Periodicity in alpha-helix lengths and C-capping preferences

Area of Science:

• Structural Biology
• Protein Biochemistry
• Computational Biology

Background:

• Alpha-helices are fundamental protein secondary structures.
• Helix length and capping motifs play crucial roles in protein folding and function.
• Predictive algorithms for protein structure often overlook helix length as a key parameter.

Purpose of the Study:

• To investigate the distribution of alpha-helix lengths in protein crystal structures.
• To analyze the relationship between helix length and N- and C-terminal capping preferences.
• To provide insights for improving computational prediction of helices and capping motifs.

Main Methods:

• Surveyed 299 high-resolution, non-homologous protein crystal structures.
• Analyzed alpha-helix lengths and identified preferred and disfavored residue counts.
• Examined amino acid preferences at N- and C-termini and C-capping motifs (Schellman, alphaL).

Main Results:

• Alpha-helices show a preference for lengths close to an integral number of turns.
• Specific residue counts (e.g., 6, 7, 10, 11) are favored, while others (e.g., 8, 9, 12) are disfavored.
• Favored length helices display distinct C-capping preferences (non-polar at C4, polar at C2) compared to disfavored lengths (non-polar at C2).
• Periodic trends in C-capping motifs correlate with side-chain burial preferences.

Conclusions:

• Helix length is a significant factor influencing alpha-helix structure and capping.
• Favored helix lengths facilitate surface exposure through specific capping strategies.
• Algorithms for predicting protein helices and C-capping should incorporate helix length as a predictive feature.