Introduction
Magainin are a class of antimicrobial peptides (AMPs) found in the African clawed frog Xenopus Laevis.
AMPs consists of 10-50 amino acids, and are produced by Eukaryotes, as part of their defence mechanism from bacteria. For informatoin about AMPs you can visit the Proteopedia page Antimicrobial peptides
Magainin 1 and 2 were discovered by Dr. Michael Zasloff and first reported in 1987. They have an alpha helix structure, and are water soluble and Potentially amphiphilic.
Magainin and Magainin 2
Magainin and Magainin 2 were discovered together, and posses very similar sequences, of 23 Amino Acid long.
Magainin: Gly-Ile-Gly-Lys-Phe-Leu-His-Ser-Ala-Gly-Lys-Phe-Gly-Lys-Ala-Phe-Val-Gly-Glu-Ile-Met-Lys-Ser
magainin 2: Gly-Ile-Gly-Lys-Phe-Leu-His-Ser-Ala-Lys-Lys-Phe-Gly-Lys-Ala-Phe-Val-Gly-Glu-Ile-Met-Asn-Ser
When the only difference is the 22th amino acid, Lys for Magainin and Asn for Magainin 2 Here we will debate abot Magainin 2 properties.
Structural highlights
In general, amphipathic helical peptides that disrupt the ionic gradient of cells are thought to do so by forming ion channels assembled from 4–6 peptide molecules. This mechanism is called the Barrel stave pore model. Then the insides of the cell leak outside and the cell dies. It was thought that this mechanism is also acountable for Magainin 2, But earlier solid-state NMR results show that its helix axis lies in the plane of phospholipid bilayers, suggesting that magainin’s mechanism for disrupting the ionic gradient may be fundamentally different. Therefore it's mechanism is still unclear.
Magainin 2 structure allows it to bind to membranes:
Magainin 2, As typical to all AMPs, Is rich with cationic residues that allow it to interact with Bacterial membranes, that are negatively charged in phosiological pH, and rich with Hydrophobic residues that allow it to interact with the membrane's phospholipids.
We can see here that the residues are organised in it's alpha helix in a way that one side contains all hydrophobic residues (shown in green), and the other side contains all cationic residues (shown in purple). this probably helps Magainin 2 to bind to the bacterial membrane and perform it's antimicrobial action.
Crystalization of Ala-Magainin
So far we had shown Magainin 2 structure based only on NMR findings, Because helical AMPs crystallography is limited, since it is hard to form crystals.
In the Hayouka et al., 2013, In order to perform crystallization of Magainin 2, changes in the sequence were maid, and Ala-Magainin 2 was contsructed. In Ala-magainin one Ser (S) and two Gly (G) residues have been changed to Ala (A) in order to increase helical propensity.
Magainin 2: Gly-Ile-Gly-Lys-Phe-Leu-His-Ser-Ala-Lys-Lys-Phe-Gly-Lys-Ala-Phe-Val-Gly-Glu-Ile-Met-Asn-Ser
Ala -magainin 2: Gly-Ile-Gly-Lys-Phe-Leu-His-Ala-Ala-Lys-Lys-Phe-Ala-Lys-Ala-Phe-Val-Ala-Glu-Ile-Met-Asn
These changes resulted in minor changes in the secondary structure. we can see here Magainin 2 residues that were changed to ala in Ala-Magainin. We can see Ala-Magainin has a few more residues in a alpha helix structure.
To form crystalyztion of Ala-Magainin 2, a racemic version (L-form and D-form) was created, as seen in the image below.(Image taken from PDB)
whilst Racemic crystallization was not successful for magainin 2, Racemic crystalization was successful for Ala-magainin. This allowed finding the peptide's structure based on crystallization, that is more accurate than NMR
