2. Definition
Apo lipoproteins are the protein components
of lipoproteins, the lipid–protein complexes
responsible for transporting lipids in the
blood. They may have additional specialized
functions that are encoded by their genes.
This review summarizes the current
knowledge on gene and protein structure and
the function of this rather complex group of
proteins.
http://www.els.net/WileyCDA/ElsArticle/refId-
a0005909.html
3. From genes to proteins
Structural organization of the human apolipoprotein (Apo) A‐I, A‐II, A‐IV, C‐I,
C‐II, C‐III and E genes. The wide bars represent the exons and are divided
into several regions: the open bars at the two ends represent the 5′ and 3′
untranslated regions; the hatched bars, the signal peptide regions; and the
solid bars, the mature peptide regions of the respective genes. The numbers
above the exons indicate the length (in nucleotides) of the exons.
4. THE DISTRIBUTION OF
APOLIPOPROTEIN E GENOTYPE
• Apolipoprotein E (ApoE) is encoded by the codominant alleles ε2, ε3, and ε4,
resulting in 6 bi-allelic genotypes
• The corresponding protein isoforms differ in their lipoprotein receptor
affinity, antioxidant activity, and inflammation modulatory properties
• APOE ε4 carriership is associated with an increased risk of atherosclerosis
and dementia
• Approximately 65%–75% of patients with Alzheimer's disease carry the ε4
allele
• http://aje.oxfordjournals.org/content/early/2015/01/21/aje.kwu442
5. Composition and structure
Lipoproteins are complex aggregates of lipids and proteins that render the lipids compatible with the
aqueous environment of body fluids and enable their transport throughout the body of all vertebrates
and insects to tissues where they are required.
Lipoproteins are synthesised mainly in the liver and intestines. Within the circulation, these aggregates
are in a state of constant flux, changing in composition and physical structure as the peripheral tissues
take up the various components before the remnants return to the liver.
The most abundant lipid constituents are triacylglycerols, free cholesterol, cholesterol esters and
phospholipids (phosphatidylcholine and sphingomyelin especially ), though fat-soluble vitamins and anti-
oxidants are also transported in this way.
Free (unesterified) fatty acids and lysophosphatidylcholine are bound to the protein albumin by
hydrophobic forces in plasma and in effect are detoxified.
the lipoprotein aggregates should be described in terms of the different protein components or
apoproteins (or 'apolipoproteins'), as these determine the overall structures and metabolism, and the
interactions with receptor molecules in liver and peripheral tissues.
the practical methods that have been used to segregate different lipoprotein classes have determined
the nomenclature.
6. Classification
the main groups are classified as chylomicrons (CM), very-low-density
lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins
(HDL), based on the relative densities of the aggregates on ultracentrifugation.
these classes can be further refined by improved separation procedures, and
intermediate-density lipoproteins (IDL) and subdivisions of the HDL (e.g. HDL1,
HDL2, HDL3 and so forth) are often defined, and each of these may have
distinctive apoprotein compositions and biological properties that are relevant
to cardiovascular disease
Density is determined largely by the relative concentrations of triacylglycerols
and proteins and by the diameters of the broadly spherical particles, which vary
from about 6000Å in CM to 100Å or less in the smallest HDL. An alternative
nomenclature is based on the relative mobilities on electrophoresis on agarose
gels. Thus, α, pre-β and β lipoproteins correspond to HDL, VLDL and LDL,
respectively.
