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Plasma
lipoproteins are large complexes of lipids and proteins involved in the
transport of lipids across different tissues and in lipid metabolism (Mahley et al., 1984). Depending on the density
by which they are separated following ultracentrifugation, plasma lipoproteins
are classified as: chylomicrons; very low density lipoproteins (VLDL); low
density lipoproteins (LDL); intermediate density lipoproteins (IDL); high
density lipoproteins (HDL) (Kwan et al.,
2007). Apolipoproteins form the protein portion of plasma lipoproteins (Mahley et al., 1984). Several apolipoproteins
have been identified in humans (e.g. Apo A (I, II, IV); Apo B (48, 100); Apo C (I, II, III, IV); Apo D; Apo
E; Apo F; Apo L (1-6); Apo M), and they associate with different types
of lipoproteins (table 1).

Apolipoproteins play several
roles in lipid metabolism and are essential in regulating the function and
structure of lipoproteins. They are involved in the transport lipids
(cholesterol, phospholipids, triglycerides) across different tissues, lipid
uptake by binding to specific cell surface receptors, and activation of enzymes
of lipid metabolism (Mahley et al.,
1984). Apo A-I is an activator of the enzyme lecithin:cholesterol
acyltransferase (LCAT), which esterifies free cholesterol in reverse
cholesterol transport to the liver, while Apo B-100 allows LDL particles to be
recognised and taken up by the LDL-receptor on hepatocytes (Kwan et al., 2007). Other apolipoproteins
have less clear functions. For example, the apolipoprotein L family are highly
expressed in the placenta and may play a role in exchange and transport of
lipids across this and other tissues, however, their main functions have not
been fully determined (Page et al., 2001).

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Apo A-I, A-IV and E share
both structural and physiochemical similarities. They are members of the exchangeable
apolipoproteins, which have the ability to move and transfer between
lipoprotein particles (Segrest et al., 1992).
The aim of this project will be to identify apolipoprotein sequences and to determine how have
apolipoproteins A-I, A-IV and E evolved in different species, and to compare common
structural features of Apo A-I, A-IV, E and L1. The hypothesis behind this work
is that apolipoproteins are highly conserved in different taxa due to the
essential functions they carry out in lipid transport and metabolism, and that
their evolution follows the evolutionary path expected from the tree of life. Evidence
from previous studies suggest that the exchangeable apolipoproteins have
evolved from a common ancestral genes by means of gene duplication (Segrest et al. 1992), and that apolipoproteins
have undergone speciation events early in the evolution of vertebrates (Babin
et al. 1997).

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