Catalytic activity, stability, unfolding, and degradation pathways of engineered and reconstituted myoglobins

Structural and functional insights into the reaction specificity of catalase-related hydroperoxide lyase: A shift from lyase activity to allene oxide synthase by site-directed mutagenesis

Two highly identical fusion proteins, an allene oxide synthase-lipoxygenase (AOS-LOX) and a hydroperoxide lyase-lipoxygenase (HPL-LOX), were identified in the soft coral Capnella imbricata. Both enzymes initially catalyze the formation of 8R-hydroperoxy-eicosatetraenoic acid (8R-HpETE) from arachidonic acid by the C-terminal lipoxygenase (LOX) domain. Despite the fact that the defined catalytically important residues of N-terminal catalase-related allene oxide synthase (cAOS) domain are also conserved in C. imbricata hydroperoxide lyase (cHPL), their reaction specificities differ. In the present study, we tested which of the amino acid substitutions around the active site of cHPL are responsible for a control in the reaction specificity. The possible candidates were determined via comparative sequence and structural analysis of the substrate channel and the heme region of coral cAOSs and C. imbricata cHPL. The amino acid replacements in cHPL—R56G, ME59-60LK, P65A, F150L, YS176-177NL, I357V, and SSSAGE155-160PVKEGD—with the corresponding residues of cAOS were conducted by site-directed mutagenesis. Although all these mutations influenced the catalytic efficiency of cHPL, only F150L and YS176-177NL substitutions caused a shift in the reaction specificity from HPL to AOS. The docking analysis of P. homomalla cAOS with 8R-HpETE substrate revealed that the Leu150 of cAOS interacts with the C5-C6 double bond and the Leu177 with the hydrophobic tail of 8R-HpETE. We propose that the corresponding residues in cHPL, Phe150 and Ser177, are involved in a proper coordination of the epoxy allylic radical intermediate necessary for aldehyde formation in the hydroperoxide lyase reaction.

Characterization of the second conserved domain in the heme uptake protein HtaA from Corynebacterium diphtheriae

HtaA is a heme-binding protein that is part of the heme uptake system in Corynebacterium diphtheriae. HtaA contains two conserved regions (CR1 and CR2). It has been previously reported that both domains can bind heme; the CR2 domain binds hemoglobin more strongly than the CR1 domain. In this study, we report the biophysical characteristics of HtaA-CR2. UV-visible spectroscopy and resonance Raman experiments are consistent with this domain containing a single heme that is bound to the protein through an axial tyrosine ligand. Mutants of conserved tyrosine and histidine residues (Y361, H412, and Y490) have been studied. These mutants are isolated with very little heme (≤5%) in comparison to the wild-type protein (~20%). Reconstitution after removal of the heme with butanone gave an alternative form of the protein. The HtaA-CR2 fold is very stable; it was necessary to perform thermal denaturation experiments in the presence of guanidinium hydrochloride. HtaA-CR2 unfolds extremely slowly; even in 6.8M GdnHCl at 37°C, the half-life was 5h. In contrast, the apo forms of WT HtaA-CR2 and the aforementioned mutants unfolded at much lower concentrations of GdnHCl, indicating the role of heme in stabilizing the structure and implying that heme transfer is effected only to a partner protein in vivo.

Interactions between heme and tau-derived R1 peptides: binding and oxidative reactivity

The interaction of hemin with the first 18-amino acid repeat in tau protein has been investigated at both the N-terminal free-amine (R1τ) and N-acetylated (AcR1τ) forms for its potential relevance in traumatic brain injury and possibly other neurodegenerative diseases. The binding properties of hemin-R1τ and hemin-AcR1τ were compared with those of the hemin complex with amyloid-β peptide fragment 1-16 (Aβ16) and synthetic hemins. AcR1τ and R1τ bind with moderate affinity to both monomeric and dimeric hemin to form 1 : 1 complexes, but for the acetylated peptide, the affinity is one order of magnitude larger (K1 = 3.3 × 10(6) M(-1)). The binding constants were similar to that of Aβ16 for hemin, but unlike the latter, neither of the two R1τ peptides forms a 2 : 1 complex with hemin. This is mostly due to electrostatic repulsion between R1τ chains, and in particular the C-terminal proline-15 kink, while structural features of the hemin-R1τ complexes do not seem to play a role. In fact, the same features are observed for the interaction between ferric heme and peptide R1τ*, where the P15 residue is replaced by an alanine. Imidazole neither binds to [hemin(R1τ)] nor [hemin(AcR1τ)], whereas small ligands such as CN and CO easily bind to the ferric and ferrous forms of the complexes, respectively. A detailed comparative study of the peroxidase activity of [hemin(R1τ)] and [hemin(AcR1τ)] shows that such activity is very low. Thus, the association between heme and unfolded neuronal peptides does not, per se, involve a significant gain of toxic pseudo-enzymatic activity. However, under conditions of heavy heme release occurring on traumatic brain injury or when this activity is prolonged for long time, it can contribute to neuronal oxidative stress. In addition, the presence of hemin increases the aggregation propensity of R1τ.