7. physical properties
The physical properties of apoproteins enable them to
bind readily at the interface between water and
phospholipids, and specifically they bind to the
phospholipids on the surface of the lipoproteins. In
effect, this outer shell of amphipathic lipids and proteins
solubilizes the hydrophobic lipid core in the aqueous
environment. Each apolipoprotein, other than apo B100,
tends to have a helical shape with a hydrophobic domain
on one side that binds to the lipid core and a hydrophilic
face that orientates to the aqueous phase.
http://lipidlibrary.aocs.org/Lipids/lipoprot/index.htm
9. Comment:
The fatty acid compositions of the main lipid classes ,As might be expected, the
triacylglycerols tend to contain a high proportion of saturated and monoenoic fatty
acids, while the phospholipids contain the highest proportion of polyunsaturated,
specifically arachidonate. The cholesterol esters are enriched in linoleate, reflecting
their biosynthetic origin .Minor differences only occur for each lipid among the
lipoprotein classes. The composition of the triacylglycerols of the chylomicrons
depends largely on that of the dietary fatty acids
http://lipidlibrary.aocs.org/Lipids/lipoprot/index.htm
11. structure
Lipoproteins are spherical (VLDL, LDL, HDL) to discoidal (nascent HDL) in shape with a core
of non-polar lipids, triacylglycerols and cholesterol esters, and a surface monolayer, ~20Å
thick, consisting of apoproteins, phospholipids and non-esterified cholesterol, which serves
to present a hydrophobic face to the aqueous phase
12. structure
Apo B100 and apo B48 large and water-insoluble and they
are the only non-exchangeable apoproteins, which are
assembled into triacylglycerol-rich lipoproteins with their
lipid components in the intestines or liver. Cholesterol
esters are required for proper folding of apo B. With 4536
amino acid residues, apo B100 is one of the largest
monomeric proteins known; apo B48 represents the N-
terminal 48% of apo B100. These stay with their lipid
aggregates during their passage in plasma and the various
metabolic changes that occur, until they are eventually
removed via specific receptors. The remaining soluble or
exchangeable apoproteins, such as apo E, apo A4, apo C3,
apo A5, and apo A1, are much smaller in molecular weight
and can exchange between lipoprotein classes and acquire
lipids during circulation.
13. Apo A1
is the main protein component of HDL, and is synthesised within the liver
(70%) and intestine (30%). It is a 28-kDa single polypeptide consisting of 243-
amino acids, which has no disulfide linkages or glycosylation. Apart from the
44 amino acid N-terminal region, the protein is arranged as eight α-helical
segments of 22 amino acids with two 11-mer repeats, and in some instances
these are separated by proline residues. It is believed that the helices are
amphipathic, each with a hydrophobic face that interacts with lipids and a
polar face that interacts with the aqueous phase. The molecule probably
exists in several conformational forms.
Apo A2 is the second most important HDL apolipoprotein, and it exists as a
homodimer with two polypeptide chains, each 77 amino acids in length and
linked by a disulfide bond.
Human apo A4 is the largest member of the exchangeable apolipoprotein
family and is a 376-amino acid glycoprotein, which is synthesised in intestinal
enterocytes and secreted as a constituent of chylomicrons
14. Apo protein(a) is an Adhesive Protein
The apo(a) molecule contains the arginine-glycine-aspartate sequence
characteristic of adhesive proteins
Properties such as organ morphogenesis, differentiation, and
growth have recently been discovered to be associated with a
number of proteins present in the extracellular matrix and the
blood, such as fibronectin, collagen, laminin, vitronectin,
osteopontin including also coagulation factors such as fibrinogen
and von Willebrand factor. These ‘adhesive proteins’ contain a
characteristic amino-acid sequence: arginine-glycine-aspartate
(RGD), by means of which they interact with integrins, a family of
cell surface receptors for adhesive proteins.
15. Detection of apo(a) in human
sperm
o When human sperm was analysed in our
laboratory, both the seminal plasma and the
cellular fraction were found to contain apo(a).
o We could not detect any apo B associated with
this apo(a). The detection of apo(a) in human
sperm is the proof that isolated apo(a) is
actually produced and secreted into a body fluid
independently of apoB and lipoprotein particles.
16. The role of the adhesive protein apo(a) under pathological
conditions
Apo(a) co-ordinates the interaction between cellular systems and the
extracellular matrix during repair processes. Apo(a) is involved in tissue
reformation during acute repair processes such as wound-healing and Lp(a)
plasma levels are known to be elevated during the post operative phase.
