Crystallins are adapted not to
crystallise; the eye lens needs to be a highly concentrated solution, but it
needs to avoid small crystals or aggregates, since they would scatter light and
make the lens opaque. Our lenses achieve this by mixing together several
different crystallins, which together form a uniform, glassy solution. The
protein molecules are arranged in a way which means that their refractive index
is nearly the same as glass- which makes the lens transparent. This is due to the small size of the protein
molecules, less than 10 nm in diameter, and their close packing at high
concentration
The lens contains three major types of
crystallins, making up about 90% of the protein. Alpha crystallins are the most
common. They are composed of two similar types of protein chain, which
associate to form large spherical complexes containing about 40 chains. These
large spheres repel one another and distribute themselves throughout the lens
cells. Beta crystallins, shown here from, also form oligomeric complexes
(contains a limited number of monomers), typically formed of two or six copies
of the chain. There are several similar beta crystallins, which can mix and
match to form a bunch of different types of oligomers. Finally, gamma
crystallins are monomeric, and serve as a weak glue to gently bind the alpha
crystallins together.
Our crystallin proteins need to
last our entire life, so the lens contains a powerful method to protect them.
Alpha crystallin acts as chaperone, finding damaged proteins and binding to
them before they can form translucent or opaque complexes. Unfortunately, in
spite of this protection, the damage builds up as we age, as crystallins are
broken or unfolded or oxidized. Slowly, the damage leads to progressive
build-up of opaque aggregates, leading to cataracts.
