Download The structure of a lipoprotein or the complementarity of neutrons and

Biologists are trying to understand the very
©1996 - Institut Laue-Langevin
The structure of a lipoprotein
or the complementarity
of neutrons and X-rays
Lipovitellin
complex mechanism of life and its modifications:
This is a storage lipoprotein which in our case was
illness.
purified from the eggs of a fish, the lamprey.
This can be approached by learning about the role
Lipovitellin is synthesized in the liver, then transported
of proteins, these large molecules with multiple
in the blood to the eggs where, in some cases, it
functions, on an atomic level,. This is a difficult task.
crystallizes spontaneously.
Major laboratories sometimes need several years to
The team of Prof. L. Banaszak (USA) succeeded in
separate enough pure protein to make a crystal and
growing crystals to millimeter dimensions and to
finally obtain its atomic structure (the nature and
determine its structure using X-rays (Fig. 2). This
relative positions of the atoms), mostly by X-ray
structure shows the bonds between each unit (amino
techniques.
acid) of the polymeric chain of the storage protein, but
In some cases these techniques, when used alone,
does not show the stored lipid which constitutes about
are unadapted to the task and neutrons can be of
15% of the molecular mass.
considerable help, as we shall see in the case of a
This is due to the fact that the lipid is almost invisible
lipoprotein called lipovitellin.
to X-rays since its scattering power is almost the same
as that of the water which surrounds the sample and
Lipoproteins
The lipoproteins are protein molecules which are
associated with lipid molecules (fats). There are two
which is indispensable for its conservation.
We will now explain how neutrons help resolve this
problem.
main types of blood lipoproteins; those which are used
to transport lipids from one place to another and those
Why neutrons?
which are used to store lipids for later use.
The transport lipoproteins are very well known and
same scattering power. The lipid is almost invisible
often implicated in cardio-vascular desease. The study
since, in a certain way, it is seen by the X-rays as being
of their structure at high resolution, is very difficult
the same shade as the water. We say that the contrast is
because of purification and crystallization problems.
zero.
To neutrons, the contrast is still zero, but if we
The storage lipoproteins are biologically similar to
their transport counterparts and have the advantage of
Figure 1. The DB21 diffractometer for the study of biological
crystals.
To X-rays, the lipid and water have almost the
replace the normal water (H2O) by heavy water1 (D2O)
being easier to purify and crystallize. They are often
found in eggs of birds, frogs and fish.
1
The normal hydrogen atom (H) and its heavy isotope (D),
have the same chemical properties. They are seen as identical
the contrast then becomes very high. The lipid and the
of their fluidity, only the global shape of the volume
arranged in this cavity, we have superimposed two lipid
protein then have very different contrasts, and we can
they occupy, but this is enough to prove that there is
molecules in yellow (Fig. 3).
mask one or the other at will by using the appropriate
enough place to store about 40 lipid molecules grouped
mixture of H2O/D2O. This technique is called contrast
variation.
together as in a cellular membrane.
X-rays and neutrons hand in hand
It is a fact that, in fundamental research, just as in
life, it is often necessary to take a second look at
Measuring with neutrons
phenomena to understand them. Here we have used two
The method consists of measuring the neutrons
very complementary types of radiation (X-rays and
diffused (deviated) by a single crystal of lipoprotein,
neutrons) each of which sees a different aspect of the
with a volume less than 1 mm3, immersed in a little
same sample so as to understand better the storage of
water.
lipids in lipovitellin.
This is made possible by the diffractometer DB21 at
the ILL (Fig. 1). This is a unique instrument (the only
one of its kind in the world), since it was especially
designed for the study of biological crystals, that is,
very large molecules, and because it takes advantage of
the high flux of neutrons produced by the reactor of the
ILL.
The results
Figure 2. Structure of lipovitellin. The mean diameter of
the molecule is 50 Å (50 10-8 cm).
For the lipovitelline, the measurements were carried
In green: structure of the protein determined by X-
out for crystals immersed in different mixtures of
rays. The representation is somewhat simplified
H2O/D2O (20%, 40%, 60%, etc.).
because the molecule contains more than 10,000 atoms.
After analysing the data collected using neutrons,
added to that collected using X-rays, the structure
represented in figure 2 is obtained.
We cannot distinguish the lipid molecules because
Only the polypeptide chain and the bonds between
amino acids are shown. This is enough to show the
external shape of the molecule and the folding of the
chain.
Figure 3. The structure of lipovitellin seen from another
angle. We have superimposed two molecules of lipid (in
yellow) over the lipid part (in violet) to show how they
can occupy the cavity in lipovitellin.
In violet: the lipid part of the structure, determined by
neutron diffraction. The contours show that the lipids
by X-rays because they have the same number of electrons,
but since their nuclei are different they have different
scattering powers for neutrons.
are contained in a protein cavity which protects them
from the aqueous media. To show how they are
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