Globin X: A target for surfactant-like molecules.

Abstract

Globin X (GbX) is recognized as a novel, intrinsically hexacoordinate, member of the vertebrate globin family. Previously we showed that GbX is significantly more stable than pentacoordinate vertebrate globins (Hb and Mb) at elevated temperatures, acidic pH as well as in the presence of denaturants. Such a unique feature of this protein led us to examine how the GbX interacts with surfactant-like molecules such as perfluorooctanoic acid (PFOA) and sodium dodecyl sulfate (SDS). In addition to GbXWT protein, the effect of intraprotein disulfide bridge and the heme iron pentacordination on GbX interactions with surfactant-like molecules was investigated by characterizing the GbXC65A and GXH90V constructs. The impact of surfactant on the structural properties of GbX was evaluated based on changes in the Soret band region and the emission properties of intrinsic Trp residues. The Kd values for SDS binding to the deoxy form of GbXWT, GbXC65A, and GbXH90V were determined to be 251 ± 4 μM, 188 ± 7 μM, and 358 ± 91 μM respectively, indicating that the intramolecular disulfide bond and hexacoordination do not have a significant effect on the surfactant-protein complex. However, the met form of GbXWT and GbXC65A binds SDS with Kd of 1426 ± 224 μM and 2106 ± 26 μM, pointing towards a lower affinity of the met form of the protein for SDS surfactant. Interestingly, PFOA produces a minimal change in the Soret band region, suggesting a distinct mechanism of binding. Based on the Trp emission data, the Kd values for PFOA binding to the met form of the protein were determined to be 40 ± 28 μM for GbX WT and 25 ± 1 μM for GbXC65A. The molecular mechanism of surfactant-like molecules binding to GbX was also evaluated using molecular dynamic simulations.