Lipids Types: Simple, Compound and Derived Lipids
The main simple lipids are triglycerides also known as triacylglycerols , steryl esters, and wax esters. Hydrolysis of these lipids yields glycerol and fatty acids, sterols and fatty acids, and fatty alcohols plus fatty acids, respectively. The most important of these simple lipids for food scientists are the triglycerides. Triglycerides are esters of the trihydric alcohol glycerol with three fatty acids Figure 1.
Many of the properties of triglycerides are dependent on the component fatty acids. Thus, the melting point of the triglyceride reflects the melting point of the component fatty acids, with three high-melting-point fatty acids yielding a high-melting triglyceride. Unsaturation in the fatty acids makes the triglyceride susceptible to autoxidation, just as the fatty acid itself would be.
All triglycerides are susceptible to hydrolysis in the presence of a catalyst. Acids, bases, or enzymes belonging to the hydrolase class, especially lipases, may act as the catalyst for the hydrolysis of triglycerides.
Steryl esters always occur together with sterols in plant, animal, or microbiological tissues. Wax esters may accumulate in considerable amounts in some biological tissues and this class comprises the main constituent of beeswax and jojoba oil. Grace Brannigan 1 , in Current Topics in Membranes , The lipids most commonly occupying annular binding sites will be affected by the protein's preferred membrane domain.
Like cholesterol, unsaturated lipids have significant effects on organization and viscoelastic behavior of the surrounding membrane. This sensitivity may reflect an elastic effect: Computational equilibration of annular lipids in a nonrandom mixture, particularly one that involves phase separation, requires CG-simulations.
We used a single nAChR from Torpedo in a range of ternary lipid mixtures, with each mixture containing DPPC, cholesterol, and a third lipid which could be one of twelve species of glycerophospholipid. In all systems in which domain formation occurred, we observed nAChR partition into cholesterol-poor l do domains Figs. However, as illustrated in Fig. It is more likely to occupy annular sites on the l o side of the nAChR and nonannular sites. Among phospholipids, n-3 PUFAs were far more likely to displace cholesterol from both annular and nonannular sites, relative to saturated or n-6 PUFA phospholipids.
This likelihood was only weakly increased by replacing PC headgroups with PE and shorter acyl chains with longer acyl chains. Goldson-Barnaby, in Pharmacognosy , There are various classifications of lipids that exist in living tissue. One of the more straightforward was developed by Bloor in .
In this classification, lipids are divided into three groups: Upon hydrolysis, simple lipids directly yield two types of products per mole: Simple lipids are further divided into neutral fats or acylglycerols and waxes. The term neutral fats is generally used to describe fatty acid esters of glycerol. These may be mono-, di-, or triesters of glycerol, the latter being the major ones found in nature.
TAGs are also known as triacylglycerides and triglycerides, the latter being the least acceptable term, chemically, though very frequently used in nutrition literature. Animal and plant fats and oils are comprised mostly of this group of compounds. They are esters comprised of three fatty acid molecules attached to a glycerol backbone. Compound lipids give rise to at least three types of primary products upon hydrolysis.
Phosphatides, sphingolipids, glycolipids, and sulfolipids are included in this group, of which phospholipids are the most abundant .
Fatty acids, phospholipids, and cholesterol are discussed further below. Waxes are discussed in Chapter 22 , Waxes. Lipids may be divided into three groups: Fatty acids may be termed, short, medium, long, or very long-chained based on the number of carbons 2—4, 6—10, 12—18, and 20—24 carbon atoms, respectively. No strict convention exists for the classification of fatty acids based on chain lengths, and so several different versions of this classification can be found in the literature [5,6].
Naturally occurring fatty acids generally have an even number of carbons arranged in a straight chain with most having 14—24 carbons present. Fatty acids with odd numbers or branched chains are more characteristically found in microorganisms and dairy fats. Dairy fats and tropical oils possess significant amounts of short-chain fatty acids. Fatty acids may also be described as saturated having no carbon—carbon double bonds or unsaturated .
Double bonds in fatty acids naturally occur in the cis -configuration and are separated by a methylene i. Subsequently, if the position of the first double bond is known, those of all the others may be easily predicted, with the exception of conjugated trans fats such as conjugated linoleic acids CLA found in ruminant fats. Double bonds may be numbered starting from the carboxylic acid end or the methyl end named omega or n- end; Fig. Although fatty acids are formally named based on the number of carbons and double bonds present, they are often given common names based on their source.
Trans fats are uncommon in nature, with the exception of ruminant fat including cows and sheep , which contains vaccenic acid as the main trans fatty acid, produced as a result of incomplete biohydrogenation of linoleic and linolenic acids by microorganisms in the rumen. Industrial trans fatty acids are produced by partial hydrogenation of vegetable or fish oils. Trans fatty acids from industrial sources are known to lower high-density lipoprotein cholesterol HDL-C , raise low-density lipoprotein cholesterol LDL-C , and increase the risk of coronary heart disease CHD.
