In: Biology
Describe the relationship between protein structure and function.
Answer . The three-dimensional structure of a protein defines
not only its size and shape, but also its function. One
characteristic that affects function is the hydrophobicity of a
protein, which is determined by the primary and secondary
structure. For example, let's look at membrane proteins. Membranes
contain large amounts of lipids, which are notoriously hydrophobic
(water and oil don't mix). The membrane-spanning regions of
membrane proteins are typically alpha helices, made of hydrophobic
amino acids. These hydrophobic regions interact favorably with the
hydrophobic lipids in the membrane, forming stable membrane
structures.
Hemoglobin is a soluble protein - found in the cytoplasm of red
blood cells as single molecules - which bind oxygen and carry it to
the tissues. In sickle cell anemia, a mutation in the beta-globin
protein of the red blood cell increases its hydrophobicity and
causes the mutant protein molecules to stick to each other,
avoiding the aqueous environment. Chains of hemoglobin change the
shape of the red blood cell from round to a sickle shape, which
causes the cells to collect in narrow blood vessels.
Figure 2. Active site | |
The folding of a protein allows for interactions between amino
acids that may be distant from each other in the primary sequence
of the protein. In enzymes, some of these amino acids form a site
in the structure that catalyzes the enzymatic reaction. This site,
called the active site of the enzyme, has amino acids that bind
specifically to the substrate molecule, also called a ligand(Fig.
2). In a similar manner, certain sites in cell receptor proteins
bind to specific ligand molecules that the receptor
recognizes.
Alterations in amino acids that may be distant from each other in
the primary sequence can lead to changes in folding. It may also
cause changes in chemical interactions among amino acids at the
active site, which alter the enzyme activity or binding of the
ligands to receptor proteins. Binding of ligands to an active site
requires specific amino acids. Therefore, an active site in a new
enzyme that belongs to the same family as a known enzyme can
usually be identified by its similarity to the active site of the
known protein. Computer programs can use the information from a
database of known enzymes to predict the active site of a new
protein using a template-based method, similar to that described
above for determining the three-dimensional structure of a protein.
Once the program has identified the potential ligand-binding sites,
other programs can test the fit and the binding ability of
thousands of possible ligand molecules - even theoretical ligands
that may not yet exist. This has tremendous possibilities for the
design of new drugs, particularly for cancer therapy.