Heme-bound SiaA from Streptococcus pyogenes: Effects of mutations and oxidation state on protein stability

The protein SiaA (HtsA) is part of a heme uptake pathway in Streptococcus pyogenes. In this report, we present the heme binding of the alanine mutants of the axial histidine (H229A) and methionine (M79A) ligands, as well as a lysine (K61A) and cysteine (C58A) located near the heme propionates (based on homology modeling) and a control mutant (C47A). pH titrations gave pKa values ranging from 9.0 to 9.5, close to the value of 9.7 for WT SiaA. Resonance Raman spectra of the mutants suggested that the ferric heme environment may be distinct from the wild-type; spectra of the ferrous states were similar. The midpoint reduction potential of the K61A mutant was determined by spectroelectrochemical titration to be 61±3mV vs. SHE, similar to the wild-type protein (68±3mV). The addition of guanidine hydrochloride showed two processes for protein denaturation, consistent with heme loss from protein forms differing by the orientation of the heme in the binding pocket (the half-life for the slower process ranged from less than half a day to two days). The ease of protein unfolding was related to the strength of interaction of the residues with the heme. We hypothesize that kinetically facile but only partial unfolding, followed by a very slow approach to the completely unfolded state, may be a fundamental attribute of heme trafficking proteins. Small motions to release/transfer the heme accompanied by resistance to extensive unfolding may preserve the three dimensional form of the protein for further uptake and release.

Adsorption and conformational change of myoglobin on biomimetic hydroxyapatite nanocrystals functionalized with alendronate

The chemical conjugation of bisphosphonates (BPs), specifically alendronate, to hydroxyapatite could be an effective means to impart to it fine-tuned bioactivity. Horse heart myoglobin (Mb), a well-characterized protein, has been adsorbed onto biomimetic hydroxyapatite nanocrystals (nHA) and onto the nHA/alendronate conjugate powdered samples. The obtained materials have potential use in bone implantation and as prospective drug-delivery devices. The kinetic absorption of Mb onto nHA is dramatically affected by its functionalization with alendronate. The covering of the nHA surface by alendronate inhibits the adsorption of myoglobin. The adsorption mechanisms of the protein were studied by spectroscopic techniques (UV-vis and surface-enhanced Raman spectroscopy). The results indicate that the protein changes conformation upon adsorption on the inorganic substrate. In particular, the interaction with nHA alters the coordination state of the iron in the heme through the formation of a hexacoordinated low-spin Mb heme, possibly involving the distal histidine. Instead, the covering of the nHA surface by alendronate does not adsorb the protein but preserves the coordination state of the heme moiety. This study could be of significance either in the field of biomaterials science, in particular, to fine tune a bone-specific drug delivery device and to test nHA as a new support for heterogeneous catalysis, improving the understating of enzyme immobilization.

Thermostability, solvent tolerance, catalytic activity and conformation of cofactor modified horseradish peroxidase

Artificial prosthetic groups, HeminD1 and HeminD2, were designed and synthesized, which contain one benzene ring and one carboxylic group or two carboxylic groups at the terminal of each propionate side chain of hemin, respectively. HeminD1 and HeminD2 were reconstituted with apo-HRP successfully to produce the two novel HRPs, rHRP1 and rHRP2, respectively. The thermal and solvent tolerances of native and reconstituted HRPs were compared. The cofactor modification increased the thermostability both in aqueous buffer and some organic solvents, and also enhanced the tolerance of some organic solvents. To determine the conformation stability, the unfolding of native and reconstituted HRPs by heat was investigated. Tm was increased from 70.0 degrees C of nHRP to 75.4 degrees C of rHRP1 and 76.5 degrees C of rHRP2 after cofactor modification. Kinetic studies indicated that the cofactor modification increased the substrate affinity and catalytic efficiency both in aqueous buffer and some organic solvents. The catalytic efficiency for phenol oxidation was increased by approximately 55% for rHRP1 in aqueous buffer, and it was also increased by approximately 70% for rHRP1 in 10% ACN. Spectroscopic studies proved that the cofactor modification changed the microenvironment of both heme and tryptophan, increased alpha-helix content, and increased the tertiary structure around the aromatic residue in HRP. The improvements of catalytic properties are related to these changes of the conformation. The introduction of the hydrophobic domain as well as the retention of the moderate carboxylic group in active site is an efficient method to improve the thermodynamic and catalytic efficiency of HRP.

