Conformational disorder of the distal leucine in monomeric Glycera hemoglobins and implications for oxygen binding

Structural features of Glycera dibranchiata monomer hemoglobins. Primary sequences of monomer hemoglobin components II and III

Primary sequences for the remaining two members (GMH2, GMH3) of the group of three major monomeric hemoglobins from the marine annelid Glycera dibranchiata have been obtained. Full sequences of each 147-amino acid globin were achieved with a high degree of confidence using standard Edman technology in combination with molecular mass determinations of the intact globins and of the cyanogen bromide cleavage fragments using electrospray ionization mass spectrometry. When minor assumptions concerning Q/E identities are made these new results indicate the likely correspondence of GMG2 with the protein represented by the first Glycera dibranchiata monomer hemoglobin complete sequence [Imamura et al., (1972), J. Biol. Chem. 247, 2785-2797]. When these new sequences are combined with the previously determined primary sequence for the third major monomer hemoglobin, GMH4 [Alam et al., J. Protein Chem. (1994), 13, 151-164], it becomes clear that these three (GMG2-4) are truly distinct proteins, contrary to previous suggestions. Surprisingly, our results show that none of these three primary sequences is identical to the published sequence of the refined monomer hemoglobin crystal structure protein; however, there is a strong correspondence to the GMG2 sequence. The present sequencing results, in combination with the published GMH4 sequence, confirm the presence of a distal Leu in place of the more commonly encountered distal His in all three of the major monomer hemoglobins isolated in this laboratory and indicate that the unusual B10 Phe occurs only in GMH4. Analysis of the sequences presented here, along with comparison of amino acid content for Glycera dibranchiata monomer hemoglobins isolated from three different laboratories, and comparison of NMR results from two laboratories suggest further correspondence which unify disparate published isolations.

Iron ligand recognition by monomeric hemoglobins

Binding affinities of monomeric Glycera dibranchiata hemoglobin for some anions and heterocyclic amines, including imidazoles, pyrazole, triazole and tetrazole have been evaluated and compared with those of sperm whale and horse heart myoglobin. The proteins' affinities for substituted heterocyclic amines are strongly influenced by the steric bulk and flexibility of the aromatic ring. The ligand coordination mode depends on the heme oxidation state, iron(III) amine adducts being more stable than the iron(II) adducts, the higher affinities of stronger Brønsted-Lowry bases reflecting their essentially sigma-donor character. The bifunctional molecule morpholinoethylisocyanide acts as a redox-state-dependent ambidentate ligand, binding as an N-donor to iron(III), but as a C-donor to iron(II). pH-Dependences of the ESR and optical spectra of the azole adducts reveal iron-linked ionisations and spin-equilibria in the heme pocket. Enthalpy and entropy changes for the binding process were estimated for several ligands, and mutually compensatory behaviour is observed globally for delta H degree and delta S degree. At the compensation temperature theta, the binding affinities of monomeric Glycera dibranchiata hemoglobin and sperm whale myoglobin are similar and associated with free energy changes delta G degree (theta) approximately -9 +/- 1 kJ mol-1 for the heterocyclic and anionic ligands.

Expression of recombinant monomer hemoglobins (component IV) from the marine annelid Glycera dibranchiata: evidence for primary sequence positional regulation of heme rotational disorder