Chronic repair processes are characteristic for all pathological states and they are
sustained by chronic ascorbate deficiency. In this situation adhesive proteins play
a particular role. They interact specifically with cellular systems such as
monocytes, T cells as well as thrombocytes and thereby play a critical role in
inflammatory, infectious, hemostatic and many other processes
The adhesive protein apo(a), on the basis of its physiological properties, may play
an important role in disease containment, tissue reorganisation and chronic
repair processes in general. The elevation of Lp(a) plasma levels as established for
cancer, cardiovascular, inflammatory and many other diseases is additional
confirmation for this concept.
In this context it is noteworthy that apo(a) can interact with a variety of other
adhesive proteins, such as fibrinogen, collagen, and fibronectin. The interaction of
apo(a) with fibronectin is of particular interest.
17. Apo(a) and fibrnectin
Fibronectin is one of the best-characterised adhesive proteins. It is present in
plasma and other body fluids, with a particularly high concentration in the
seminal fluid. Fibronectin is involved in cellular migration during embryogenesis,
morphogenesis, differentiation, and growth of many systems. In particular it has
been shown to be involved in development and differentiation of the brain and
the peripheral nervous system. Other functions comprise platelet aggregation,
thrombus formation, and wound healing
Apo(a) and fibronectin share common structural and functional properties. Both
molecules consist of numerous repeated segments, including kringle structures,
and possibly share common ancestral genes. Both apo(a) and fibronectin can bind
to fibrin and collagen. A particularly well-characterised region is the cell-binding
region of fibronectin, which contains an RGD sequence and is critically involved in
the interaction of fibronectin with integrins and different cell systems.
http://www4.dr-rath-
foundation.org/THE_FOUNDATION/About_Dr_Matthias_Rath/publications/pub08.htm
18. the normal function of the APOB
gene
The APOB gene provides instructions for making two versions of the apolipoprotein B protein, a
short version called apolipoprotein B-48 and a longer version known as apolipoprotein B-100.
Both of these proteins are components of lipoproteins, which are particles that carry fats and
fat-like substances (such as cholesterol) in the blood.
Apolipoprotein B-48 is produced in the intestine, where it is a building block of a type of
lipoprotein called a chylomicron. As food is digested after a meal, chylomicrons are formed to
carry fat and cholesterol from the intestine into the bloodstream. Chylomicrons are also
necessary for the absorption of certain fat-soluble vitamins such as vitamin E and vitamin A.
Apolipoprotein B-100, which is produced in the liver, is a component of several other types of
lipoproteins. Specifically, this protein is a building block of very low-density lipoproteins (VLDLs),
intermediate-density lipoproteins (IDLs), and low-density lipoproteins (LDLs). These related
molecules all transport fats and cholesterol in the bloodstream.
Low-density lipoproteins are the primary carriers of cholesterol in the blood. Apolipoprotein B-
100 allows these particles to attach to specific receptors on the surface of cells, particularly in
the liver. The receptors transport low-density lipoproteins into the cell, where they are broken
down to release cholesterol. The cholesterol is then used by the cell, stored, or removed from
the body.
19. How are changes in the APOB gene related to
health conditions?
More than 90 mutations in the APOB gene have been found
to cause familial hypobetalipoproteinemia (FHBL), a
disorder that impairs the body's ability to absorb and
transport fat. Most APOB gene mutations that cause FHBL
lead to the production of apolipoprotein B that is abnormally
short.
The severity of the condition largely depends on the length
of the abnormal apolipoprotein B. Some mutations in the
APOB gene lead to the production of a protein that is shorter
than apolipoprotein B-100, but longer than apolipoprotein
B-48. In these cases, normal apolipoprotein B-48 is still made
in the intestine. The normal-length apolipoprotein B-48 can
form chylomicrons normally, but the abnormally short
apolipoprotein B-100 produced in the liver is less able to
produce lipoproteins.
