Cyanide Binding to Truncated Hemoglobins:  A Crystallographic and Kinetic Study^,

Abstract

Cyanide is one of the few diatomic ligands able to interact with the ferric and ferrous heme-Fe atom. Here, the X-ray crystal structure of the cyanide derivative of ferric Mycobacterium tuberculosis truncated hemoglobin-N (M. tuberculosis trHbN) has been determined at 2.0 A (R-general = 17.8% and R-free = 23.5%), and analyzed in parallel with those of M. tuberculosis truncated hemoglobin-O (M. tuberculosis trHbO), Chlamydomonas eugametos truncated hemoglobin (C. eugametos trHb), and sperm whale myoglobin, generally taken as a molecular model. Cyanide binding to M. tuberculosis trHbN is stabilized directly by residue TyrB10(33), which may assist the deprotonation of the incoming ligand and the protonation of the outcoming cyanide. In M. tuberculosis trHbO and in C. eugametos trHb the ligand is stabilized by the distal pocket residues TyrCD1(36) and TrpG8(88), and by the TyrB10(20) - GlnE7(41) - GlnE11(45) triad, respectively. Moreover, kinetics for cyanide binding to ferric M. tuberculosis trHbN and trHbO and C. eugametos trHb, for ligand dissociation from the ferrous trHbs, and for the reduction of the heme-Fe(III)-cyanide complex have been determined, at pH 7.0 and 20.0 degrees C. Despite the different heme distal site structures and ligand interactions, values of the rate constant for cyanide binding to ferric (non)vertebrate heme proteins are similar, being influenced mainly by the presence in the heme pocket of proton acceptor group(s), whose function is to assist the deprotonation of the incoming ligand (i.e., HCN). On the other hand, values of the rate constant for the reduction of the heme-Fe(III)-cyanide (non)vertebrate globins span over several orders of magnitude, reflecting the different ability of the heme proteins considered to give productive complex(es) with dithionite or its reducing species SO(2)(-). Furthermore, values of the rate constant for ligand dissociation from heme-Fe(II)-cyanide (non)vertebrate heme proteins are very different, reflecting the different nature and geometry of the heme distal residue(s) hydrogen-bonded to the heme-bound cyanide.

Structural Determinants in the Group III Truncated Hemoglobin from Campylobacter jejuni

“…Moreover, the water molecule found bridging residues Tyr(B10) and Trp(E15) (filling the second small matrix cavity) matches the site occupied by the Gln 49 (E11) OE1 atom in Bs-trHb, but it is not in contact with the bound cyanide, contrary to what is observed for the Bs-trHb Gln 49 (E11) OE1 atom (9). When compared with group I trHbs, the Cj-trHbP cyanide binding mode mostly resembles that found in Mt-trHbN (29), with differences due to substitutions in the latter protein of residues Val and Gln at sites G8 and E11, respectively. Notably, the Mt-trHbN Gln(E11) side chain partly matches the Cj-trHbP water molecule bridging Tyr 19 (B10) OH and Trp 54 (E15) NE1 atoms.…”

New Insights on the Electronic and Molecular Structure of Cyanide-Ligated Iron(III) Porphyrinates

“…[49,50,51] The IR C-N stretch provides a useful monitor of the bonding sites and ligand environment. [52,53] As has been noted for hemoproteins, the C-N stretching band in cyano-substituted iron(III) porphyrinates is very weak.…”

Protein structure in the truncated (2/2) hemoglobin family

“…In group I 2/2HbNs the hydrogen bonded network involves mostly residues at B10, E7, and E11 topological sites (23). For example, in Mt-2/2HbN a direct TyrB10-O 2 hydrogen bond occurs, stabilized by GlnE11 interacting with TyrB10 (9,24,26,31). Raman spectroscopy indicates that in Mt-2/2HbN the hemeFe-bound O 2 , CO, and OH -ligands are stabilized by hydrogen bonding to TyrB10 (24,26).…”