A description of the efficient high-level expression of the monomer hemoglobin (GMG4) from Glycera dibranchiata is presented. The cDNA described by Simons and Satterlee [Simons, P.C., & Satterlee, J.D. (1989) Biochemistry 28, 8525-8530] was subcloned into an expression system, and conditions were found that led to the production of large amounts of soluble apoprotein (rec-gmg). These conditions included lowering the temperature during the induction period and growth in a rich medium with a higher ionic strength. Characterization of this reconstituted recombinant protein showed that it was not identical to the native GMH4 protein. Both UV-visible and 1H NMR data indicated differences within the holoprotein (rec-gmh) heme pocket compared to the native protein, the major difference being that two nonidentical heme orientations are significantly populated in rec-gmh. This phenomenon has been seen previously in other heme proteins, where these heme orientational isomers are described by a 180-deg rotation about the heme alpha-gamma meso axis. This work prompted the production of a complete chemical sequence for the native GMH4 [Alam S.L., Satterlee, J. D., & Edmonds, C. G. (1994) J. Protein Chem. 13, 151-164], which showed that the expressed rec-gmg protein differed at three primary sequence positions (41, 95, and 123) from the native component IV globin (GMG4). Subsequently, we have produced the triple-revertant mutations required to express the recombinant wild-type protein (recGMG4). The physical characteristics of the active site in the holoprotein (recGMH4) are identical to those of the native protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete amino acid sequence of the Glycera dibranchiata monomer hemoglobin component IV: structural implications

The globin derived from the monomer Component IV hemoglobin of the marine amnelid, Glycera dibranchiata, has been completely sequenced, and the resulting information has been used to create a structural model of the protein. The most important result is that the consensus sequence of Component IV differs by 3 amino acids from a cDNA-predicted amino acid sequence thought earlier to encode the Component IV hemoglobin. This work reveals that the histidine (E7), typical of most heme-containing globins, is replaced by leucine in Component IV. Also significant is that this sequence is not identical to any of the previously reported Glycera dibranchiata monomer hemoglobin sequences, including the sequence from a previously reported crystal structure, but has high identity to all. A three-dimensional structural model for monomer Component IV hemoglobin was constructed using the published 1.5 A crystal structure of a monomer hemoglobin from Glycera dibranchiata as a template. The model shows several interesting features: (1) a Phe31 (B10) that is positioned in the active site; (2) a His39 occurs in an interhelical region occupied by Pro in 98.2% of reported globin sequences; and (3) a Met41 is found at a position that emerges from this work as a previously unrecognized heme contact.

Thermochromism of heme adducts of Glycera hemoglobin and some other monomeric heme proteins

The thermally induced difference spectra of myoglobin (Mb) and Glycera dibranchiata hemoglobin (Hbm) derivatives and of cytochrome-c were recorded between 4 degrees and 30 degrees C in the 390-750 nm range. Thermodynamic parameters were estimated and upper and lower temperature limiting spectra were deduced for the various heme protein derivatives' equilibria. The effective iron d-electron population divides the hemes broadly into two different groups of behavior type. In the first group, Hbm(III)N3, Hbm(III), Mb(III)(H2O), and Cytc(III) show equilibria between two spin states. The weakest coupling between the heme and the globin occurs among the second group, for Hbm(II)CO and Mb(II)CO, which in the higher temperature limit undergoes averaging of the carbonyl tilt, while an axially elongated geometry is probably accessed for Hbm(II)NO and Mb(II)NO. Examples of the less common situation of increased absorption intensity and/or low-spin states at higher temperature were found in both groups. In the case of the methyl thioglycolate low-spin adducts of Hbm(III), an acid/base equilibrium involving thioglycolate deprotonation occurs. Apparent enthalpy-entropy compensation is exhibited by all these heme derivatives, and it is suggested that the delta H degrees and delta S degrees values relate to the intimacy of coupling between the heme structure and the solvent-dependent microconformation of the globin.