Reactivity and endogenous modification by nitrite and hydrogen peroxide: does human neuroglobin act only as a scavenger?

NGB (human neuroglobin), a recently discovered haem protein of the globin family containing a six-co-ordinated haem, is expressed in nervous tissue, but the physiological function of NGB is currently unknown. As well as playing a role in neuronal O2 homoeostasis, NGB is thought to act as a scavenger of reactive species. In the present study, we report on the reactivity of metNGB (ferric-NGB), which accumulates in vivo as a result of the reaction of oxyNGB (oxygenated NGB) with NO, towards NO2- and H2O2. NO2- co-ordination of the haem group accounts for the activity of metNGB in the nitration of phenolic substrates. The two different metNGB forms, with and without the internal disulfide bond between Cys46 (seventh residue on the inter-helix region between helices C and D) and Cys55 (fifth residue on helix D), exhibit different reactivity, the former being more efficient in activating NO2-. The kinetics of the reactions, the NO2--binding studies and the analysis of the nitrated products from different substrates all support the hypothesis that metNGB is able to generate an active species with the chemical properties of peroxynitrite, at pathophysiological concentrations of NO2- and H2O2. Without external substrates, the targets of the reactive species generated by the metNGB/NO2-/H2O2 system are endogenous tyrosine (resulting in the production of 3-nitrotyrosine) and cysteine (oxidized to sulfinic acid and sulfonic acid) residues. These endogenous modifications were characterized by HPLC-MS/MS (tandem MS) analysis of metNGB after reaction with NO2- and H2O2 under various conditions. The internal S-S bond affects the functional properties of the protein. Therefore metNGB acts not only as scavenger of toxic species, but also as a target of the self-generated reactive species. Self-modification of the protein may be related to or inhibit its postulated neuroprotective activity.

Redox reactivity of the heme Fe3+/Fe2+ couple in native myoglobins and mutants with peroxidase-like activity

The reaction enthalpy and entropy for the one-electron reduction of the ferric heme in horse heart and sperm whale aquometmyoglobins (Mb) have been determined exploiting a spectroelectrochemical approach. Also investigated were the T67R, T67K, T67R/S92D and T67R/S92D Mb-H variants (the latter containing a protoheme-L: -histidine methyl ester) of sperm whale Mb, which feature peroxidase-like activity. The reduction potential (E degrees ') in all species consists of an enthalpic term which disfavors Fe(3+) reduction and a larger entropic contribution which instead selectively stabilizes the reduced form. This behavior differs from that of the heme redox enzymes and electron transport proteins investigated so far. The reduction thermodynamics in the series of sperm whale Mb variants show an almost perfect enthalpy-entropy compensation, indicating that the mutation-induced changes in DeltaH(o')(rc) and DeltaS(o')(rc) are dominated by reduction-induced solvent reorganization effects. The modest changes in E degrees ' originate from the enthalpic effects of the electrostatic interactions of the heme with the engineered charged residues. The small influence that the mutations exert on the reduction potential of myoglobin suggests that the increased peroxidase activity of the variants is not related to changes in the redox reactivity of the heme iron, but are likely related to a more favored substrate orientation within the distal heme cavity.

Non-covalent modification of the heme-pocket of apomyoglobin by a 1,10-phenanthroline derivative

To expand the repertoire of artificial enzymes that are constructed by replacing the natural prosthetic group of hemoproteins with non-natural cofactors, we examined incorporation of a non-porphyrinic ligand (1) into the heme-pocket of apomyoglobin in a non-covalent fashion. Ligand 1 is a highly conjugated 1,10-phenanthroline derivative, which shares some structural features with protoporphyrin IX; for example, molecular size and arrangement of hydrophobic and anionic parts. Addition of apomyoglobin to a solution of 1 induces clear changes in the absorption spectrum of 1, suggesting one-to-one incorporation of 1 into the heme cavity of apomyoglobin with an affinity of 6.3 x 10(6)M(-1). We found that the hydrolytic activity of apomyoglobin toward p-nitrophenyl hexanoate was greatly suppressed because of the incorporation of 1 into the heme-pocket.