobic thing, and so the molecules rearrange themselves so as to minimise disruption of water.
Triglycerides (Triacylglycerols)
Triglycerides, or triacylglycerols, are essentially three fatty acids joined to glycerol via ester linkages. Glycerol is a 3-carbon hydrocarbon with an -OH group on each carbon. There are also diglycerides, in which there are only two fatty acids joined to glycerol, and monoglycerides, in which there is only one fatty acid joined to glycerol.
Here are some of the properties of triglycerides:
1. Weak amphiphiles
Triglycerides are only weakly amphiphilic. This is because the -COOH groups have been esterified and therefore the only part free to interact with water is the =O group, which won't ionise in water. Furthermore, there are multiple hydrophobic chains. Hence the hydrophobic part dominates and triglycerides are generally insoluble. Triglycerides do not form micelles and are found instead as spherical oily droplets in the cytoplasm of cells.
2. Store energy
As triglycerides do not draw in water of hydration, they can be stored without osmotic effect. This is in contrast with glycogen, which despite drawing in far less water than many individual glucose molecules, still has a small osmotic effect. Fatty acids are also found in a highly reduced form, meaning that they can release a lot of energy when they are oxidised in the mitochondria. In fact, they carry twice as much energy as an equivalent weight of glucose.
3. Factors affecting melting temperature
The factors affecting the melting temperature of triglycerides are essentially the same as those affecting the melting temperature of fatty acids in general, seeing that triglycerides are essentially just three fatty acids joined together on a glycerol backbone. One important point of note is that triglycerides found in plants tend to be more unsaturated than those of animals, but triglycerides found in both generally have enough unsaturated fatty acids to be liquid at around 37 degrees (which is roughly what our body temperature is).
Cholesterol
Cholesterol is an important lipid. It is quite different to the fatty acids that I've talked about so far in that cholesterol is not linear- instead, it is made up of four rings plus a branched side chain. Three of the rings have six carbons while the fourth has five. Cholesterol has an -OH group on the opposite side of the branched chain. This -OH group is the only hydrophilic group in the whole 27-carbon molecule, so cholesterol is generally insoluble in water and does not form micelles.
Cholesterol has important functions in forming cell membranes, as it can slot in between membrane lipids and make them pack together more closely (more on that later). It is also an important substrate for forming bile salts and steroid hormones. Cholesterol can also be converted to cholesteryl ester (which is also insoluble) for storage.
Phospholipids
Most lipids in the lipid bilayer of cells are phospholipids. Phospholipids are essentially diglycerides with a phosphate group esterified onto the third carbon of glycerol. As phosphate is negatively charged at biological pH, phospholipids are strong amphiphiles. Another interesting point is that usually the first fatty acid chain of phospholipids is saturated while the second is unsaturated.
Phosphatidic acid is a simple phospholipid with two fatty acid chains and a phosphate group. Other groups can be joined onto the phosphate group to make different kinds of phospholipids. For example, a choline group can be added to form phosphatidylcholine, or the amino acid serine can be added to form phosphatidylserine. Many of these groups are also polar and some also carry charges, contributing further to the amphiphilic nature of phospholipids.
Phospholipids are pretty important in biology, so I'll probably cover them in a bit more detail later. Let's have a look at one of their major functions first:
Phospholipid Bilayers
One of the unique features of phospholipids is that they can form bilayers. Phospholipid bilayers are basically two layers of phospholipids, with hydrophobic parts facing inwards, towards each other, and hydrophilic parts facing outwards, towards the solutions on either side. (There's also a note on the slide that says that the fatty acids in these phospholipids have more than 8 carbons, but I don't know if this is a hard and fast rule or if more carbons is just better for stability under biological conditions.) This structure is important in cell membranes. Cell membranes often also have proteins and other molecules such as cholesterol in them. I'll probably talk about this more at a later time.
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