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Papers In Press, published online ahead of print May 22, 2006
J. Biol. Chem, 10.1074/jbc.M601832200
Submitted on February 27, 2006
Accepted on May 22, 2006

The mechanism of direct heme transfer from the streptococcal cell surface protein Shp to HtsA of the HtsABC transporter

Tyler K. Nygaard, George C. Blouin, Mengyao Liu, Maki Fukumura, John S. Olson, Marian Fabian, David M. Dooley, and Benfang Lei

Veterinary Molecuar Biology, Montana State University, Bozeman, MT 59718

Corresponding Author: blei{at}montana.edu

The heme-binding proteins Shp and HtsA are part of the heme acquisition machinery found in Streptococcus pyogenes. The hexacoordinate heme (Fe(II)-protoporphyrin IX) or hemochrome form of holoShp (hemoShp) is stable in air in Tris-HCl buffer, pH 8.0, binds to apoHtsA with a Kd of 120 ± 18 mu M and transfers its heme to apoHtsA with a rate constant of 28 ± 6 s-1 at 25°C, pH 8.0. The hemoHtsA product then autooxidizes to the hexacoordinate hemin (Fe(III)-protoporphyrin IX) or hemichrome form (hemiHtsA) with an apparent rate constant of 0.017 ± 0.002 s-1. HemiShp also rapidly transfers hemin to apoHtsA through a hemiShp:apoHtsA complex (Kd = 48 ± 7 mu M) at a rate ~40,000 times greater than the rate of simple hemin dissociation from hemiShp into solvent (ktransfer = 43 ± 3 s-1 versus k-hemin = 0.0003 ± 0.00006 s-1). The rate constants for hemin binding to and dissociation from HtsA (k'hemin ˜ 80 mu M-1s-1, k-hemin = 0.0026 ± 0.0002 s-1) are 50 and 10-fold greater than the corresponding rate constants for Shp (k'hemin ˜ 1.6 mu M-1s-1, k-hemin = 0.0003 s-1), which implies that HtsA has a more accessible active site. However, the affinity of apoHtsA for hemin (Khemin ˜ 31,000 µM-1) is roughly 5-fold greater than that of apoShp (Khemin ˜ 5,300 mu M-1), accounting for the net transfer from Shp to HstA. These results support a direct, rapid, and affinity-driven mechanism of heme and hemin transfer from the cell surface receptor Shp to the ABC transporter system.


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