The IsdC protein from Staphylococcus aureus uses a flexible binding pocket to capture heme

Abstract

Staphylococcus aureus scavenges heme-iron from host hemoproteins using iron-regulated surface determinant (Isd) proteins. IsdC is the central conduit through which heme is passed across the cell wall and binds this molecule using a NEAr Transporter (NEAT) domain. NMR spectroscopy was used to determine the structure of IsdC in complex with a heme analog, zinc-substituted protoporphyrin IX (ZnPPIX). The backbone coordinates of the ensemble of conformers representing the structure exhibit a root mean square deviation to themeanstructure of 0.53 ± 0.11 Å. IsdC partially buries protoporphyrin within a large hydrophobic pocket that is located at the end of its β-barrel structure. The central metal ion of the analog adopts a pentacoordinate geometry in which a highly conserved tyrosine residue serves as a proximal ligand. Consistent with the structure and its role in heme transfer across the cell wall, we show that IsdC weakly binds heme (KD = 0.34 ± 0.12 μM) and that ZnPPIX rapidly dissociates from the protein at a rate of 126±30 s-1. NMR studies of the apo-form of IsdC reveal that a 310 helix within the binding pocket undergoes a flexible to rigid transition as heme is captured. This structural plasticity may increase the efficiency of heme transfer across the cell wall by facilitating protein-protein interactions between apoIsdC and upstream hemoproteins. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.