20. continue
Other mutations result in a protein that is shorter than
both apolipoprotein B-48 and apolipoprotein B-100. In
these cases, no normal-length apolipoprotein B protein is
produced. The severely shortened protein is not able to
form lipoproteins in the liver or the intestine. Generally, if
both versions of the protein are shorter than
apolipoprotein B-48, the signs and symptoms are more
severe than if some normal length apolipoprotein B-48 is
produced. All of these protein changes lead to a reduction
of functional apolipoprotein B. As a result, the
transportation of dietary fats and cholesterol is decreased
or absent. A decrease in fat transport reduces the body's
ability to absorb fats and fat-soluble vitamins from the
diet, leading to the signs and symptoms of FHBL.
21. hypercholesterolemia - caused
by mutations in the APOB gene
At least five mutations in the APOB gene are known to cause a form of inherited
hypercholesterolemia called familial defective apolipoprotein B-100 (FDB). This
condition is characterized by very high levels of cholesterol in the blood and an
increased risk of developing heart disease. Each mutation that causes this
condition changes a single protein building block (amino acid) in a critical region
of apolipoprotein B-100. The altered protein prevents low-density lipoproteins
from effectively binding to their receptors on the surface of cells. As a result,
fewer low-density lipoproteins are removed from the blood, and cholesterol
levels are much higher than normal. As the excess cholesterol circulates
through the bloodstream, it is deposited abnormally in tissues such as the skin,
tendons, and arteries that supply blood to the heart (coronary arteries). A
buildup of cholesterol in the walls of coronary arteries
greatly increases a person's risk of having a heart attack.
22. Where is the APOB gene
located?
Cytogenetic Location: 2p24-
p23
Molecular Location on
chromosome 2
23. What other names do people use for
the APOB gene or gene products?
apoB-48
apoB-100
APOB_HUMAN
apolipoprotein B (including Ag(x) antigen)
http://ghr.nlm.nih.gov/gene/APOB
24. Apo B
is a protein that is involved in the metabolism of lipids and is the main protein
constituent of lipoproteins such as very low-density lipoprotein (VLDL) and
low-density lipoprotein (LDL, the "bad cholesterol"). Concentrations of apo B
tend to mirror those of LDL-C.
A health practitioner may order both an apo A-I (associated with high-density
lipoprotein (HDL), the "good" cholesterol) and an apo B to determine an apo
B/apo A-I ratio. This ratio is sometimes used as an alternative to a total
cholesterol/HDL ratio to evaluate risk for developing CVD.
Apo B levels may be ordered to monitor the effectiveness of lipid treatment as
an alternative to non-HDL-C (non-HDL-C is the total cholesterol concentration
minus the amount of HDL).
In rare cases, an apo B test may be ordered to help diagnose a genetic problem
that causes over- or under-production of apo B.
25. When is it ordered?
Apo B may be measured, along with an apo A-I or
other lipid tests, when a health practitioner is trying
to evaluate someone's risk of developing CVD and
when a person has a personal or family history of
heart disease and/or abnormal lipid levels,
especially when the person has significantly
elevated triglyceride levels.
Sometimes apo B is ordered to monitor a person
who is undergoing treatment for high cholesterol.
26. What does the test result mean?
Elevated levels of apo B correspond to elevated levels of LDL-C and to non-HDL-C and are
associated with an increased risk of cardiovascular disease (CVD). Elevations may be due to a high-
fat diet and/or decreased clearing of LDL from the blood.
Some genetic disorders are the direct (primary) cause of abnormal levels of apo B. For example,
familial combined hyperlipidemia is an inherited disorder causing high blood levels of cholesterol
and triglycerides. Abetalipoproteinemia, also called Apolipoprotein B deficiency or Bassen-
Kornzweig syndrome, is a very rare genetic condition that can cause abnormally low levels of apo B.
For more on some of these disorders, see the Related Pages tab.
