Forces that Determine Protein Structure

What forces determine the 3-dimensional structure of proteins?

Disulfide Bonds (or Disulfide Bridges)

These bonds form when two cysteine molecules join together to form a bond between their two sulfur atoms (shown below). Such bonds cross-link polypeptide chains or parts of chains. Because they are nonpolar covalent bonds, they are very strong bonds and not easily broken. They are considered to be part of primary structure, although they do contribute to the three-dimensional conformation of the protein.

Hydrogen Bonds

Hydrogen bonds form when a hydrogen atom is covalently bonded to one electronegative atom (usually oxygen or nitrogen) and is simultaneously attracted to another electronegative atom. This attraction is the hydrogen bond (shown as dashed lines below).

Ionic Bonds

A charged R-group in one part of a polypeptide chain can attract an oppositely charged R-group in another part of the chain. The force of such an electrostatic attraction is the ionic bond, sometimes called a salt linkage, salt bridge, or electrostatic bond.

Hydrophobic Interactions

Non polar R-groups tend to cluster together in water. These weak associations are called hydrophobic interactions. When the polypeptide chain folds, the hydrophobic R-groups tend to be close to each other in the interior of the folded chain, whereas hydrophilic R-groups tend to be on the outside, attracted to water. The diagram below illustrates some hydrophobic interactions between R-groups. Hydrophobicity of various amino acids.