Hemoglobin is known to be a source of peptides involved in several functions. The peptide FLSFPTTKTYFPHFDLSHGSAQVKGHGAK (Hb33-61) is a proteolytic product of the bovine hemoglobin α-chain found in the gut content of the cattle tick, Boophilus microplus, and it possesses antimicrobial activity. Since in the past we showed that the amidated form of Hb33-61, Hb33-61a, is active against a few Gram-positive bacteria and fungi strains at micromolar concentration [Fogaça et al. (1999) J. Biol. Chem. 274, 25330-25334], we have been prompted to shed more light on its functional and structural features. Here we show that the peptide is able to disrupt the bacterial membrane of Micrococcus luteus A270. As for its structure, it has a random conformation in water, and it does not interact with zwitterionic micelles. On the other hand, it binds to negatively charged micelles acquiring a finite structural organization. The 3D structure of Hb33-61a bound to SDS micelles exhibits a nonconventional conformation for an antimicrobial peptide. The backbone is characterized by the presence of a β-turn in the N-terminus and by a β-turn followed by a α-helical stretch in the C-terminus. A hinge, whose spatial organization is stabilized by side-chain-side-chain interactions, joins these two regions. Interestingly, it preserves structural features present in the corresponding segment of the bovine hemoglobin α-chain. Hb33-61a does not possess a well-defined amphipathic nature, and H/D exchange experiments show that while the C-terminal region is embedded in the SDS micelle, one face of the N-terminal half is partly exposed to the solvent.
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