Fats are triacylglycerols containing saturated fatty acids - solid at room temp - usually from animal source (however, coconut & palm oil are saturated).
Oils are triacylglycerols containing mono- or polyunsaturated fatty acids - liquid at room temp
- usually from plant sources (however, fish oils are polyunsaturated).
Phospholipids are triacylglycerols that have had a FA replaced with a phosphate linked FA group.
The major dietary sterol is cholesterol.
Excessive dietary fat intake is associated with obesity, diabetes, cancer, hypertension and atherosclerosis.
Not more than 35% of energy intake should come from fat. Saturated fat should not make up more than 15% of the total fat intake.
Omega-3 fatty acids (20 carbons) from fish may protect against atherosclerosis. American Heart association recommends 2-3 fish meals per weak. Fish oil supplements should be avoided because they may be contain concentrated toxins accumulated by the fish.
FUNCTIONS OF LIPIDS :
Major components of cell membranes.
Required to solubilise fat soluble vitamins
Biosynthetic precursors (e.g. steroid hormones from cholesterol)
Protection (e.g. kidneys are shielded with fat in fed state)
Stomach - lingual lipase and gastric lipase attack triacylglycerols and hydrolyse a limited number of FA.
Small Intestine - acid chyme (stomach contents) stimulates mucosa cells to release hormone (choleocystokinin) which stimulates gall bladder and pancreas to release bile and digestive enzymes respectively (bile acids help emulsify fat droplets thus increasing their surface area).
Other mucosa cells release secretin which causes pancreas to release bicarbonate rich fluid to neutralise chyme.
Enzymic digestion of lipids in small intestine 2- monoacylglycerol
Enzymic digestion generates more polar products that form mixed micelles of free fatty acids, 2-monoacylglycerol, cholesterol & bile salts that are adsorbed (except bile salts which pass through to ileum – see later).
Once adsorbed fatty acids and 2-monoacylglycerol are recombined to form triacylglycerol.
Triacylglycerol + cholesterol + phospholipid + proteins form a lipoprotein complex called a chylomicron which transports the lipids in the circulation.
Lipid transport in the circulation Lipids are insoluble in plasma. In order to be transported they are combined with specific proteins to form lipoproteins: Non polar lipids in core (TAG and cholesterol esters ) Proteins (apoproteins) Cholesterol
Apoproteins are only weakly associated with a particular lipoprotein and are easily transferred to another lipoprotein of a different class. Apoproteins have various functions including:
Binding sites for receptors
Activators or co-enzymes for enzymes involved with lipid metabolism
The five classes of lipoprotein (all contain characteristic amounts TAG, cholesterol, cholesterol esters, phospholipids and apoproteins) Increasing density B100 Formed in circulation by partial breakdown of IDL. Delivers cholesterol to peripheral tissues 22 Low density lipoproteins (LDL) A, C(I,II,III), D, E Liver. Removes “used” cholesterol from tissues and takes it to liver. Donates apolipoproteins to CM and VLDL 8 High density lipoproteins (HDL) B100, C(I,II,III) , E Liver. Transport of endogenously synthesised TAG 43 Very low density lipoproteins (VLDL) A, B48, C(I,II,III) E Intestine. Transport of dietary TAG 500 Chylomicrons (CM) Major apoliproteins Source and function Diameter (nm) Class
Assembled in intestinal mucosal cells.
They enter the lymphatic system and enter the blood via the thoracic duct.
They contain mostly TAG.
Nascent chylomicrons contain apoprotein B48 but pick up others apoproteins from high density lipoproteins (HDL) once they enter the circulation.
Chylomicrons Cholesterol ester Apoprotein TAG (86%) Cholesterol phospholipid Function is to transport dietary TAG to the adipose tissues where it can be stored as fat or to muscles where the constituent fatty acids can be used for energy.
This animation shows how chylomicrons are metabolised once they enter the circulation from the lymphatic system HDL CII E HDL CMR B48 CII E Taken up by liver (via LDL receptors) Lipoprotein lipase Capillary wall (endothelial surface) Tissues CM B48 CM B48 CII E CMR B48 CII E
The nascent chylomicron enters the circulation carrying apoprotein B48 and a large cargo of triacylglycerol (with a little cholesterol, cholesterol ester and phospholipid). It receives apoproteins E and CII from high density lipoprotein (HDL). The chylomicrons travel around the circulatory system until they associate with lipoprotein lipase, an enzyme bound to the enodothelial surface. This association is mediated through apoprotein CII. The lipoprotein lipase hydrolyses the triacylglycerol to liberate free fatty acids which diffuse into the local tissues. As triacylglycerol is lost, the chylomicron shrinks and forms a chylomicron remnant (CMR). The released fatty acids can be reassembled into triaclyglycerol for storage as fat or oxidised to produce ATP. The chylomicron remnant dissociates from the lipoprotein lipase and returns apoprotein CII to HDL. The remnant is finally taken up by the liver through receptor mediated endocytosis (liver receptors recognise B48 and E). Dietary cholesterol delivered to the liver in CMR can be repackaged in VLDLs and sent out to tissues or converted to bile salts. Dietary cholesterol inhibits cholesterol synthesis by the liver.
Synthesised in intestine.
Contain mostly dietary TAG (with a little cholesterol, cholesterol ester & phospholipid.
Transport TAG to tissues and deliver remaining cholesterol & cholesterol ester to the liver.
Very low density lipoproteins (VLDL)
Synthesised in the liver. Contain mostly TAG but with a significant amount of cholesterol and cholesterol ester.
