The iron-type nitrile hydratase activator protein is a GTPase

Role of second-sphere arginine residues in metal binding and metallocentre assembly in nitrile hydratases

Two conserved second-sphere βArg (R) residues in nitrile hydratases (NHase), that form hydrogen bonds with the catalytically essential sulfenic and sulfinic acid ligands, were mutated to Lys and Ala residues in the Co-type NHase from Pseudonocardia thermophila JCM 3095 (PtNHase) and the Fe-type NHase from Rhodococcus equi TG328-2 (ReNHase). Only five of the eight mutants (PtNHase βR52A, βR52K, βR157A, βR157K and ReNHase βR61A) were successfully expressed and purified. Apart from the PtNHase βR52A mutant that exhibited no detectable activity, the kcat values obtained for the PtNHase and ReNHase βR mutant enzymes were between 1.8 and 12.4 s-1 amounting to <1% of the kcat values observed for WT enzymes. The metal content of each mutant was also significantly decreased with occupancies ranging from ∼10 to ∼40%. UV-Vis spectra coupled with EPR data obtained on the ReNHase mutant enzyme, suggest a decrease in the Lewis acidity of the active site metal ion. X-ray crystal structures of the four PtNHase βR mutant enzymes confirmed the mutation and the low active site metal content, while also providing insight into the active site hydrogen bonding network. Finally, DFT calculations suggest that the equatorial sulfenic acid ligand, which has been shown to be the catalytic nucleophile, is protonated in the mutant enzyme. Taken together, these data confirm the necessity of the conserved second-sphere βR residues in the proposed subunit swapping process and post-translational modification of the α-subunit in the α activator complex, along with stabilizing the catalytic sulfenic acid in its anionic form.

Moving metals: How microbes deliver metal cofactors to metalloproteins

First row d-block metal ions serve as vital cofactors for numerous essential enzymes and are therefore required nutrients for all forms of life. Despite this requirement, excess free transition metals are toxic. Free metal ions participate in the production of noxious reactive oxygen species and mis-metalate metalloproteins, rendering enzymes catalytically inactive. Thus, bacteria require systems to ensure metalloproteins are properly loaded with cognate metal ions to maintain protein function, while avoiding metal-mediated cellular toxicity. In this perspective we summarize the current mechanistic understanding of bacterial metallocenter maturation with specific emphasis on metallochaperones; a group of specialized proteins that both shield metal ions from inadvertent reactions and distribute them to cognate target metalloproteins. We highlight several recent advances in the field that have implicated new classes of proteins in the distribution of metal ions within bacterial proteins, while speculating on the future of the field of bacterial metallobiology.

Possible functions of CobW domain-containing (CBWD) genes in dinoflagellates using Karlodinium veneficum as a representative

Since > 91% of dinoflagellates are proven auxotrophs of vitamin B₁₂ and the cobalamin synthetase W (CobW) is a key gene involved in vitamin B₁₂ synthesis pathway, a number of CobW domain-containing (CBWD) genes in dinoflagellates (DinoCBWDs) were surprisedly found from our transcriptomic and meta-transcriptomic studies. A total of 88 DinoCBWD genes were identified from the genomes and transcriptomes of four dinoflagellates, with five being cloned for full-lengths and characterized using the cosmopolitan and ecologically-important dinoflagellates Karlodinium veneficum and Scrippsiella trochoidea (synonym of Scrippsiella acuminata). DinoCBWDs were verified being irrelevant to vitamin B₁₂ biosynthesis due to their transcriptions irresponsive to vitamin B₁₂ levels and their phylogenetic positions. A comprehensive phylogenetic analysis demonstrated 75 out of the 88 DinoCBWD genes identified belong to three subfamilies of COG0523 protein family, of which most prokaryotic members are reported to be metallochaperones and the eukaryotic members are ubiquitously found but mostly unknown for their functions. Our results from K. veneficum demonstrated DinoCBWDs are associated with metal homeostasis and other divergent functions, with four KvCBWDs involving in zinc homeostasis and KvCBWD1 likely functioning as Fe-type nitrile hydratase activator. In addition, conserved motif analysis revealed the structural foundation of KvCBWD proteins that are consistent with previously described CBWD proteins with GTPase activity and metal binding. Our results provide a stepping-stone toward better understanding the functions of DinoCBWDs and the COG0523 family.

Discovery of the ATPase Activity of a Cobalt-Type Nitrile Hydratase Activator and Its Promoting Effect on Enzyme Maturation

An activator protein and a metal ion are two factors known to be indispensable for the maturation of nitrile hydratase (NHase). Here, the third key factor, adenosine triphosphate (ATP), was identified to play an important role in the activation of Co-type NHase. Free phosphate measurements revealed that the Co-type activator protein can hydrolyze ATP/GTP with appreciable performance and that such catalytic performance is related to NHase activity. Computational analysis and site-directed mutagenesis identified several potential hot spot residues involved in the binding of ATP to Co-type activator protein, and an E60A/W61A/D62A/I139A/T141A combinatorial variant reduced the ATPase activity to 18% of its original level. Further NHase activation studies using the combinatorial variant demonstrated that although the ATPase activity of the Co-type activator protein correlated with NHase activity, a low ATP concentration of 0.5 mmol/L was optimal for NHase activation, with higher ATP concentrations potentially inhibiting NHase activity.

