Double-stranded helical twisted β-sheet channels in crystals of gramicidin S grown in the presence of trifluoroacetic and hydrochloric acids
✍ Scribed by Llamas-Saiz, Antonio L. ;Grotenbreg, Gijsbert M. ;Overhand, Mark ;van Raaij, Mark J.
- Publisher
- International Union of Crystallography
- Year
- 2007
- Tongue
- English
- Weight
- 1003 KB
- Volume
- 63
- Category
- Article
- ISSN
- 0907-4449
No coin nor oath required. For personal study only.
✦ Synopsis
Gramicidin S is a nonribosomally synthesized cyclic decapeptide antibiotic with twofold symmetry (Val-Orn-Leud-Phe-Pro) 2 ; a natural source is Bacillus brevis. Gramicidin S is active against Gram-positive and some Gram-negative bacteria. However, its haemolytic toxicity in humans limits its use as an antibiotic to certain topical applications. Synthetically obtained gramicidin S was crystallized from a solution containing water, methanol, trifluoroacetic acid and hydrochloric acid. The structure was solved and refined at 0.95 A resolution. The asymmetric unit contains 1.5 molecules of gramicidin S, two trifluoroacetic acid molecules and ten water molecules located and refined in 14 positions. One gramicidin S molecule has an exact twofold-symmetrical conformation; the other deviates from the molecular twofold symmetry. The cyclic peptide adopts an antiparallel -sheet secondary structure with two type II 0 -turns. These turns have the residues d-Phe and Pro at positions i + 1 and i + 2, respectively. In the crystals, the gramicidin S molecules line up into doublestranded helical channels that differ from those observed previously. The implications of the supramolecular structure for several models of gramicidin S conformation and assembly in the membrane are discussed.