The Structural Rule Distinguishing a Superfold: A Case Study of Ferredoxin Fold and the Reverse Ferredoxin Fold.

Area of Science:

• Protein structure and evolution
• Structural bioinformatics
• Computational biology

Background:

• Superfolds are protein structures found across diverse superfamilies, but the rules distinguishing them remain elusive.
• Previous research has explored structural characteristics of superfolds without definitive success.
• The ferredoxin fold is a well-known example of a superfold.

Purpose of the Study:

• To identify the distinguishing structural rule between superfolds and ordinary protein folds.
• To analyze the ferredoxin fold and its reverse as a case study.
• To investigate the role of secondary structure interactions in fold stability.

Main Methods:

• Database analysis of protein structural preferences for alpha-beta (αβ) and beta-alpha (βα) units.
• Rosetta folding simulations to assess physical realizability of protein folds.
• Comparative analysis of the ferredoxin fold and its reverse counterpart.

Main Results:

• Structural preferences for αβ- and βα-units stabilize the ferredoxin fold by separating helices.
• In the reverse ferredoxin fold, these preferences cause structural conflict, limiting its occurrence.
• Rosetta simulations indicated the ferredoxin fold is more physically realizable than its reverse.
• A known rule, the right-handedness of the βαβ-unit, is sometimes broken to prevent frustration.

Conclusions:

• Minimal structural conflict, or minimum frustration, is proposed as the key rule distinguishing superfolds.
• This principle explains the prevalence of the ferredoxin fold and rarity of its reverse.
• The minimum frustration rule is as significant as the right-handedness rule for βαβ-units in protein structure.