Mini-myoglobin. Electron paramagnetic resonance and reversible oxygenation of the cobalt derivative

A molecule for all seasons: The heme

If life without heme-Fe were at all possible, it would definitely be different. Indeed this complex and versatile iron-porphyrin macrocycle upon binding to different “globins” yields hemeproteins crucial to sustain a variety of vital functions, generally classified, for convenience, in a limited number of functional families. Over-and-above the array of functions briefly outlined below, the spectacular progress in molecular genetics seen over the last 30 years led to the discovery of many hitherto unknown novel hemeproteins in prokaryotes and eukaryotes. Here, we highlight a few basic aspects of the chemistry of the hemeprotein universe, in particular those that are relevant to the control of heme-Fe reactivity and specialization, as sculpted by a variety of interactions with the protein moiety.

The 109 residue nerve tissue minihemoglobin from Cerebratulus lacteus highlights striking structural plasticity of the alpha-helical globin fold

A very short hemoglobin (CerHb; 109 amino acids) binds O(2) cooperatively in the nerve tissue of the nemertean worm Cerebratulus lacteus to sustain neural activity during anoxia. Sequence analysis suggests that CerHb tertiary structure may be unique among the known globin fold evolutionary variants. The X-ray structure of oxygenated CerHb (R factor 15.3%, at 1.5 A resolution) displays deletion of the globin N-terminal A helix, an extended GH region, a very short H helix, and heme solvent shielding based on specific aromatic residues. The heme-bound O(2) is stabilized by hydrogen bonds to the distal TyrB10-GlnE7 pair. Ligand access to heme may take place through a wide protein matrix tunnel connecting the distal site to a surface cleft located between the E and H helices.

The mini-hemoglobins in neural and body wall tissue of the nemertean worm, Cerebratulus lacteus

Hemoglobin (Hb) occurs in circulating red blood cells, neural tissue, and body wall muscle tissue of the nemertean worm, Cerebratulus lacteus. The neural and body wall tissue each express single major Hb components for which the amino acid sequences have been deduced from cDNA and genomic DNA. These 109-residue globins form the smallest stable Hbs known. The globin genes have three exons and two introns with splice sites in the highly conserved positions of most globin genes. Alignment of the sequences with those of other globins indicates that the A, B, and H helices are about one-half the typical length. Phylogenetic analysis indicates that shortening results in a small tendency of globins to group together regardless of their actual relationships. The neural and body wall Hbs in situ are half-saturated with O2 at 2.9 and 4.1 torr, respectively. The Hill coefficient for the neural Hb in situ, approximately 2.9, suggests that the neural Hb self-associates in the deoxy state at least to tetramers at the 2-3 mM (heme) concentration estimated in the cells. The Hb must dissociate upon oxygenation and dilution because the weight-average molecular mass of the HbO2 in vitro is only about 18 kDa at 2-3 microM heme concentration. Calculations suggest that the Hb can function as an O2 store capable of extending neuronal activity in an anoxic environment for 5-30 min.

Structural and functional roles of modules in hemoglobin. Substitution of module M4 in hemoglobin subunits

The alpha- and beta-subunits of human hemoglobin consist of the modules M1, M2 + M3, and M4, which correspond to the exons 1, 2, and 3, respectively (Go, M. (1981) Nature 291, 90-92). To gain further insight into functional and structural significance of the modules, we designed two kinds of chimeric hemoglobin subunits (chimeric alphaalphabeta- and betabetaalpha-subunits), in which the module M4 was replaced by the partner subunits. CD spectra in the far-UV region showed that the secondary structure of the chimeric alphaalphabeta-subunit drastically collapsed, while the chimeric betabetaalpha-subunit conserved the native globin structure (Wakasugi, K., Ishimori, K., Imai, K., Wada, Y., and Morishima, I. (1994) J. Biol. Chem. 269, 18750-18756). SAXS data also suggested a partially disordered structure of the chimeric alphaalphabeta-subunit. Based on tryptophan fluorescence spectra and computer modeling from x-ray structures of native globins, steric constraint between Trp14 and Tyr125 would be induced in the chimeric alphaalphabeta-subunit, which would perturb the packing of the A- and H-helices and destabilize the globule structure. On the other hand, such a steric constraint was not found for the counterpart chimeric subunit, the betabetaalpha-subunit. The different stabilities of these module-substituted globins imply that modules would not always be stable "structural" units, and interactions between modules are crucial to construct stable globin subunits.

