Targeting Candida albicans O-acetyl-L-homoserine sulfhydrylase (Met15p) in antifungal treatment

Fungal infections are a serious threat to public health as they are becoming increasingly frequent. A major problem stems also from a rising fungal resistance to currently available antifungal therapies, therefore novel molecular targets are highly desirable. Exploration of enzymes participating in the biosynthesis pathways of essential amino acids such as L-methionine (L-Met) may provide new insights into pharmaceutical development. The MET15 gene from Candida albicans, encoding O-acetyl-L-homoserine sulfhydrylase (Met15p), an enzyme catalyzing the second step in that pathway, was cloned and expressed in two versions: as N and C-terminal oligo-His-tagged fusion proteins. The recombinant enzymes revealed appropriate activity, and catalyzed conversion of O-acetyl-L-homoserine and a sulfide ion to produce L-homocysteine. A new RP-HPLC-DAD method, using the enzymatic reaction product pre-column derivatization with 5,5'-dithio-bis-(2-nitrobenzoic acid) was developed and used by us to determine Met15p activity. Newly synthesized compounds as well as two commercially available exhibited a Met15p inhibitory effect which was related to antifungal activity. Fungal cells' sensitivity to inhibitors depending on the presence or absence of L-Met in the medium clearly indicated Met15p targeting. Moreover, the synergistic effect of the first methionine biosynthetic enzyme affecting inhibitor and Met15p inhibitors indicate that methionine biosynthesis pathway enzymes are promising molecular targets.