Abstract

Structural and functional properties of ferrous Mycobacterium tuberculosis (Mt-Nb) and human (Hs-Nb) nitrobindins (Nbs) were investigated. At pH 7.0 and 25.0 °C, the unliganded Fe(II) species is penta-coordinated and unlike most other hemoproteins no pH-dependence of its coordination was detected over the pH range between 2.2 and 7.0. Further, despite a very open distal side of the heme pocket (as also indicated by the vanishingly small geminate recombination of CO for both Nbs), which exposes the heme pocket to the bulk solvent, their reactivity toward ligands, such as CO and NO, is significantly slower than in most hemoproteins, envisaging either a proximal barrier for ligand binding and/or crowding of H2O molecules in the distal side of the heme pocket which impairs ligand binding to the heme Fe-atom. On the other hand, liganded species display already at pH 7.0 and 25 °C a severe weakening (in the case of CO) and a cleavage (in the case of NO) of the proximal Fe-His bond, suggesting that the ligand-linked movement of the Fe(II) atom onto the heme plane brings about a marked lengthening of the proximal Fe-imidazole bond, eventually leading to its rupture. This structural evidence is accompanied by a marked enhancement of both ligands dissociation rate constants. As a whole, these data clearly indicate that structural–functional relationships in Nbs strongly differ from what observed in mammalian and truncated hemoproteins, suggesting that Nbs play a functional role clearly distinct from other eukaryotic and prokaryotic hemoproteins.

Highlights

  • Globins are a superfamily of evolutionary conserved heme-proteins that bind, sense, and transport diatomic gases [1,2,3,4,5,6,7,8,9]

  • The highly solvent exposed heme-Fe-atom is at the root of the fast auto-oxidation rate of Mt-Nb(II), Arabidopsis thaliana Nb (At-Nb)(II) and Hs-Nb(II), which is similar to that of Rhodnius prolixus nitrophorins (Rp-NPs) and 104–105 times higher than that of mammalian globins [22,29,30]

  • The kinetic and thermodynamic behavior of Nb(II) forms can be investigated under appropriate con

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Summary

Introduction

Globins are a superfamily of evolutionary conserved heme-proteins that bind, sense, and transport diatomic gases [1,2,3,4,5,6,7,8,9]. Most of these proteins (e.g., hemoglobin (Hb) and myoglobin (Mb)) are characterized by eight α-helical segments shaped around the heme with a 3/3 fold [1,7]. Over the last two decades, monomeric all-β-barrel and mixed-α/β hemoproteins, displaying globin-like reactivity, have been characterized.

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