Proteins – Tertiary Structures

There are two notable tertiary structures – α (ALPHA) helix and BETA pleated sheet.

α Helix

  • Right handed helix much like that of a DNA helix.
  • Each amino acid side chain (R group) is 100 degrees relative to the last side chain, outside of the helix. This means there are 3.6 residues per turn and 5.4 angstroms per turn/level. On the sketch below, each R stands for a different amino acid side chain.

A couple of alterations:

  • Glycine residues will disrupt the α helix as it has no chiral carbon. The lack of a chiral carbon in Glycine makes it very flexible.
  • Proline has a cyclic side chain which restricts the rotation of phi to ~50°. There is also no H atom on the N end of the amino acid so Hydrogen bonding does not occur between residues.

Amphipathic Helices:

  • Helixes can end up with hydrophobic residues on one side and polar (hydrophilic) on the other – essentially giving the helix two faces. The image below illustrates R1, R4, R7 and R8 as hydrophobic, and R2, R4, R5, and R6 as hydrophilic.
  • This means helices can be constructed to generate lipid (hydrophobic) or water (hydrophilic) soluble proteins.

– β Pleated Sheet

There are two types of pleated sheet – Parallel and Anti-Parallel.

  • Parallel sheet has successive polypeptide strands in the same direction.
  • Anti-Parallel sheet has successive polypeptide strands in opposite directions.

These strands are typically 5-10 amino acids long, and the pleated sheet is formed by a continuous series twisted into these strands.

It has been suggested that the anti-parallel configuration is more stable.