The role of nucleoside triphosphate hydrolase metallochaperones in making metalloenzymes

Metalloenzymes catalyze a diverse set of challenging chemical reactions that are essential for life. These metalloenzymes rely on a wide range of metallocofactors, from single metal ions to complicated metallic clusters. Incorporation of metal ions and metallocofactors into apo-proteins often requires the assistance of proteins known as metallochaperones. Nucleoside triphosphate hydrolases (NTPases) are one important class of metallochaperones and are found widely distributed throughout the domains of life. These proteins use the binding and hydrolysis of nucleoside triphosphates, either adenosine triphosphate (ATP) or guanosine triphosphate (GTP), to carry out highly specific and regulated roles in the process of metalloenzyme maturation. Here, we review recent literature on NTPase metallochaperones and describe the current mechanistic proposals and available structural data. By using representative examples from each type of NTPase, we also illustrate the challenges in studying these complicated systems. We highlight open questions in the field and suggest future directions. This minireview is part of a special collection of articles in memory of Professor Deborah Zamble, a leader in the field of nickel biochemistry.

COG0523 proteins: a functionally diverse family of transition metal-regulated G3E P-loop GTP hydrolases from bacteria to man

Transition metal homeostasis ensures that cells and organisms obtain sufficient metal to meet cellular demand while dispensing with any excess so as to avoid toxicity. In bacteria, zinc restriction induces the expression of one or more Zur (zinc-uptake repressor)-regulated Cluster of Orthologous Groups (COG) COG0523 proteins. COG0523 proteins encompass a poorly understood sub-family of G3E P-loop small GTPases, others of which are known to function as metallochaperones in the maturation of cobalamin (CoII) and NiII cofactor-containing metalloenzymes. Here, we use genomic enzymology tools to functionally analyse over 80 000 sequences that are evolutionarily related to Acinetobacter baumannii ZigA (Zur-inducible GTPase), a COG0523 protein and candidate zinc metallochaperone. These sequences segregate into distinct sequence similarity network (SSN) clusters, exemplified by the ZnII-Zur-regulated and FeIII-nitrile hydratase activator CxCC (C, Cys; X, any amino acid)-containing COG0523 proteins (SSN cluster 1), NiII-UreG (clusters 2, 8), CoII-CobW (cluster 4), and NiII-HypB (cluster 5). A total of five large clusters that comprise ≈ 25% of all sequences, including cluster 3 which harbors the only structurally characterized COG0523 protein, Escherichia coli YjiA, and many uncharacterized eukaryotic COG0523 proteins. We also establish that mycobacterial-specific protein Y (Mpy) recruitment factor (Mrf), which promotes ribosome hibernation in actinomycetes under conditions of ZnII starvation, segregates into a fifth SSN cluster (cluster 17). Mrf is a COG0523 paralog that lacks all GTP-binding determinants as well as the ZnII-coordinating Cys found in CxCC-containing COG0523 proteins. On the basis of this analysis, we discuss new perspectives on the COG0523 proteins as cellular reporters of widespread nutrient stress induced by ZnII limitation.

Proposed mechanism for post-translational self-modification of Co-NHase based on Co2+ diffusion limitation

BACKGROUND

Nitrile hydratase (NHase), was an excellent biocatalyst for the synthesis of amide compounds. NHase was typical heterodimeric metalloprotein, required of the assistance of activator for active expressions. In this work, we found a special Co-NHase HBA from Caldalkalibacillus thermarum, which had the ability of post-translational self-modification and could incorporate Co²⁺ into the catalytic center in the absence of activator.

METHOD AND RESULTS

We simulated the movement of Co²⁺ in silico and established a hypothetical model to predict the Co²⁺ incorporation efficiency (X_Co ) of NHases. According to the simulation results, NHase mutants with different positive charge distribution were constructed. Compared with wild-type, the Co²⁺ incorporation efficiency of K1 (M10K) was increased by 2.1-fold from 0.36 to 0.76, and the specific activity was increased by 3.2-fold from 136.3 to 432.0 U/mg, while mutant K1H1 (M10K, D11H) and K2H2 (M10K, D11H, E20K, N21H) lost the ability of post-translation self-modification.

CONCLUSIONS AND IMPLICATIONS

The interactions of positively charged residues near the catalytic center, such as lysine with strong electrostatic repulsive interaction, arginine with weak electrostatic repulsive interaction and histidine with metal affinity, could limit the free diffusion of Co²⁺ in NHase and affect the efficiency of post-translational self-modification. This work also provided an effective strategy for protein engineering of NHases and other metalloenzymes. This article is protected by copyright. All rights reserved.

