Structural Basis of Heme Acquisition from Human Hemoglobin by the Gram‐Positive Pathogen Streptococcus pyogenes

Abstract

Our research seeks to learn how Gram‐positive pathogens acquire iron from the human host, a key process required for bacterial pathogenesis and a potential drug target. In particular, the focus of our work is the mechanism of heme extraction from hemoglobin (Hb), the main source of iron in the human body. In an effort to decipher this process, we are using a combination of biochemical and biophysical methods including X‐ray crystallography, isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR), analytical ultra‐centrifugation (AUC), and heme transfer assays. Current efforts are devoted to studying this process in Streptococcus pyogenes (S. pyogenes) given its relevance as a human pathogen. We have recently characterized a novel Hb receptor from S. pyogenes ‐ the Streptococcal hemoprotein receptor (Shr). Shr appears to contain a similar domain architecture to the well‐characterized Hb receptors IsdH and IsdB found in Staphylococcus aureus. Unlike IsdH and IsdB, Shr mediates Hb binding through a domain of unknown function (DUF). Here we present the structure of this DUF domain in isolation and progress toward determining its structure in complex with Hb. The crystal structure of the DUF domain in isolation reveals a novel Hb‐binding domain structurally unrelated to Hb‐binding domains from other Gram‐positive pathogens. In addition, we have collected biochemical data on this domain suggesting that it slows the rate of heme release from Hb. We hypothesize that the DUF domain may slow the spontaneous process of heme release by Hb after erythrocyte lysis, allowing heme to be subsequently utilized downstream in the S. pyogenes heme acquisition pathway. This work is significant as it describes a novel mechanism of heme acquisition by a Gram‐positive pathogen. Future studies will be devoted to understanding the mechanism of heme extraction by the full‐length Shr protein.Support or Funding InformationThis work was supported by National Institutes of Health Grant AI52217 (to R.T.C.). R.M. was supported by a Cellular and Molecular Biology Training Grant (Ruth L. Kirschstein National Research Service Award GM007185). NMR equipment used in this research was purchased using funds from shared equipment grant NIH S10OD016336.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.