Structure-function relationships in histidine-rich antimicrobial peptides from Atlantic cod

Gad-1 and Gad-2 are antimicrobial peptide (AMP) sequences encoded by paralogous genes. They are rich in histidine, which suggests that their activity might be pH-dependent. We examined their structure-function relationships with a view to learning how to improve AMP therapeutic ratios. Activity assays with Gram-negative bacteria and cancer cell lines demonstrate that Gad-2 is substantially more active at slightly acidic pH than it is at neutral pH. By contrast, the activity of Gad-1 at lower pH is similar to its activity at pH 7. Circular dichroism spectra indicate that the greater functional plasticity of Gad-2 correlates with a greater structural plasticity; Gad-2's percent helicity varies dramatically with altered pH and lipid environment. Interestingly, Gad-2's highest levels of helicity do not correspond to the conditions where it is most active. High resolution solution NMR structures were determined in SDS micelles at pH 5, conditions that induce an intermediate level of helicity in the peptides. Gad-1 is more helical than Gad-2, with both peptides exhibiting the greatest helical tendencies in their central region and lowest helicity in their N-termini. The high resolution structures suggest that maximum activity relies on the appropriate balance between an N-terminal region with mixed hydrophobic/hydrophilic structure features and an amphipathic central and C-terminal region. Taken together with previous studies, our results suggest that to improve the therapeutic ratio of AMPs, consideration should be given to including sequential histidine-pairs, keeping the overall charge of the peptide modest, and retaining a degree of structural plasticity and imperfect amphipathicity.