First-in-class inhibitors of SbnA reduce siderophore production in Staphylococcus aureus

Siderophore production, along with heme scavenging by hemophores, is one of the main mechanisms exploited by bacteria to achieve an adequate iron supply. Staphylococcus aureus produces two main siderophores, staphyloferrin A (SA) and staphyloferrin B (SB), with the latter produced only by the most invasive, coagulase-positive S. aureus strains. Along the seven steps of the SB biosynthetic pathway, N-(2-amino-2-carboxyethyl)-l-glutamate synthase (SbnA) catalyzes the crucial formation of the intermediate N-(2-amino-2-carboxyethyl)-l-glutamate from O-phospho-L-serine and glutamate. Our functional characterization of the enzyme highlighted that citrate inhibits SbnA with an inhibitory constant (K_i) in the order of magnitude of the physiological concentration of the metabolite. We searched for inhibitors of SbnA within citrate analogues and identified 2-phenylmaleic acid (2-PhMA) as the best hit, with a K_i of 16 ± 2 μm and a mechanism of inhibition that is competitive with O-phospho-L-serine for active site binding. The methyl ester of 2-PhMA at a 2 mm concentration was effective in inhibiting siderophore biosynthesis in S. aureus. These results pave the way for the discovery of promising inhibitors of iron acquisition that might find application as innovative antimicrobials.