Lipids
6.2 Structure of lipids
6.2.1 Classification of lipids
So what is a lipid? Lipids can be classified into three types: simple lipids, compound lipids and derived lipids. Figure 6.5 is a schematic of their classification. From an energy perspective, and as sport scientists, we are mainly interested in the fats, although from a health perspective we are interested in lipoproteins and from a regulatory and growth perspective we are interested in the steroids. Lipids are not defined by their structure, but rather are defined by being soluble in non- polar solvents such as ether. Therefore, we should
Classification of Lipids
Simple Lipids
Fats
Phospho- lipids
Glyco- lipids
Lipo- Proteins Waxes
Compound Lipids
Derived lipids Steroids Others
Figure 6.5 Classification of lipids
be able to separate lipids from carbohydrates and protein by dissolving fats in ether, wherein any carbohydrates and proteins present would fail to dissolve.
Lipids are stored in the body astriglycerides or triacylglycerols (abbreviated to TAGs), whereas the useable form of energy are the fatty acids.
Lipids can be found as TAGs in adipose tissue, in muscle, and in blood in the form of lipoproteins.
TAGs have to be broken down to fatty acids and glycerol before being used as an energy source. This process is known as lipolysis (see Section 6.3.1).
A triglyceride or triacylglycerol is in effect a glycerol molecule which provides the backbone to which are attached three fatty acyl units (see Figure 6.6). Glycerol, when released, is normally
CH2 O C R1
R2
R3 O
CH O C O
CH2 O C O G
L Y C E R O L
Fatty Acyl unit Fatty Acyl unit
Fatty Acyl unit
TRIACYLGLYCEROL TRIGLYCERIDE.
Figure 6.6 Structure of a triacylglycerol (triglyceride)
H C C C C C C C C C C C C C C C C H
H H
H H
H H
H H
H H
H H
H H
H H
H H
H H
H H
H H
H H
H H
H OH Saturated Fatty Acids
O Palmitic Acid – C15H31 COOH
PALMITIC ACID
Figure 6.7 A saturated fatty acid metabolized by the liver to produce energy, or it
can be converted to glucose by gluconeogenesis, whereas fatty acids are taken up by muscle and liver, where they undergo beta-oxidation (β-oxidation) before entering the TCA cycle and undergoing further oxidation to carbon dioxide and water.
The fatty acids which make up triglycerides are monocarboxylic acids with a general formula is R-COOH, in which the R-group is usually an unbranched chain with an even number of carbon atoms and can be either saturated or unsatu- rated. If there are double bonds between the carbon atoms, then the fatty acid is unsaturated.
Figure 6.7 shows the structure of palmitic acid, which has 15 carbons in a chain, followed by a carboxylic group that is the 16th carbon, as well as oxygen and the hydroxyl group. There are no double bonds between the 15 carbon atoms, so it is asaturated fatty acid.
If you now look at the structure ofoleic acid (Figure 6.8), you will note that there is a single double bond (between the 9th and 10th carbon atoms); this makes oleic acid anunsaturated fatty acid. Beneath oleic acid, note that there are two further unsaturated fatty acids which have either two or three double bonds. Linoleic acid has two double bonds (between the 9th and 10th and between the 12th and 13th carbon atoms) whereas linolenic acid has three double bonds (between the 9th and 10th, between the 12th and 13th, and between the 15th and 16th).
Linolenic and linoleic acids, together with arachidonic acid, are known collectively as essential fatty acids; linolenic acid is commonly
known as omega-3 fatty acid and linoleic acid is known as omega-6 fatty acid. Omega-3 fatty acids are found in fish and in some seeds (e.g. flax seed), while omega-6 fatty acids are more commonly found in meat products. Being essential fatty acids implies that these fatty acids are essential to have in the diet regularly or else a deficiency symptom arises. This is due to the body being incapable of synthesizing them (or at least making sufficient amounts for health).