Glycera dibranchiata hemoglobin. Structure and refinement at 1.5 A resolution

The coelomic cells of the common marine bloodworm Glycera dibranchiata contain several hemoglobin monomers and polydisperse polymers. We present the refined structure of one of the Glycera monomers at 1.5 A resolution. The molecular model for protein and ordered solvent for the deoxy form of the Glycera monomer has been refined to a crystallographic R-factor of 12.7% against an X-ray diffraction dataset at 1.5 A resolution. The positions of 1095 protein atoms have been determined with a maximum root-mean-square (r.m.s.) error of 0.13 A, and the r.m.s. deviation from ideal bond lengths is 0.015 A and from ideal bond angles is 1.0 degree. The r.m.s. deviation of planar groups from their least-squares planes is 0.007 A, and the r.m.s. deviation for torsion angles is 1.2 degrees for peptide groups and 16.8 degrees for side-chains. A total of 153 water molecules has been located, and they have been refined to a final average occupancy of 0.80. Multiple conformations have been found for five side-chains, and a change has been suggested for the sequence at five residues. The heme group is present in the "reverse" orientation that differs only in the positions of the vinyl beta-carbons from the "normal" orientation. The doming of the heme towards the proximal side, and the bond distances and angles of the heme and proximal histidine are typical of most deoxy globin structures. The substitution of leucine for the distal histidine residue (E7) creates an unusually hydrophobic heme pocket.

cDNA cloning and predicted amino acid sequence of Glycera dibranchiata monomer hemoglobin IV

The three major monomer hemoglobins from Glycera dibranchiata erythrocytes isolated in this laboratory were sequenced from their N-termini. A stretch of amino acid sequence identity was used to determine the sequence of a mixed oligodeoxynucleotide that would be complementary to all 12 possible mRNA sequences coding for the amino acids. A cDNA library was constructed by using poly(A+) RNA from G. dibranchiata erythrocytes, the library was probed with the oligonucleotide, and the longest positive inserts found were subcloned into a sequencing plasmid and then sequenced. The first one was 745 bases long, containing 85 bases of 5'-untranslated RNA, an open reading frame of 444 bases coding for 148 amino acids, and a 3'-untranslated region of 216 bases. The predicted amino acid sequence matches the first 25 amino acids of G. dibranchiata monomer globin component IV. The sequence contains an N-terminal methionine plus 18 other mostly conservative sequence changes compared to the published sequence of Imamura et al. (1972), which appears from our partial sequencing to be monomer globin component II. We confirm the presence of leucine in the E7 position, which is histidine in most myoglobins and hemoglobins.

Structural consequences of heme isomerism in monomeric hemoglobins from Glycera dibranchiata

Two-dimensional 1H-NMR methods have been used to assign heme and amino acid proton resonances in both isomeric states of the carbon monoxide complexes of two Glycera dibranchiata monomeric hemoglobins, HbA and HbB. For each hemoglobin, there are small differences in heme pocket structure in the two isomeric forms. The largest structural perturbations associated with heme isomerism involve residues close to pyrrole rings I and II. The positions relative to the heme of phenylalanine CD1 and the proximal histidine ligand are almost unaffected by heme isomerism. These residues probably play a key role in determining the location of the heme within the heme pocket.

NMR studies of the heme pocket conformations of monomeric hemoglobins from Glycera dibranchiata. Implications for ligand binding

Two-dimensional 1H-NMR methods have been used to assign side-chain resonances for the tryptophan residues and for several amino acids located in the heme pockets of the carbon monoxide complexes of the major monomeric hemoglobins from Glycera dibranchiata. The NMR spectra reveal a high degree of conservation of the heme pocket structure in the different hemoglobins. However some conformational differences are evident and residues at positions B10 and G8 on the distal side of the heme pocket are not conserved. From the present NMR studies it appears that the monomeric G. dibranchiata hemoglobin examined by X-ray crystallography [Padlan, E. A. & Love, W. (1974) J. Biol. Chem. 249, 4067-4078] corresponds to HbC. Except that the orientation of the heme in solution is the reverse of that reported in the crystal structure, there is a close correspondence between the heme pocket structure in the crystal and in solution. The proximal histidine coordination geometry is almost identical in the CO complexes of the three monomeric hemoglobins studied. Distal residues are strongly implicated in determining the observed kinetic differences in ligand binding reactions. In particular, steric crowding of the ligand binding site in hemoglobin A is probably a major factor in the slower kinetics of this component.