Folding of apominimyoglobin

The acid unfolding pathway of apominimyoglobin (apo-mini-Mb), a 108-aa fragment (aa 32-139) of horse heart apomyoglobin has been studied by means of circular dichroism, in comparison with the native apoprotein. Similar to sperm whale apomyoglobin [Hughson, F. M., Wright, P. E. & Baldwin, R. L. (1990) Science 249, 1544-1548], a partly folded intermediate (alpha-helical content approximately 35%) is populated at pH 4.2 for horse heart apomyoglobin. For this intermediate, Hughson et al. proposed a structural model with a compact subdomain involving tertiary interactions between the folded A, G, and H helices, with the remainder of the protein essentially unfolded. As described in this paper, a folding intermediate with an alpha-helical content of approximately 33% is populated at pH 4.3-5.0 also in apo-mini-Mb. The acid unfolding pathway is similarly affected in both the native and the mini apoprotein by 15% trifluoroethanol, a helix-stabilizing compound. Thus, the folding of the apo-mini-Mb intermediate is similar to that observed for the native apoprotein, in spite of the absence in the miniprotein of the A helix and of a large part of the H helix, which are crucial for the stability of apo-Mb intermediate. Our results suggest that acquisition of a folded state in apo-mini-Mb occurs through an alternative pathway, which may or may not be shared also by apo-Mb.

Mini-myoglobin: native-like folding of the NO-derivative

Mini-myoglobin is a polypeptide fragment (residues 32-139) obtained by limited proteolysis of horse heart apomyoglobin and reconstituted with the natural heme. Its functional properties are very similar to those of native myoglobin and therefore it may represent a model for testing the functional role of the protein fragment encoded by the central exon of myoglobin gene (residues 31-105). Here we have investigated some properties of the nitric oxide derivative of mini-myoglobin in comparison with those of NO-myoglobin, to provide more structural information on the heme pocket residues in addition to that already acquired by electron paramagnetic resonance of the cobalt-substituted mini-myoglobin. At pH 7.0, optical and circular dichroism Soret spectra, as well as electron paramagnetic resonance spectra reveal that the heme orientation in the pocket and the coordination state of the ferrous iron in NO-mini-myoglobin are similar to those of the whole protein. The spectra of the NO-mini-myoglobin complex are very sensitive to pH changes at variance to what is observed for the NO-myoglobin derivative in the same pH range (5.5-9.5). In particular, increasing or decreasing pH from 7.0, results in a decrease of the extinction coefficient and of the ellipticity in the Soret region and in a change of the shape of the electron paramagnetic resonance signal. The spectral changes are diagnostic for a transition from a hexa-coordinated (at pH 7.0) to a penta-coordinated heme (at pH 5.5 or 9.5), with the proximal histidine-iron bond either broken or stretched dramatically. Thus, although mini-myoglobin is able to bind NO in a geometry similar to that of the native protein, the resulting NO derivative shows a much higher pH dependence, suggesting that the two lacking side domains are mainly involved in enhancing the stability of the hemoprotein core.

Protein dynamics in minimyoglobin: is the central core of myoglobin the conformational domain?

The kinetics of CO binding to the horse myoglobin fragment Mb-(32-139), the so-called "mini-Mb," were investigated by laser flash photolysis in 0.1 M phosphate buffer and in buffer with 75% (vol/vol) glycerol. The reaction displays complex time courses that can be approximated satisfactorily only with a sum of five exponentials. The features of the kinetic components and a comparison of the deoxy-minus-carbonyl difference spectra of mini-Mb and horse Mb obtained under equilibrium conditions, with the kinetic difference spectra resulting from the global analysis of the traces recorded between 400 and 450 nm, show that CO binding to mini-Mb is accompanied by large structural changes. In view of the fact that mini-Mb is an approximation of the Mb-(31-105) fragment encoded by the central exon of the Mb gene, this finding is particularly relevant. On the basis of our data and previous reports [De Sanctis, G., Falcioni, G., Giardina, B., Ascoli, F. & Brunori, M. (1988) J. Mol. Biol. 200, 725-733; De Sanctis, G., Falcioni, G., Grelloni, F., Desideri, A., Polizo, F., Giardina, B., Ascoli, F. & Brunori, M. (1992) J. Mol. Biol. 222, 637-643], we propose that the protein fragment encoded by the central exon of the Mb gene is the domain responsible for ligand-linked conformational transitions, while the two terminal fragments dampen the amplitude of the structural changes that accompany ligand binding, thus rendering the protein stable and kinetically more efficient in its physiological function.