Approaching a complete classification of protein secondary structure

Area of Science:

• Structural biology
• Computational biophysics
• Protein science

Background:

• Protein secondary structure classification is fundamental to understanding protein folding and function.
• Existing classifications primarily focus on alpha-helices and beta-structures.
• The conformational space of polypeptide chains is vast, yet protein structures exhibit recurring patterns.

Purpose of the Study:

• To develop a comprehensive classification of protein secondary structure types.
• To analyze the conformational diversity of amino acid residues in globular proteins.
• To identify and characterize novel secondary structure elements.

Main Methods:

• Computer analysis of crystallographic structural data from the Protein Data Bank.
• Distribution analysis of amino acid conformations on the Ramachandran map.
• Identification of conformational maxima corresponding to distinct secondary structure types.

Main Results:

• A majority of amino acid residues were categorized into five main conformation types.
• The number of unique torsion angle (phi, psi) sequence variants in globular proteins is limited.
• A third type of secondary structure, the mobile (M-) conformation (extended left-handed helical), was identified.
• M-conformation fragments play a significant role in protein globule dynamics.
• Short segments of M-conformation are prevalent in globular proteins due to a small correlation radius.

Conclusions:

• The conformational repertoire of polypeptide chains in proteins is more restricted than previously assumed.
• The mobile (M-) conformation represents a significant, previously underappreciated, class of protein secondary structure.
• Understanding M-conformation is key to elucidating protein dynamics and function.