Bi(III) Binding Stoichiometry and Domain-Specificity Differences Between Apo and Zn(II)-bound Human Metallothionein 1a

Bismuth is a xenobiotic metal with a high affinity to sulfur that is used in a variety of therapeutic applications. Bi(III) induces the cysteine-rich metallothionein (MT), a protein known to form two-domain cluster structures with certain metals such as Zn(II), Cd(II), or Cu(I). The binding of Bi(III) to MTs has been previously studied, but there are conflicting reports on the stoichiometry and binding pathway, which appear to be highly dependent on pH and initial metal-loading status of the MT. Additionally, domain specificity has not been thoroughly investigated. In this paper, ESI-MS was used to determine the binding constants of [Bi(EDTA)]- binding to apo-MT1a and its individual αMT fragment. The results were compared to previous experiments using βMT1a and βαMT3. Domain specificity was investigated using proteolysis methods and the initial cooperatively formed Bi2MT was found to bind to cysteines that spanned across the traditional metal binding domain regions. Titrations of [Bi(EDTA)]- into Zn7MT were performed and were found to result in a maximum stoichiometry of Bi7MT, contrasting the Bi6MT formed when [Bi(EDTA)]- was added to apo-MT. These results show that the initial structure of the apo-MT determines the stoichiometry of new incoming metals and explains the previously observed differences in stoichiometry.

ESI-MS analysis of Cu(I) binding to apo and Zn7 human metallothionein 1A, 2, and 3 identifies the formation of a similar series of metallated species with no individual isoform optimization for Cu(I)

Metallothioneins (MTs) are cysteine-rich proteins involved in metal homeostasis, heavy metal detoxification, and protection against oxidative stress. Whether the four mammalian MT isoforms exhibit different metal binding properties is not clear. In this paper, the Cu(I) binding properties of the apo MT1A, apo MT2, and apo MT3 are compared and the relative Cu(I) binding affinities are reported. In all three isoforms, Cu4, Cu6, and Cu10 species form cooperatively, and MT1A and MT2 also form a Cu13 species. The Cu(I) binding properties of Zn7-MT1A, Zn7-MT2, and Zn7-MT3 are compared systematically using isotopically pure 63Cu(I) and 68Zn(II). The species formed in each MT isoform were detected through electrospray ionization-mass spectrometry and further characterized using room temperature phosphorescence spectroscopy. The mixed metal Cu, Zn species forming in MT1A, MT2, and MT3 have similar stoichiometries and their emission spectral properties indicate that analogous clusters form in the three isoforms. Three parallel metallation pathways have been proposed through analysis of the detailed Cu, Zn speciation in MT1A, MT2, and MT3. Pathway ① results in Cu5Zn5-MT and Cu9Zn3-MT. Pathway ② involves Cu6Zn4-MT and Cu10Zn2-MT. Pathway ③ includes Cu8Zn4-MT. Speciation analysis indicates that Pathway ② is the preferred pathway for MT2. This is also evident in the phosphorescence spectra with the 750 nm emission from Cu6Zn4-MT being most prominent in MT2. We see no evidence for different MT isoforms being optimized or exhibiting preferences for certain metals. We discuss the probable stoichiometry for MTs in vivo based on the in vitro determined binding constants.

Human apo-metallothionein 1a is not a random coil: Evidence from guanidinium chloride, high temperature, and acidic pH unfolding studies

The structures of apo-metallothioneins (apo-MTs) have been relatively elusive due to their fluxional, disordered state which has been difficult to characterize. However, intrinsically disordered protein (IDP) structures are rather diverse, which raises questions about where the structure of apo-MTs fit into the protein structural spectrum. In this paper, the unfolding transitions of apo-MT1a are discussed with respect to the effect of the chemical denaturant GdmCl, temperature conditions, and pH environment. Cysteine modification in combination with electrospray ionization mass spectrometry was used to probe the unfolding transition of apo-MT1a in terms of cysteine exposure. Circular dichroism spectroscopy was also used to monitor the change in secondary structure as a function of GdmCl concentration. For both of these techniques, cooperative unfolding was observed, suggesting that apo-MT1a is not a random coil. More GdmCl was required to unfold the protein backbone than to expose the cysteines, indicating that cysteine exposure is likely an early step in the unfolding of apo-MT1a. MD simulations complement the experimental results, suggesting that apo-MT1a adopts a more compact structure than expected for a random coil. Overall, these results provide further insight into the intrinsically disordered structure of apo-MT.

Physician and Advanced Practice Clinician Burnout in Rural and Urban Settings

INTRODUCTION

Recruiting rural-practicing clinicians is a high priority. In this study, we explored burnout and contributing work conditions among rural, urban, and family practice physicians and advanced practice clinicians (APCs) in an Upper Midwestern health care system.

METHODS

The Mini Z burnout reduction measure was administered by anonymous electronic survey in March 2022. We conducted bivariate analyses of study variables, then assessed relationships of study variables to burnout with multivariate binary logistic regression.

