Biomedical Engineering Reference
In-Depth Information
their functional form. There are several levels of organization; first the interactions
between neighboring parts of the chain can lead to regular structures such as the
α
-helices or the
β
-sheets. These structures (the secondary structures) arrange them-
selves into more complex domains that are common to some extent to many differ-
ent proteins. For a given protein, these domains are arranged in a particular way to
define the full 3-D tertiary structure. Very often, these structures are not functional
by themselves; they self-assemble, and the functional protein is a multimer (some-
times called the quaternary structure) of several of these units that are then called
monomers (not to be mistaken with the monomer as a single unit of a polymer
chain) (Figure 8.5). Some proteic assemblies result from only a few monomers
(sometimes only one); some others, in the self-assembly of many of them (up to
several hundred): actin filaments for instance are made of the helical arrangement
of actin monomers; tubulin monomers organize themselves in a cylinder to form
microtubules. Actin microfilament and microtubules form the cell cytoskeleton and
have a very dynamic assembly-disassembly behavior inside the cells.
As the 3-D structure imposes the protein function, it is of great interest to ex-
perimentally access it. Practically, the techniques used are X-rays diffraction, elec-
tron microscopy, and for smaller proteins, NMR. We have seen in the preceding
part that it was already difficult to compute the shape of RNA molecules where
there are only a limited number of possible interactions. This is of course even more
the case here and it is actually quite difficult to predict the 3-D structure of a protein
from its sequence [7].
Some proteins can also be engineered to become tools in the hands of biologists.
Enzymes for instance are catalysts that are of particular importance since most of
the biological processes are highly dynamic. Members of this family include restric-
Figure 8.5
Structure of
α
-hemolysin. This complex is a heptamer that forms pores in membranes;
the “stem” crosses the lipid membrane and the “cap” is in contact with the extracellular medium.
(From [8].)
