Melianone inhibits Secreted Aspartic Proteases (SAP), a Virulence Factor During Hyphal Formation in Candida albicans

Background & Objective: Candida albicans (C.-P. Robin) Berkhout, the pathogenic yeasts’ ability to transform from yeast to hyphal forms in the bloodstream is essential during systemic infections. Among the several virulence factors studied, secreted aspartic proteinases (SAPs) involved in hyphal penetration are targets of putative hyphal inhibitors. Upregulation of SAP6 gene, (two-to 31- fold high) during budded to hyphal transition and lack of studies on its inhibition, prompted us to investigate this particular protein using in silico tools. Results: Hyphal inhibition of germinating yeast cells by melianone, a triterpenoid, from Swietenia mahagoni (L.) Jacq. (Meliaceae) was observed at 0.1 μM (IC50). One of the targets of putative hyphal inhibitors, SAP, was assayed and for the first time, 50 % of the biological SAP activity was found to be inhibited by melianone at 0.125 μM. This data on SAP inhibition led us to analyse the 3-dimensional structure for SAP6 protein that was constructed through a combination of homology modelling and ab-initio method (Phyre2) and validated before performing Induced Fit Docking (IFD). Melianone formed H-bond and hydrophobic interactions with the crucial residues (ASP108, TYR160, ALA161, ASP162, ASP294, THR297, ASP379) in the catalytic site of SAP6 with a glide energy (-)54.9327 kcal/mol upon Induced Fit Docking (IFD). Conclusion: We report here for the first time on the SAP inhibitory ability of melianone at 0.125 uM. Being a small molecular mass inhibitor, binding with high affinity to the S3 pocket sites of SAP proteins provides evidence for pre-clinical testing of such compounds against fungal pathogens. The study is a valuable insight for further research on novel and effective inhibitors targeting SAP.