6. Blood Lipids
1
Describe and understand what's the difference between apoproteins and lipoproteins.
Be able to explain lipid biochemistry and metabolism and also look at lipid disorders and
clinical disease, the diseases associated with lipid disorders (dyslipidaemia) be able to
explain the role of the clinical biochemistry laboratory in the analysis of blood lipid and also
obviously their role in the diagnosis of dyslipidaemia and to also be able to understand and
explain the treatments are associated with treating dyslipidaemia.
Major biomolecules alongside proteins, carbohydrates and nucleic acids and lipids planned
number of different roles within the body.
They are an important structural component to cells, and you know that in relation to
phospholipids with their hydrophilic head and their hydrophobic tails and they subsequently
form the main component of the plasma membrane.
They play an important role within metabolism and also in hormonal pathways, if we take
metabolism as an example free fatty acids are an important energy source within the body
and a fuel ready to channel life
They also play an important role in the absorption of nutrients such as fat-soluble vitamins
such as vitamin D, as well as acting as an important fuel source they also act as an important
energy store, and we know that in the case of triglycerides.
Triglycerides are the way which had stored within the body, and we can find them within
adipocytes and these triglycerides consisting of their glycerol and free fatty acids
subsequently act as an important and essential energy reserve in the body should the body
have to revert to those energy stores.
Fatty acids consists of a long hydrocarbon chain and they consist of a carboxyl group at one
end and fatty acids can either be saturated or unsaturated depending upon whether they
have double bonds, unsaturated fatty acids have a double bond between carbons and if it
has one or more double bonds is referred to as unsaturated fatty acid, if the fatty acid has no
double bonds it's referred to as a saturated fatty acid.
Fatty acids are an important energy source within the body they act as a fuel for metabolism
but when they're not being used as part of metabolism, we can store them as triglycerides
and triglycerides and therefore act as an energy source, we store triglycerides within
adipocytes.
They consist of a glycerol backbone linked to three fatty acids.
Cholesterol can be absorbed from our diet, but it can also be synthesised by the liver and
cholesterol has a number of important roles, but one particular role is that is an essential
component of plasma membranes, and it is important in providing some rigidity and
integrity to that plasma membrane.
Lipoproteins are the way in which we can transport lipids around the body they are the lipid
transport molecules and lipoproteins basically arise from the from the joining of lipids to
apoproteins.
Lipoproteins as lipid transport molecules increase the solubility of lipids so we can actually
transport them in the blood around the body to where they need to go.
Apo means its lipid lacking, they subsequently bind lipid and they overcome this problem
that lipids are insoluble so if you could combine an insoluble lipid to a protein, it increases
, 6. Blood Lipids
that solubility and is the combination of a lipid too and apoprotein that gives rise to a
lipoprotein.
They are multi-functional proteins, they maintain structural integrity of the lipoproteins once
they've bound to a lipid, they regulate enzymes acting on lipoproteins, they can act as
cofactors for enzymes involved in lipid metabolism, we know the apoproteins can be
recognised by various cellular receptors within the body which can subsequently then take
up particular lipoprotein.
Apoproteins can be categorised alphabetically A, B, C and E and within that we have
subgroups.
Different apoproteins are synthesised and producing different areas so group A are
produced with in the intestine and liver and group C is just the liver.
Defects or within apoproteins can subsequently give rise to dyslipidaemia or conditions or
diseases relating to changes within blood lipids.
Apoproteins are those lipid binding proteins, when you can take apoprotein then you
actually then bind it to lipid, it becomes a lipoprotein.
Lipoproteins consist of outer shell of phospholipid, free cholesterol and apoproteins and
within the core of this lipoprotein they are rich within triglycerides and cholesterol esters.
These lipoproteins have 3 roles.
They act as interorgan fuel – when these lipoproteins travel within the circulation, they can
release some of their triglycerides which are broken down into the free fatty acids and those
free fatty acids act as an important energy source for metabolising cells.
They are involved within distribution so these lipoproteins can transport fat soluble vitamins
for example such as vitamin D and because they also contain a rich core of cholesterol and
contain cholesterol also on the outside, they maintain the cholesterol pool.
That's important because cholesterol is important not only in cell membranes, it's important
in the formation of bile salts and it's also important in the formation of steroid hormones.
Classifying lipoproteins can be dependent on a number of different factors and one
particular factor is the apoprotein which determines the lipoprotein class.
We classify lipoproteins based upon their lipid composition, the apoprotein that's bound to
them and also their size.
We can classify them into five major groups of which I've showed four here in a little bit
more on the fifth in a moment so if we take four of the five major classes of lipoprotein on
the top we can then look and compare their composition, the apoproteins that they contain,
their diameter, their density and their function.
The role of these chylomicrons is to transport dietary triglycerides so when we eat fats and
we absorb the triglycerides they are packaged into chylomicrons and these chylomicrons will
carry the triglycerides around the body to where they are needed but if we compare that to
low density lipoprotein for example or LDL we subsequently see that it differs in its
composition, it contains less triglyceride and more cholesterol, it contains more
phospholipid and more protein, it has a smaller diameter and a greater density.
Unlike chylomicrons which are the main carrier of dietary triglycerides LDL is the main
carrier of cholesterol.
The main way we classify them is based upon their variations within triglycerides,
cholesterol, phospholipids, apoproteins, diameter and density.
The fifth lipoprotein class is referred to as intermediate density lipoprotein, so it sits
between LDL and HDL, and I recommend that as extra reading.
