Streptococcus pyogenes, an important pathogen that causes a wide range of diseases, possesses the sia gene cluster, which encodes proteins involved in the heme acquisition system. Although this system was previously described, the molecular mechanism of effective heme transfer remains to be elucidated. Here, we have characterized the interactions between heme and each domain of Streptococcal hemoprotein receptor (Shr) and Streptococcal heme-binding protein (Shp). Our kinetic and thermodynamic analyses suggested that effective heme transfer within this system is achieved not only by affinity-based transfer but also by the difference of the binding driving force. The biophysical characterization of the above-mentioned interaction will lead to an indication for the selection of the target for a chemical screening of inhibitors as novel antibacterial agents based on biophysical approaches. (C) 2017 Elsevier Inc. All rights reserved.