Nascent VLDL contains apoprotein B100 but pick up others from HDL in the circulation
Cholesterol Cholesterol ester TAG (55%) Protein phospholipid Function is to transport endogenously synthesised TAG to the extra hepatic tissues where it can be stored as fat or to muscles where the constituent fatty acids can be used for energy. The cholesterol is delivered to extra hepatic tissues once VLDL has been metabolised to LDL.
This animation shows how VLDL are metabolised once they enter the circulation from the liver LDL HDL CII E CII E B100 Some LDL taken up by liver (LDL receptors) Some LDL taken up by other tissues (LDL receptors). LDL delivers cholesterol and TAG to the extra hepatic tissues. Having lost TAG to tissues LDL contains a large proportion of cholesterol/cholesterol esters Lipoprotein lipase Capillary wall (endothelial surface) Tissues VLDL B100
VLDL VLDL synthesised in the liver with apoprotein B100. VLDL receives apoproteins CII and E from HDL. Like chylomicrons, VLDLs travels around the circulatory system until they associate with lipoprotein lipase, an enzyme bound to the enodothelial surface. This association is mediated through apoprotein CII. The lipoprotein lipase hydrolyses the triacylglycerol to liberate free fatty acids which diffuse into the local tissues. As triacylglycerol is lost, the VLDL shrinks and forms LDL. The released fatty acids can be reassembled into triaclyglycerol for storage as fat or oxidised to produce ATP. The LDL return apoprotein CII and E to HDL and are taken up by receptor mediated endocytosis by the extra hepatic tissues (with some being taken back up by the liver). LDL having lost most of their triacyglycerol are rich in cholesterol and cholesterol esters and represent the route by which cholesterol is transported form the liver to the tissues (although all tissues can make cholesterol to some extent).
LDL membrane receptor
Found in clathrin coated pits (endocytosis)
After endocytosis the receptor is recycled whilst the LDL (or CMR) is degraded to releasing lipid cargo. Cholesterol uptake down regulates the cells own production of cholesterol and down regulates LDL receptor synthesis .
Mutations in LDL receptors causes increased plasma LDL levels (i.e. increased cholesterol levels). This accelerates progress of atherosclerosis (Familial hyperlipedimias).
The cholesterol in LDL is often called “bad cholesterol”.
Role of LDL in atherosclerosis
Damage to endothelium (hypertension, smoking etc).
LDLs penetrate vascular wall, deposit in the intima and with time are damaged by oxidation.
Oxidised LDLs attract the attention of macrophages which ingest the LDL.
Macrophages become overloaded with lipid and become “foam” cells which die and release pools of lipid in the vessel wall (plaques).
A complex processes mediated by cytokines and growth factors causes smooth muscle cells to form a collagenous cap over the lipid (mature atherosclerotic plaque).
Cap grows and can constrict the vessel (causing angina for example).
Macrophages can degrade the cap while T cells can inhibit collagen synthesis – the cap can rupture to expose collagen and lipids
This leads to aggregation of platelets and blood clot formation.
If the coronary artery is blocked by a clot – heart attack.
Blocking of arteries in the brain causes stroke.
Antioxidants (vitamin E and C) may protect LDL from oxidation and so protect against heart attack and stroke.
Synthesised in liver.
Contain mostly dietary TAG (with a little cholesterol, cholesterol ester & phospholipid.
Converted to LDL which contain an increased proportion of cholesterol & cholesterol ester (due to loss of TAG).
Transport TAG and cholesterol from liver to tissues.
Cholesterol in LDL referred to as “bad cholesterol” since LDLs are implicated in atherosclerosis
High density lipoprotiens
Act as a reservoir for apoproteins which can be donated or received from other lipoproteins.
Also play a vital role in scavenging “used” cholesterol (reverse cholesterol transport):
HDL Peripheral tissues HDL apoproteins “ used” cholesterol transferred to HDL and converted to cholesterol ester Liver HDL receptor mediated endocytosis by liver Cholesterol can be converted to bile salts for excretion or repackaged in VLDL for redistribution VLDL HDL LDL LDL LDLreceptor mediated endocytosis some cholesterol ester transferred to circulating VLDL
High density lipoprotiens summary
HDL carries “used” cholesterol (as CE) back to the liver. Also donate some CE to circulating VLDL for redistribution to tissues.
HDL taken up by liver and degraded. The cholesterol is excreted as bile salts or repackaged in VLDL for distribution to tissues.
Cholesterol synthesis in the liver is regulated by the cholesterol arriving through HDL (and dietary cholesterol returned by chylomicron remnants).
Cholesterol (CE) in HDL is referred to as “good cholesterol”.
Excess cholesterol in liver is converted to bile salts. Bile salts are polar constitute the major excretory pathway of cholesterol.
Stored in the gall bladder along with free cholesterol and phospholipids ready to be secreted into the small intestine. Bile salts are strongly amphipathic and act as detergents to help break up (emulsify) dietary lipids. About 30g of bile salts and 2g of cholesterol are secreted by the bile duct every day. Cholesterol is present in bile at virtually the limit of its solubility and it can crystallise out to form gall stones)
Almost all of this is actively reabsorbed so that only about 1-2g of bile salts and cholesterol are lost each day in the faeces. The reabsorbed bile salts and cholesterol are transported back to the liver bound to albumin and are recycled
This cycle of secretion and reabsorption constitutes the enterohepatic circulation.
Fibre can bind bile salts and cholesterol and prevent its reabsorption. This is considered to be one of the health benefits of fibre. Other treatments based on resins.
Sitostanol ester (from plants) has a similar structure to cholesterol and stops cholesterol entering micellar phase and reduces its adsorbtion in to the blood stream (Benecol margarine).
Cholesterol Cholesterol ester Esterification at the OH position with a fatty acid (catalysed by LCAT enzyme) Back