Recently there has been some publicity sur- rounding the use of so-called ‘trans-fats’ or hydrogenated fats in foods. Trans-fats have been linked to increases in coronary heart disease and some cancers, and so it is clearly not advisable to eat them if possible. So what is a trans-fat? A saturated fatty acid is a straight chain molecule, whereas an unsaturated fatty acid is usually bent. Have a look at Figures 6.7 and 6.9 to note the difference between palmitic and oleic acids.
Palmitic acid, as a saturated fatty acid, is a straight chain, whereas oleic acid is bent at the double bond and this structure is the naturally occurring cis-form of oleic acid. However, oleic acid can be straightened if hydrogen is forced onto the double bonds (hydrogenation) to make it saturated. In this state, it becomes the trans-form.
If there are a number of fatty acids in the cis- form, it is difficult for them to lie next to each other structurally, but if they are in the trans-form they can lie close together. In the trans-form, they appear more like saturated fatty acids which have a high melting point. This can confer a degree of stability on the structure, and hence trans-fats are used in confectionery products as stabilizers.
C C C C C C C C C C C C C C C C C H
H H
H H
H H
H H
H H
H
H H H
H H
H H
H H
H H
H H
H H
H OH Monounsaturated Fatty Acids
O
C OH
O Oleic Acid — C17H33 COOH
OLEIC ACID C
H H
H H
H
C C C C C C C C C C C C C C C C H
H H
H H
H H
H H
H H
H
H H H
H
H H H
H H
H H
H H
H Polyunsaturated fatty Acids
LINOLEIC ACID (Omega-6 fatty acid) C
H H
H H
H
C C C C C C C C C C C C C C C C C H
H
H H H
H
H H H H H
H
H H H
H
H H H
H OH O
LINOLENIC ACID (Omega-3 fatty acid) C
H H
H H
H
C C C C C C C C C C H H H H H H H H H H
ARACHIDONIC ACID
H H H H
H H
C O OH C C C
H
H H C
H H
H H
H H
C H
H
C H
H C H
H C H
H C H
H
Figure 6.8 Unsaturated fatty acids
cis and Trans Forms of Oleic acid
Oleic acid, trans form Synthetic trans form
‘Normal’ cis from
COOH H3C
H H
109 1
18
Figure 6.9 Cis- and trans- fatty acid
Phospholipids
Glycerol
Phosphoric acid FA
FA
Figure 6.10 Structure of a phospholipid
6.2.2 Compound lipids
Fatty acids and glycerol thus together make up the simple lipids, in so far as they contain carbon hydrogen and oxygen only. However, if we were to incorporate a phosphate or nitrogen group to the fatty acid, we would no longer have a simple lipid – instead we would have a compound lipid.
This can be seen in Figure 6.10 with thephospho- lipids. Phospholipids contain glycerol, fatty acids and phosphoric acid, and they form an integral part of the plasma membrane.
Other types of compound lipids includelipopro- teins, such as chylomicrons, very low density lipoprotein (VLDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL).
These are in effect lipids such as triglycerides and cholesterol surrounded by protein (see Table 6.1 and Figure 6.11). The greater the lipid content in relation to the protein content, the less dense is the lipoprotein, and so chylomicrons are the least dense and HDLs are the most dense. Furthermore in relation to their size, the less dense lipoprotein molecules (chylomicrons) are larger than the more dense lipoproteins (HDL).
Chylomicrons are formed as a result of the digestion of lipids and are produced by the intestinal cells from the absorbed fatty acids in our food before being synthesized to TAGs, which make up by far the largest constituent.
Chylomicrons are then taken to the liver and to
other tissues, where the triglycerides are broken down bylipoprotein lipase(LPL) in the blood to release the fatty acids and then transported into the cell for storage.
6.2.3 Derived lipids
The final group of lipids are the derived lipids, which include cholesterol and the steroids. From Figure 6.12, you can see that the structure of cholesterol is very similar to the steroid hormones.
In fact, cholesterol is crucial for the synthesis of steroid hormones such as testosterone, pro- gesterone, and oestrogen (the sex hormones). A diet totally lacking in cholesterol would prove problematic for sex hormone production.