Abnormal levels of apo B can also be caused by underlying conditions or other factors (secondary
causes). Increased levels of apo B are seen, for example, in:
Diabetes
Use of drugs such as: androgens, beta blockers, diuretics, progestins (synthetic progesterones)
Hypothyroidism
Nephrotic syndrome (a kidney disease)
Pregnancy (levels increase temporarily and decrease again after delivery)
27. Apo B levels
may be decreased with any condition that affects lipoprotein
production or affects its synthesis and packaging in the liver. Lower
levels are seen with secondary causes such as:
Use of drugs such as: estrogen (in post-menopausal women), lovastatin,
simvastatin, niacin, and thyroxine
Hyperthyroidism
Malnutrition
Reye syndrome
Weight reduction
Severe illness
Surgery
Cirrohsis
An increased ratio of apo B to apo A-I may indicate a higher risk of
developing CVD.
http://labtestsonline.org/understanding/analytes/apob/tab/test/
28. Effects of apoB truncation on
lipid affinity Several laboratories have reported a number of naturally occurring apoB gene mutations that
produce truncated forms of apoB that range in length from 9% (apoB-9) to 89% (apoB-89) of full-
length apoB-100
Because many domains throughout the entire length of apoB appear to be involved in lipid binding,
truncated apoB is expected to form denser lipid-poor particles. For example, Parhofer et al.
demonstrated that apoB-89, apoB-75, and apoB-54.8 were secreted into the VLDL fraction, whereas
apoB-31 was secreted into HDL. In general, the shorter the mutant apoB protein, the denser and
more lipid poor the particle
In fact, studies on the expression of truncated forms of apoB in cultured cells have indicated that the
size of the secreted particles and the amount of lipid per particle are proportional to the length of
the truncated apoB
29. Some elevations of apo B-100 (and LDL-C) are due to mutations in the APOB gene that cause
it to produce apo B-100 that is not recognized as easily by LDL receptors. Others are in the LDL
receptor system of the liver cell that recognizes apo B-100. These genetic defects impede the
clearing of LDL from the blood and result in accumulations of LDL in the circulation, increasing
the risk of heart disease.
Chylomicrons, the lipoprotein particles that carry dietary lipids to the liver, contain a
lipoprotein called apolipoprotein B-48. It is about half the size of apo B-100 and is structurally
related to apo B-100. It is not considered a risk factor for atherosclerosis and is not measured
as part of the apo-B test. The apo B test is specific for apo B-100.
http://labtestsonline.org/understanding/analytes/apob/tab/test/
30. APO E
is an O-linked glycoprotein in three isoforms
and is synthesised by many tissues, including
liver, brain, adipose tissue, and artery wall, but
most is present in plasma lipoproteins derived
primarily from the liver. It is involved in many
aspects of lipid and lipoprotein homeostasis,
both for the triacylglycerol-rich lipoproteins and
HDL, but it is also important for lipid
metabolism in brain and other tissues. In
addition, it is believed to have some non-lipid
related functions, for example on immune
response and inflammation.
31. The function of APO E
Apolipoprotein E has many functions in the body. When it is synthesized by the liver
as part of VLDL it functions in the transport of triglycerides to the liver tissue. It is also
incorporated into HDL (as HDL-E) and functions in cholesterol distribution among cells.
It is also incorporated into intestinally synthesized cholymicrons and transports
dietary triglycerides and cholesterol. It is involved in lipid metabolism by mediating
the receptor binding of apo-E lipoproteins to the LDL receptor. Receptor binding
begins the cellular uptake of lipoproteins to be used in intracellular cholesterol
metabolism
Apolipoprotein E is synthesized in several areas of the body. Approximately three-
fourths of the plasma apo-E is synthesized in the liver. Liver apo-E is produced
primarily by hepatic parenchymal cells, and it becomes a component of VLDL. The
brain also produces a large amount of apo-E. Approximately one-third of the liver
levels of apo-E are made by astrocytes, the star-shaped branching neuroglial cells that
are found in the brain. Apo-E is also synthesized in the spleen, lungs, adrenals, ovaries,
kidneys, muscle cells, and in macrophages (Mahley). The apo-E synthesized from
macrophages is involved in reverse cholesterol transport, local redistribution of
cholesterol, and protection against the development of artherosclerotic lesions
http://wwwchem.csustan.edu/chem4400/SJBR/Dawn971.htm
32. Isoforms of APO E
There are three different isoforms of apolipoprotein E: apo-E 2, apo-E 3, And apo-E 4.