The major trans fat in partially hydrogenated vegetable oil products is elaidic acid, whereas trans isomers of C Although the effects of trans fatty acids from natural sources are less clear, some research suggest that all trans fats have a similar effect on plasma cholesterol levels. Research in this area is however limited and, in some cases, conflicting [8,9]. Fatty acids commonly found in foods are shown in Table Fatty acids may be termed, short, medium, long, or very long-chained based on the number of carbons 2—4, 6—10, 12—18, and 20—24 carbon atoms, respectively , and as saturated having no carbon-carbon double bonds or unsaturated possessing at least one carbon-carbon double bond.
Most of these double bonds occur naturally in the cis -configuration. Trans fats are uncommon in nature, with the exception of ruminant fat. Trans fatty acids from industrial sources are known to lower HDL cholesterol, raise LDL cholesterol, and increase the risk of coronary heart disease. Some research suggest that all trans fats have a similar effect on plasma cholesterol levels. Unsaturated fatty acids are liquids, as are all SFAs with less than 10 carbon atoms, and the higher the degree of unsaturation, the lower the melting point of a fatty acid.
Long-chain omega-3 fatty acids are characteristically found in large amounts in fish oils compared to fats from terrestrial animals Tables Oleic acid and linoleic acid LA are the main fatty acids in most vegetable oils. Palmitic, stearic, and oleic acids are dominant in terrestrial animal fats, while palmitic, oleic, eicosapentaenoic EPA , and docosahexaenoic DHA acids are among the dominant fatty acids in marine oils.
Oleic and LAs are the main fatty acids in most vegetable oils. Phospholipids are the most abundant type of lipid constituents in cell membranes, their chief role involving structural integrity of the membrane bilayer.
They are glycerol esters in which most of the times two of the glyceride OH groups are linked to fatty acids while the other is attached to a phosphate group. The phosphate is then linked to a simple, polar organic molecule. The majority of phospholipids are comprised of a diacylglycerol, a phosphate group, and a simple organic molecule, such as ethanolamine or choline.
As they possess an abundance of LA, phospholipids are susceptible to autooxidation . Palmitic acid is also commonly present. Phosphatidylcholine PC , which is often referred to as lecithin, is the most abundant class of lipids in animal cell membranes, accounting for nearly half of the total. Similarly they are the major components of plant membranes.
Lecithin serves as a surface-active agent in the production of emulsions. Commercially, egg yolk is the most important animal source of lecithin with soy being the most important plant source. PC is the main plasma phospholipid and an important component of lipoproteins, especially HDL. It strongly influences the circulation of different classes of lipoprotein, more so the very-low-density lipoproteins VLDL. PC is the biosynthetic precursor of phosphatidic acid, lysophosphatidylcholine platelet-activating factor, and phosphatidylserine.
Additionally, it provides the choline for sphingomyelin one of many sphingolipids biosynthesis. Phosphatidylserine, phosphatidylethanolamine, and lysophosphatidylcholine are among the more abundant phospholipids.
Phospholipids are the most abundant type of lipid constituents in cell membranes, with PC which is often referred to as lecithin being the most abundant class. Commercially, egg yolk is the most important animal source of lecithin, with soy being the most important plant source.
Cholesterol is the most dominant sterol in animal fats and oils, being present in vegetable oils in negligible amounts . In addition to synthesis by the body within the liver , it may also be obtained from the diet via the consumption of animal foods.
Cholesterol has a number of important biological roles and is required for human life and health. It resides mainly in membranes, the brain being the most concentrated source of cholesterol among animal organs. Due to its water insolubility, cholesterol has to be combined with water-soluble proteins in order to be transported in the body, thereby forming lipoproteins .
High levels of cholesterol in the blood can lead to the development of atherosclerosis, and has been associated with a myriad of health problems.
Functions of cholesterol however include modulation of membrane fluidity and permeability, maintenance of the structural integrity of membranes, development and functioning of central nervous system, sperm development, and embryonic development.
Additionally, it is a precursor in the biosynthesis of vitamin D, steroid hormones, and bile acids. The latter facilitate the digestion and absorption of lipids and the prevention of cholesterol buildup in the bile. This is owing to their strong emulsifying properties. The fat-soluble vitamin D cholecalciferol is known mostly for its role in calcium and phosphorus metabolism, and, by extension, bone development and the prevention of rickets. Humans and other animals naturally produce this vitamin from cholesterol in their skin upon exposure to sunlight.
It may also be obtained from the diet only small amounts are naturally present in most foods or dietary supplements. Good food sources of vitamin D include fatty fish, such as salmon, sardines, and herring, beef, eggs, and fortified foods, such as cereal and milk. Lipids owe their functional versatility to their hydrophobic structure.
Because of the presence of fairly long carbon chains, lipids tend to associate with each other and have limited or no solubility in water. Fatty acids and cholesterol are simple lipids , whereas triglycerides and phospholipids are complex lipids Fig.