lipids and metabolism

Question 1

Fatty acids: an Introduction

Answer 1

Made up of mostly carbon and hydrogen, lipids are NON-POLAR and so don’t dissolve in water i.e. they are HYDROPHOBIC. (hydrophilic: a substance dissolves in water

Lipids include:
simple fats
Hydrogenation hardens a fat (i.e. butter)
compound fatty acids e.g. phospholipids (Dr Sillence)
waxes
sterols e.g. cholesterol
fat-soluble vitamins A, D

Upon hydrolysis lipids yield glycerol and fatty acids, i.e. triglyceride fats: an animals energy story of fat.

Question 2

Fat is stored in adipose cells

Answer 2

Adipose cells are also known as adipocytes or fat cells.

Adipose cells contain a large lipid droplet (mainly triacylglycerols) and a flattened, squashed nucleus.

Adipose cells can quadruple in size when we put on weight.

Question 3

Lipids

Answer 3

Most lipids contain, or are derived from fatty acids and they perform many important biological functions:

Triacylglycerols (triglycerides): a fuel store of the body (fat).

Phospholopids: crucial constituents of biological membranes

Steroids : cholesterol and bile function

Eicosanoids metabolised from lipids, include: prostaglandins: signalling molecules within cells.

Question 4

Triglycerides

Answer 4

the hydrophobic side chain is normally depicted by an “R”
fatty acids are esterfied to glycerol to make a fat
They may be homogenous or heterogenous with respect to the fatty acid
e.g.
Glycerol tristearate (tristearin)
Glycerol tripalmitate (tripalmitin)
Glycerol trioleate (triolein)

Question 5

Fatty acid numbering

Answer 5

Underlined = number of carbon atoms

bold = number of double bonds in the aliphatic chain; parentheses = location of double bonds, counted from the carbon (1st) of the carboxyl.

Question 6

Saturated / unsaturated

Answer 6

The different fats are identified from their fatty acids (FA’s). The most commonly occurring FA’s are simple, saturated and unbranched i.e. palmitic and stearic acids:

Mono-unsaturated: one C=C double bond

Poly-unsaturated: two or more C=C double bonds

Question 7

Unsaturated

Answer 7

Unsaturated fatty acids include oleic acid (unsaturated with an even number of carbons, C18), also branched and even cyclic ones in nature.

Question 8

Transport breakdown

Genetic deficinces in Carnitine transport

Answer 8

Different tissue groups can be affected including muscle, kidney and heart (but not liver where a different fatty acid transporter system apparently takes place). The condition results in very low levels of available carnitine

Treatment can involve diet i.e. containing low levels of long chained fatty acids and avoiding fasting, where the tissues require fatty acid oxidation for energy.

Question 9

Analysis

Answer 9

Gas Chromatography (GC):  
Solid / Gas phase

Thin Layer Chromatography
(TLC): solid phase (silica)

High Performance Liquid Chromatography HPLC: solid phase / Liquid

Droplet Counter current Chromatography: Liquid to Liquid phase

Question 10

Steroids

Answer 10

The biochemist Chevreul found that in animal fats, there was a small residue that could not be saponified

Saponification means the hydrolysis of the fatty acids using NaOH giving the products free glycerol and the sodium salt of the fatty acid (e.g. sodium searate – “soap”) – hence saponification – “soap making”.

The structure of steroids is based on the three ringed compound perhydrocyclopentanophenanthrene

Question 11

Steroids: Cholesterol

Answer 11

Key points of  cholesterol are:
Hydroxyl (-OH) on C3.
Double bond between C5 and C6.
Methyl groups on C10 and C13.
1 (C20),5 (C25) dimethyl hexane side-chain

The properties of cholesterol depend on the properties of the functional groups:

In vivo, cholesterol is not especially active. It tends to accumulate as gallstones or coating coronary arteries. However, its derivatives are important.

For example, bile acids (salts):
cholic acid
glycholic acid and
taurocholic acid (NEXT SLIDE).
The –OH may be esterified, reduced or chlorinated.
The double bond may be hydrogenated, brominated etc.
The side-chain may be modified or substituted

The ring structure of cholesterol cannot be metabolised to CO2 and H2O in humans. Excretion is via the liver and gallbladder into the intestine as bile acids.
To convert to a bile acid (salt) the cholesterol is metabolised with a carboxyl group (-COOH, cholic acid). This group is esterified, usually with one of two amino acids (reacting with the amino grp. of a) glycine or b) taurine) to form a peptide link

Question 12

Steroids: Bile acid function

Answer 12

The function of bile acids are as detergents.
the compounds have a hydrophobic “end” (the ring structure) and a hydrophilic “end” (the ionisable amino acid). They are excreted in the bile and help to emulsify (render them soluble) fats prior to digestion.

The formation of bile acids is the normal end product of cholesterol metabolism.