Apo-E 3 is the parent form and all others are compared to it. Apo-E 2 is different from
apo-E 3 because a cysteine is substituted for arginine at residue 158. Apo-E 2 is
associated with Type III Hyperproteinemia (where there is an excess of protein in the
blood plasma) and it does not bind to the lipid receptor. In fact, apo-E 2 shows less than
2% of the normal receptor binding activity. Apo-E 4 has an arginine substituted for
cysteine at residue 112. This residue is outside of the strongest lipid binding area and
the substitution doesn't.affect the lipid binding ability of the apolipoprotein.
Functionally, apo-E 4 still has 100% of normal receptor binding activity .
Apoliooprotein E uses different metabolic pathways in the body. one of these pathways
is endocrine-like, and involves the redistribution of lipids among cells of different organs.
It takes lipids from the areas where the lipid is synthesized and distributes them to other
areas where the lipids are used or stored. Another pathway in paracrine-like, where the
lipids are transported among cells in the same organ or tissue. Apo- E is also involved in
various pathways that are unrelated to lipid transport, such as the stimulation of
lymphocytes.
http://wwwchem.csustan.edu/chem4400/SJBR/Dawn971.htm
33. Apo D
is atypical in that it is very different in structure
from other apolipoproteins, and it is expressed
widely in mammalian tissues (most others are
produced mainly in liver and intestine). In plasma,
it is present mainly in HDL and to a lesser extent
in LDL, where it may function as a multi-ligand
binding protein capable of transporting small
hydrophobic molecules such as arachidonic acid,
steroid hormones, and cholesterol for metabolism
or signalling.
34. APOD occurs in the macromolecular
complex with lecithin-cholesterol
acyltransferase. It is probably involved
in the transport and binding of bilin.
Appears to be able to transport a
variety of ligands in a number of
different contexts.
http://www.uniprot.org/uniprot/P050
90
35. The apolipoprotein (apo)
B/apoA-I ratio
represents the balance between apoB-rich
atherogenic particles and apoA-I-rich
antiatherogenic particles, and this ratio is
considered to be a marker of cardiovascular risk.
Although many studies have demonstrated the
importance of the apoB/apoA-I ratio in predicting
the presence or absence of cardiovascular disease,
less is known about apoB/apoA-I ratio as a marker
of plasma atherogenicity.
36. The main cause of errors in the estimation of
the lipoprotein-related risk of cardiovascular
disease when using the conventional lipid
indexes is due to the wide variance of
cholesterol within the low-density
lipoprotein and high-density lipoprotein
molecules; this variance is due to the active
exchange of lipid components between
lipoproteins.
the lipoprotein cholesterol concentration
does not always correspond to lipoprotein
concentration
37. ApoB is an essential structural component of very low-
density lipoproteins, intermediate-density lipoproteins, and
low-density lipoproteins.
Because each particle of these lipoproteins contains one
molecule of apoB, the total atherogenic particles number
can be accurately estimated by measuring the plasma level
of this apoprotein. In contrast, apoA-I, the major structural
constituent of antiatherogenic high-density lipoproteins,
exchanges between lipoproteins and the number of
apoprotein molecules varies between lipoprotein particles.
levels of apoA-I in plasma are strongly correlated with HDL
levels.
apoB and apoA-I are assumed to be superior markers for
lipoprotein abnormalities
