Protein Structure Information Sheet Template

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Protein Structure
Protein Structure
Secondary structure refers to the shape of a folding protein due exclusively to hydrogen bonding between its backbone
amide and carbonyl groups. Secondary structure does not include bonding between the R-groups of amino acids,
hydrophobic interactions, or other interactions associated with tertiary structure. The two most commonly encountered
secondary structures of a polypeptide chain are α-helices and beta-pleated sheets. These structures are the first major
steps in the folding of a polypeptide chain, and they establish important topological motifs that dictate subsequent tertiary
structure and the ultimate function of the protein.
α-Helices
An α-helix is a right-handed coil of amino-acid residues on a polypeptide chain, typically ranging between 4 and 40
residues. This coil is held together by hydrogen bonds between the oxygen of C=O on top coil and the hydrogen of N-H
on the bottom coil. Such a hydrogen bond is formed exactly every 4 amino acid residues, and every complete turn of
the helix is only 3.6 amino acid residues. This regular pattern gives the α-helix very definite features with regards to the
thickness of the coil and the length of each complete turn along the helix axis.
The structural integrity of an α-helix is in part dependent on correct steric configuration. Amino acids whose R-groups are
too large (tryptophan, tyrosine) or too small (glycine) destabilize α-helices. Proline also destabilizes α-helices because
of its irregular geometry; its R-group bonds back to the nitrogen of the amide group, which causes steric hindrance. In
addition, the lack of a hydrogen on Proline's nitrogen prevents it from participating in hydrogen bonding.
Another factor affecting α-helix stability is the total dipole moment of the entire helix due to individual dipoles of the C=O
groups involved in hydrogen bonding. Stable α-helices typically end with a charged amino acid to neutralize the dipole
moment.
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