Question 13

Metabolism of arachidonic acid

Answer 13

Metabolism of arachidonic acid from lipids by a phospholipase:
Fatty acid cyclooxygenases (COX):
COX-1 and COX-2 (Cox-3)➡️
Initiate biosynthesis of prostanoids:
The prostaglandins and thromboxanes
Cox-1: prostaglandins protecting the lining of the stomach
Cox-2: prostaglandins responsible for inflammation / pain

Question 14

metabolism of triglycerides :

Adipose cells (fat cells)

Answer 14

Specialised as a site of triacylglycerol synthesis and storage.
Droplets of triacylglycerol coalesce to form a large globule, filling most of the cell volume.
Adipose cells can be transported to tissues by the blood. While muscles store triacyglycerols for their own energy needs.

Question 15

Pancreatic lipases

Answer 15

In order to begin the process of fat metabolism, triacylglycerols must be broken down into their component fatty acids and glycerol.
This occurs primarily in the duodenum during digestion and liberates fatty acids from Adipose cells.
Pancreatic lipases are responsible for the release of fatty acids from triacylglycerols by hydrolysis.

Question 16

Lipids in an aqueous environment

Answer 16

Dietary triacylglycerols are emulsified by bile salts i.e. cholic acid, glycholate and taurocholic acid, in the intestinal lumen.

The triacylglycerols are incorporated into micelles composed of bile salts with the ester bonds oriented towards the outer surface of the micelle.

Finally, digestion products are carried in micelles to the intestinal epithelium for transport across the plasma membrane.
If the production of bile salts is inadequate, i.e. due to liver disease, large amounts of fat is excreted in the faeces: steatorrhea. (name based upon stearic acid, a common fatty acid).

Question 17

Dietary lipid transport.

Answer 17

Once in the intestinal mucosal cells, the triacylglycerides are re-synthesised from fatty acids and monoacylglycerols and packaged into chylomicrons (lipoprotein transport particles).
There is also a protein component (adolipoproteins) to chylomicrons
Released into the lymph system➡️blood.
Chylomicrons also transport fat soluble vitamins and cholesterol

Question 18

Chylomicron

Answer 18

Following digestion they have been re-constituted as triacylglycerides in the Chylomicrons and have been transported to adipose tissue.

From here they are hydrolysed again to release the fatty acids = this involves the mobilisation etc (next slide) and they involve the hormone sensitive lipases to release fatty acids into the blood stream.

Question 19

Utilisation of fatty acids as fuels.

Answer 19

Three stages:
MOBILISATION

ACTIVATED AND TRANSPORTATION

BREAKDOWN TO ACETYL CoA, which is then processed in the citric acid cycle.

Question 20

MOBILISATION

Answer 20

Triacylglycerides must be hydrolysed before they can be utilised as fuels.
Hormonally controlled lipases and phosphorylation of two key proteins:
Perilipin A, a fat-droplet-associated protein. The phosphorylation of perilipin A restructures fat droplets so that triacylglycerols are more accessible to the:

Hormone-sensitive lipase. This phosphorylated lipase hydrolyses triacylglycerides to free fatty acids.

Question 21

ACTIVATED AND TRANSPORTION

Answer 21

Fatty acids are oxidised in the mitochondria.
They are first activated through the formation of a thioester linkage to coenzyme A before mitochondrial entry.
This reaction is catalyzed by carnitine acyltransferase I, which is bound to the outer mitochondrial membrane.

The acyl groups is transferred back to coenzyme A on the matrix side of the membrane, catalysed by carnitine acyltransferase II.

Question 22

BREAKDOWN to Acetyl CoA

Answer 22

A saturated Acetyl CoA is degraded by a recurring sequence of four reactions:
Oxidation by flavin adenine dinucleotide (FAD).
Hydration.
Oxidation by NAD+.
Thiolysis by coenzyme A. (Thiolysis is a reaction with a thiol (RSH) that cleaves one compound into two).

Fatty acid chain is shortened by two C’s.
This Oxidation takes place at the BETA -carbon of the fatty acid. Therefore this series of reactions is called the BETA-oxidation pathway.
Overall, the complete oxidation of an acetyl CoA, i.e. Palmitate (C16 = 8 molecules acetyl CoA), yields 106 ATP molecules via NADH and FADH2.

Question 23

Unsaturated and odd-chain fatty acids require additional steps for degradation

Answer 23

BETA-Oxidation accomplishes the complete degradation of saturated fatty acids having an even number of carbon atoms.

This is the case for most fatty acids. However additional steps are required for unsaturated and unevenly C-numbered fatty acids.

Question 24

Unsaturated

Answer 24

Most of the reactions remain the same as for saturated fatty acids.
The additional steps are two only and require the enzymes of: an isomerase and a reductase
Odd-numbered double bonds are handled by the isomerase.

Even-numbered ones by the reductase.

Question 25

Odd-chain fatty acids

Answer 25

Fatty acids with an odd number of carbon atoms are in the minority.
They are oxidised in the same way as fatty acids with an even number.

However, propionyl CoA, rather than acetyl CoA molecules are produced in the final stages of degradation.