What
are crystals?
Crystals are all around us in our everyday lives.
Not only can we see crystals in rock formations, but with the aid of a
microscope one can see that grains of sugar and salt are great examples
of what crystals look like. These types of crystals often have a
unique three dimensional shape, called a lattice,
that is very strong due to the type of atoms and molecules they are composed
of. Although we can not see protein crystals without the help of
a special kind of microscope, they too have a unique three dimensional
structure.
What
are proteins?
Proteins are one of the most important substances
in living organisms. They are composed of chains of amino
acids joined by peptide bonds. There
are 20 different amino acids and they can appear in any sequence in a protein.
Therefore proteins can have many different functions in an organism, and
take many different shapes. The fact that there are so many different
shaped proteins can make it difficult to determine the structure of a specific
protein.
Why
are scientists interested in protein crystals?
Since proteins are so essential
to our bodies, scientists are interested in determining the structures
of protein crystals to learn more about their functions. Unlike rock
crystals which are strong because they are composed of minerals, protein
crystals are very fragile and soft because they are composed of amino acids.
The fact that they are so fragile can make them difficult to work with.
This is where x-ray crystallography comes into play.
How
do scientists crystallize proteins?
In order to analyze a protein's
structure using x-ray crystallography, they must first grow the crystals
using special techniques. One technique for growing crystals is called
the hanging drop technique. Using the special equipment below and
carefully varying the concentration of protein solution, scientists and
induce the growth of tiny crystals hanging on the underside of a slide.
The formation of these crystals is called nucleation. Once enough
pure protein crystals have been obtained, they can be analyzed using x-ray
crystallography.
How
does X-Ray Crystallography work? 
As explained by the diagram below, the tiny protein
crystals are bombarded with x-rays. These x-rays pass through the
crystals and send signals to a detector. With the information gathered
by the detector, scientists can use computers to reconstruct the three
dimensional structure of the protein. With this computer generated
reconstruction of the protein structure, all kinds of information can be
gathered about the protein.
Group 2
The Biology Project
University of Arizona
Tucson, AZ. 85719