Role of amino acid substitutions on proteolytic stability of histatin 5 in the presence of secreted aspartyl proteases and salivary proteases

Histatin 5 (Hst5) is a 24-amino-acid peptide naturally present in human saliva that has been proposed as a potential antifungal therapeutic. However, Hst5 is susceptible to degradation by secreted aspartyl proteases (Saps) produced by Candida albicans, which could limit its efficacy as a therapeutic. To better understand the role of the lysine residues of Hst5 in proteolysis by C. albicans Saps (Sap1, Sap2, Sap3, Sap5, Sap6, Sap9, and Sap10), we studied variants of Hst5 with substitutions to leucine or arginine at the lysine residues (K5, K11, K13, and K17). Sap5, Sap6, and Sap10 did not degrade Hst5 or the variants. However, we observed degradation of the peptides by Sap1, Sap2, Sap3, and Sap9, and the degradation depended on the site of substitution and the substituent residue. Some modifications, such as K11L and K13L, were particularly susceptible to proteolysis by Sap1, Sap2, Sap3, and Sap9. In contrast, the K17L modification substantially increased the stability and antifungal activity of Hst5 in the presence of Saps. We used mass spectrometry to characterize the proteolysis products, which allowed us to identify fragments likely to have maintained or lost antifungal activity. We also evaluated the proteolytic stability of the Hst5 variants in saliva. Both K17L and K5R showed improved stability; however, the enhancements were modest, suggesting that further engineering is required to achieve significant improvements. Our approach demonstrates the potential of simple, rational substitutions to enhance peptide efficacy and proteolytic stability, providing a promising strategy for improving the properties of antifungal peptides.