Protein Folds. All-Alpha Topologies

The RCSB.org server holds more than 150000 PDB files but there are only a few thousand different types of protein folds. So it is curious to see a few simplest. Based on Tertiary Protein Structure and Folds and Protein Folds.

The Lone Helix:
1GCN - glucagon, a hormone involved in regulating sugar metabolism in mammals (as does insulin). See also glucagon-like peptide 2 - 2L63.
PDB vdW B&L rib HET 1Å= HF AA FPS

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The Helix-turn-helix motif:
1ROP - RNA-binding protein Rop (a part of dimer really), which is found in certain plasmids (small circular molecules of double-stranded DNA occurring in bacteria and yeast) and involved in their replication.
The Four-helix Bundle:
In four-helix-bundle proteins the interfaces between the helices consist mostly of hydrophobic residues while polar side chains on the exposed surfaces interact with the aqueous environment. The ligand-binding sites of these proteins occur between the alpha helices.
1CCD - Clara cell protein.
2IJK - ColE1 Rom protein dimer.
1A7E - Myohemerythrin.
1FHA - Ferritin.
4ZF7 - Interleukin-2.
1HUW - human growth factor.
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