Calculating metalation in cells reveals CobW acquires CoII for vitamin B12 biosynthesis while related proteins prefer ZnII

Protein metal-occupancy (metalation) in vivo has been elusive. To address this challenge, the available free energies of metals have recently been determined from the responses of metal sensors. Here, we use these free energy values to develop a metalation-calculator which accounts for inter-metal competition and changing metal-availabilities inside cells. We use the calculator to understand the function and mechanism of GTPase CobW, a predicted Co^II-chaperone for vitamin B₁₂. Upon binding nucleotide (GTP) and Mg^II, CobW assembles a high-affinity site that can obtain Co^II or Zn^II from the intracellular milieu. In idealised cells with sensors at the mid-points of their responses, competition within the cytosol enables Co^II to outcompete Zn^II for binding CobW. Thus, Co^II is the cognate metal. However, after growth in different [Co^II], Co^II-occupancy ranges from 10 to 97% which matches CobW-dependent B₁₂ synthesis. The calculator also reveals that related GTPases with comparable Zn^II affinities to CobW, preferentially acquire Zn^II due to their relatively weaker Co^II affinities. The calculator is made available here for use with other proteins.

Recent Advances and Promises in Nitrile Hydratase: From Mechanism to Industrial Applications

Nitrile hydratase (NHase, EC 4.2.1.84) is one type of metalloenzyme participating in the biotransformation of nitriles into amides. Given its catalytic specificity in amide production and eco-friendliness, NHase has overwhelmed its chemical counterpart during the past few decades. However, unclear catalytic mechanism, low thermostablity, and narrow substrate specificity limit the further application of NHase. During the past few years, numerous studies on the theoretical and industrial aspects of NHase have advanced the development of this green catalyst. This review critically focuses on NHase research from recent years, including the natural distribution, gene types, posttranslational modifications, expression, proposed catalytic mechanism, biochemical properties, and potential applications of NHase. The developments of NHase described here are not only useful for further application of NHase, but also beneficial for the development of the fields of biocatalysis and biotransformation.

Mechanistic Insights into the Metal-Dependent Activation of ZnII-Dependent Metallochaperones

Members of the COG0523 subfamily of candidate GTPase metallochaperones function in bacterial transition-metal homeostasis, but the nature of the cognate metal, mechanism of metal transfer, and identification of target protein(s) for metal delivery remain open questions. Here, we explore the multifunctionality of members of the subfamily linked to delivering Zn^II to apoprotein targets under conditions of host-imposed transition-metal depletion. We examine two zinc-uptake repressor (Zur)-regulated COG0523 family members, each from a major human pathogen, Acinetobacter baumannii (AbZigA) and Staphylococcus aureus (SaZigA), in an effort to develop a model for Zn^II metallochaperone activity. Zn^II chelator competition experiments reveal one high-affinity (K_Zn1 ≈ 10¹⁰-10¹¹ M^-1) metal-binding site in each GTPase, while AbZigA and SaZigA are characterized by an additional one and two (lower-affinity) metal-binding sites, respectively. Co^II titrations reveal that both metallochaperones have similar electronic absorption characteristics that indicate the presence of two tetrahedral metal coordination sites. High-affinity metal binding at the CXCC motif activates the GTPase activity of both enzymes, with Zn^II more effective than Co^II. Both GTPases bind the product, GDP, more tightly in the apoprotein than the Zn^II-bound state and exhibit what is best described as a "locked" conformation around the GTP substrate. Negative thermodynamic linkage is observed between nucleotide binding and metal binding, leading to a new mechanistic model for COG0523-catalyzed metal delivery.

Metallochaperones and metalloregulation in bacteria

Bacterial transition metal homoeostasis or simply 'metallostasis' describes the process by which cells control the intracellular availability of functionally required metal cofactors, from manganese (Mn) to zinc (Zn), avoiding both metal deprivation and toxicity. Metallostasis is an emerging aspect of the vertebrate host-pathogen interface that is defined by a 'tug-of-war' for biologically essential metals and provides the motivation for much recent work in this area. The host employs a number of strategies to starve the microbial pathogen of essential metals, while for others attempts to limit bacterial infections by leveraging highly competitive metals. Bacteria must be capable of adapting to these efforts to remodel the transition metal landscape and employ highly specialized metal sensing transcriptional regulators, termed metalloregulatory proteins,and metallochaperones, that allocate metals to specific destinations, to mediate this adaptive response. In this essay, we discuss recent progress in our understanding of the structural mechanisms and metal specificity of this adaptive response, focusing on energy-requiring metallochaperones that play roles in the metallocofactor active site assembly in metalloenzymes and metallosensors, which govern the systems-level response to metal limitation and intoxication.