Protein Structural Organization and Amino Acid Properties

Backbone Conformation and the Ramachandran Diagram

While the side chains (R-groups) provide the chemical identity, the backbone of the protein determines its shape. The polypeptide backbone is constrained by rotation around two specific dihedral (torsional) angles:

• $\phi$ (phi): The angle of rotation around the $C\alpha-N$ bond.
• $\psi$ (psi): The angle of rotation around the $C\alpha-C$ bond.

Because of steric hindrance (atoms bumping into each other), not all combinations of $\phi$ and $\psi$ are possible. To visualize these allowed orientations, scientists use a Ramachandran Diagram. Specific regions of this diagram correspond to specific secondary structures:

• One region is characteristic of $\alpha$-helices.
• Another region is characteristic of $\beta$-strands.

Secondary Structure: Helices and Sheets

Secondary structures are stabilized by patterns of hydrogen bonds between the carbonyl oxygen ($C=O$) and the amide nitrogen ($N-H$) of the peptide backbone.

$\alpha$-Helices and $N_M$ Nomenclature

An $\alpha$-helix is a common right-handed spiral. It is characterized by an $n+4$ hydrogen bonding cycle, meaning the $C=O$ of residue $i$ forms a hydrogen bond with the $N-H$ of residue $i+4$.

To describe different types of helices, we use $N_M$ nomenclature:

• $N$ represents the number of amino acid residues that make up one complete turn of the helix.
• $M$ represents the number of atoms involved in the hydrogen bond cycle.

For example, a standard $\alpha$-helix is often designated as a $3.6_{13}$ helix, indicating roughly 3.6 residues per turn and a 13-atom ring formed by the hydrogen bond.

$\beta$-Sheets and Strands

$\beta$-strands are more extended than helices. When multiple $\beta$-strands align side-by-side, they form $\beta$-sheets. These sheets can be organized in two ways:

• Parallel: All strands run in the same direction (e.g., N-terminus to C-terminus).
• Antiparallel: The strands run in opposite directions.

Sidechain Locations and the Hydrophobic Effect

The "inside" versus "outside" placement of amino acids is governed by the hydrophobic effect—the tendency of non-polar molecules to aggregate in aqueous solution to minimize contact with water.

The spatial distribution of residues follows predictable patterns based on their chemical properties:

Predicting Location: The Hydropathy Scale

To predict whether a residue will be buried or exposed, scientists use a hydropathy scale.

• Residues with more negative values on the scale are more hydrophobic.
• These hydrophobic residues are likely to be found in the protein interior or within transmembrane regions (the parts of a protein that span a lipid membrane).

Worked Example: If you are analyzing a protein sequence and encounter a stretch of residues with highly negative hydropathy values, you should predict that this segment likely forms a transmembrane domain or is buried deep within the protein core.