Carbethoxylation of coordinated histidine by diethylpyrocarbonate

Highly Regioselective and Stereoselective Hydroxylation of Free Amino Acids by a 2-Oxoglutarate-Dependent Dioxygenase from Kutzneria albida

Hydroxyl amino acids have tremendous potential applications in food and pharmaceutical industries. However, available dioxygenases are limited for selective and efficient hydroxylation of free amino acids. Here, we identified a 2-oxoglutarate-dependent dioxygenase from Kutzneria albida by gene mining and characterized the encoded protein (KaPH1). KaPH1 was estimated to have a molecular weight of 29 kDa. The optimal pH and temperature for its l-proline hydroxylation activity were 6.5 and 30 °C, respectively. The K _m and k _cat values of KaPH1 were 1.07 mM and 0.54 s^-1, respectively, for this reaction by which 120 mM l-proline was converted to trans-4-hydroxy-l-proline with 92.8% yield (3.93 g·L^-1·h^-1). EDTA, [1,10-phenanthroline], Cu²⁺, Zn²⁺, Co²⁺, and Ni²⁺ inhibited this reaction. KaPH1 was also active toward l-isoleucine for 4-hydroxyisoleucine synthesis. Additionally, the unique biophysical features of KaPH1 were predicted by molecular modeling whereby this study also contributes to our understanding of the catalytic mechanisms of 2-oxoglutarate-dependent dioxygenases.

His-containing plant metallothioneins: comparative study of divalent metal-ion binding by plant MT3 and MT4 isoforms

Metallothioneins (MTs) are a superfamily of Cys-rich, low-molecular weight metalloproteins that bind heavy metal ions. These cytosolic metallopeptides, which exist in most living organisms, are thought to be involved in metal homeostasis, metal detoxification, and oxidative stress protection. In this work, we characterise the Zn(II)- and Cd(II)-binding abilities of plant type 3 and type 4 MTs identified in soybean and sunflower, both of them being His-containing peptides. The recombinant metal-MT complexes synthesised in Zn(II) or Cd(II)-enriched Escherichia coli cultures have been analysed by ESI-MS, and CD, ICP-AES, and UV spectroscopies. His-to-Ala type 3 MT mutants have also been constructed and synthesised for the study of the role of His in divalent metal ion coordination. The results show comparable divalent metal-binding capacities for the MTs of type 3, and suggest, for the first time, the participation of their conserved C-term His residues in metal binding. Interesting features for the Zn(II)-binding abilities of type 4 MTs are also reported, as their variable His content may be considered crucial for their biological performance.

Caenorhabditis elegans metallothionein isoform specificity--metal binding abilities and the role of histidine in CeMT1 and CeMT2

Two metallothionein (MT) isoforms have been identified in the model nematode Caenorhabditis elegans: CeMT1 and CeMT2, comprising two polypeptides that are 75 and 63 residues in length, respectively. Both isoforms encompass a conserved cysteine pattern (19 in CeMT1 and 18 in CeMT2) and, most significantly, as a result of their coordinative potential, CeMT1 includes four histidines, whereas CeMT2 has only one. In the present study, we present a comprehensive and comparative analysis of the metal [Zn(II), Cd(II) and Cu(I)] binding abilities of CeMT1 and CeMT2, performed through spectroscopic and spectrometric characterization of the recombinant metal-MT complexes synthesized for wild-type isoforms (CeMT1 and CeMT2), their separate N- and C-terminal moieties (NtCeMT1, CtCeMT1, NtCeMT2 and CtCeMT2) and a DeltaHisCeMT2 mutant. The corresponding in vitro Zn/Cd- and Zn/Cu-replacement and acidification/renaturalization processes have also been studied, as well as protein modification strategies that make it possible to identify and quantify the contribution of the histidine residues to metal coordination. Overall, the data obtained in the present study are consistent with a scenario where both isoforms exhibit a clear preference for divalent metal ion binding, rather than for Cu coordination, although this preference is more pronounced towards cadmium for CeMT2, whereas it is markedly clearer towards Zn for CeMT1. The presence of histidines in these MTs is revealed to be decisive for their coordination performance. In CeMT1, they contribute to the binding of a seventh Zn(II) ion in relation to the M(II)(6)-CeMT2 complexes, both when synthesized in the presence of supplemented Zn(II) or Cd(II). In CeMT2, the unique C-terminal histidine abolishes the Cu-thionein character that this isoform would otherwise exhibit.

Protein surface mapping using diethylpyrocarbonate with mass spectrometric detection

The reliability and information content of diethylpyrocarbonate (DEPC) as a covalent probe of protein surface structure has been improved when used appropriately with mass spectrometric detection. Using myoglobin, cytochrome c, and beta-2-microglobulin as model protein systems, we demonstrate for the first time that DEPC can modify Ser and Thr residues in addition to His and Tyr residues. This result expands the capability of DEPC as a structural probe because about 25% of the sequence of the average protein can now be covered using this covalent labeling reagent. In addition, we establish a new approach based on mass spectrometry to ensure the structural integrity of proteins during amino acid-specific covalent labeling reactions. This approach involves monitoring the extent of modification as a function of reagent concentration and allows any small-scale or local perturbations caused by the covalent label to be readily identified and avoided. Results indicate that these dose-response plots are much more reliable and generally applicable probes of possible protein structural changes than fluorescence or circular dichroism spectroscopies. These dose-response plots also provide a means of quantitatively comparing the reactivity of each modified residue. On the basis of comparisons to known X-ray crystal structures, we find that the solvent accessibility of the reactive atom in the side chain and the presence of a nearby charged residue most affect modification rates. Finally, this improved surface mapping method has been used to determine the effect of Cu(II) binding on the structure of beta-2-microglobulin. Results confirm that Cu(II) binds His31, but not any of the other three His residues, and changes the solvent accessibility of residues near His31 and near the N-terminus.

Mapping Cu(II) binding sites in prion proteins by diethyl pyrocarbonate modification and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometric footprinting

Although Cu(II) ions bind to the prion protein (PrP), there have been conflicting findings concerning the number and location of binding sites. We have combined diethyl pyrocarbonate (DEPC)-mediated carbethoxylation, protease digestion, and mass spectrometric analysis of apo-PrP and copper-coordinated mouse PrP23-231 to "footprint" histidine-dependent Cu(II) coordination sites within this molecule. At pH 7.4 Cu(II) protected five histidine residues from DEPC modification. No protection was afforded by Ca(II), Mn(II), or Mg(II) ions, and only one or two residues were protected by Zn(II) or Ni(II) ions. Post-source decay mapping of DEPC-modified histidines pinpointed residues 60, 68, 76, and 84 within the four PHGGG/SWGQ octarepeat units and residue 95 within the related sequence GGGTHNQ. Besides defining a copper site within the protease-resistant core of PrP, our findings suggest application of DEPC footprinting methodologies to probe copper occupancy and pathogenesis-associated conformational changes in PrP purified from tissue samples.