Protein Tertiary Structure Explained

Tertiary structure is the three-dimensional arrangement of all atoms making up the protein. The tertiary structure of a protein is directly responsible for its biological properties. For proteins that consist of a single polypeptide chain (lacking quaternary structure), the tertiary structure represents the maximum structural information obtainable.

Types of Tertiary Structure

There are two types of tertiary structure:

• Fibrous tertiary structure: Proteins where one dimension is much larger than the other two.
• Globular tertiary structure: More frequent type, where one dimension predominates over the others, and its shape is roughly spherical.

Forces Stabilizing Tertiary Structure

The forces that stabilize the tertiary structure of a protein are established between the various amino acid side chains that compose it. The bonds themselves can be of two types: covalent and non-covalent.

Covalent Bonds

Covalent bonds may be due to:

• Formation of a disulfide bond between two Cys side chains.
• Formation of an amide bond between the side chains of Lys and a dicarboxylic amino acid.

Non-Covalent Bonds

Non-covalent bonds may be of four types:

• Electrostatic forces between ionized side chains with opposite charges.
• Hydrogen bonds between side chains of polar amino acids.
• Hydrophobic interactions between nonpolar side chains.
• Forces of polarity due to dipole-dipole interactions.

As a result of these interactions in globular proteins with tertiary structure:

• Side chains with nonpolar character are oriented inside the molecule, avoiding interactions with the solvent, and form a compact core with hydrophobic character.
• Side chains of polar amino acids are located on the surface of the molecule, interacting with water and allowing the protein to remain in solution.

Not all these interactions contribute equally to the maintenance of tertiary structure. Obviously, the bond that provides more stability is covalent, and among non-covalent interactions, hydrophobic interactions are the most important, as they require close proximity between the nonpolar groups of these amino acids.

Protein Denaturation

When these interactions disappear, the tertiary structure of the protein becomes unstable and loses its characteristic three-dimensional structure, so that it loses its function and often precipitates. This phenomenon is known as denaturation.

There are distinct regions within the tertiary structure of proteins that act as autonomous units of folding and/or protein denaturation. The total or partial loss of structural levels above the primary structure is called denaturation, which may be reversible or irreversible.

TAMPONS