RESULTS

Of 1118 clinicians (63% response rate), 589 physicians and 496 APCs were included in this study (n = 1085). Most were female (56%), physicians (54%), and White (86%), while 21% were in family practice, 46% reported burnout, and 349 practiced rurally. Rural and urban clinician burnout rates were comparable (45% vs 47%). Part-time work protected against burnout for family practice and rural clinicians, but not urban clinicians. In multivariate models for rural clinicians, stress (OR: 8.53, 95% CI: 4.09 to 17.78, P < .001), lack of workload control (OR: 3.06, 95% CI: 1.47-6.36, P = .003), busy/chaotic environments (OR: 2.53, 95% CI: 1.29-4.99, P = .007), and intent to leave (OR: 2.18, 95% CI: 1.06-4.45, P = .033) increased burnout odds. In family practice clinicians, stress (OR: 13.43 95% CI: 4.90-36.79, P < .001) also significantly increased burnout odds.

CONCLUSIONS

Burnout was comparable between rural and urban physicians and APCs. Part-time work was associated with decreased burnout in rural and family practice clinicians. Addressing burnout drivers (stress, workload control, chaos) may improve rural work environments, reduce turnover, and aid rural clinician recruitment. Addressing stress may be particularly impactful in family practice.

Structural motifs in the early metallation steps of Zn(II) and Cd(II) binding to apo-metallothionein 1a

Many proteins require a metal cofactor to function and these metals are often involved in the protein folding process. The protein metallothionein (MT) has a dynamic structure capable of binding to a variety of metals with different stoichiometries. The most well-understood structure is the seven-metal, two domain structure formed upon metallation using Zn(II) or Cd(II). However, the partially metallated states and the pathways to form these clusters are less well-understood, although it is known that the pathways are pH dependent. Using stopped flow methods, it is shown that the metallation rates of the less cooperative Zn(II) binding pathway is much more impacted by low pH conditions that that of the more cooperative Cd(II) binding pathway. Electrospray ionization mass spectrometry (ESI-MS) methods reveal specific mixtures of bridging and terminally bound MxSy structures form in the first few metallation steps. Using a combination of methods, the data show that the result of unfolding this intrinsically disordered apo-MT structure using guanidinium chloride is that the formation of preliminary bridging structures that form in the first few metallation steps is impeded. The data show that more terminally bound structures form. Our conclusion is that the compact conformation of the native apo-MT at physiological pH allows for rapid formation of complex metal-thiolate structures with high affinity that provides protection from oxidation, a function that is suppressed upon unfolding. Overall, these results highlight both the importance of the apo-MT structure in the metallation pathway, but also the differences in Zn(II) and Cd(II) binding under different conditions.

Supermetalation of Cd-MT3 beyond the two-domain model

The flexibility of mammalian metallothioneins (MTs) has contributed to the difficulty in obtaining structural information for this family of metalloproteins that bind divalent metals with its twenty cysteines. While the two-domain structure for Cd7MT is well-established as a Cd4S11 and Cd3S9, a third structure has been reported when 8 Cd(II) ions bind to MT1. Isoform 3 of the MT family, MT3, has been of interest to the research community since its isolation as a growth inhibitory factor isolated in brain tissue, and has since been noted as a prominent participant in the mediation of neurodegenerative diseases and regular brain development. The differences between MT3 and the other isoforms of MT include an additional hexapeptide insertion of acidic residues in the α domain as well as the introduction of two prolines in the β domain. It is unclear whether these changes impact the metalation properties of MT3. We report the formation of a Cd8MT3 species is characterized by electrospray ionization mass spectrometry and UV-visible absorption spectroscopy. We report that the spectroscopic properties of this supermetalated Cd8MT3 are similar to those of the supermetalated Cd8MT1, with a clear indication of changes in structure from "fully-metalated" Cd7MT3 to supermetalated Cd8MT3 from circular dichroism spectra and both 1D 113Cd and 2D 1H-113Cd HSQC NMR spectra. We conclude that the metalation properties are not impacted significantly due to the amino acid changes in MT3, and that the cysteinyl thiols are the key players in determining the capacity of metal-binding and the structure of metal-thiolate clusters.

Metallothionein-3 and carbonic anhydrase metalation properties with Zn(II) and Cd(II) change as a result of protein-protein interactions

Metallothioneins (MT) are regulators of the metals Zn(II) and Cu(I) and act as antioxidants in many organisms, including in humans. Isoform 3 (MT3) is expressed constitutively in central nervous tissue and has been shown to have additional biological functions, including the inhibition of neuronal growth, the regulation of apoptosis, and cytoskeleton modulation. To facilitate these functions, protein-protein interactions likely occur. These interactions may then impact the metalation status of the MT and the recipient metalloprotein. Using electrospray ionization mass spectrometry and circular dichroism spectroscopy, we report that the interaction between the zinc metalloenzyme, carbonic anhydrase (CA), and MT3, impacts the metalation profiles of both apo-MT3 and apo-CA with Cd(II) and Zn(II). We observe two phases in the metalation of the apo-CA, the first of which is associated with an increased binding affinity of apo-CA for Cd/Zn(II) and the second pathway is associated with apo-CA metalated without a change in binding affinity. The weak interactions that result in this change of binding affinity are not detectable as a protein complex in the ESI-mass spectral data or in the circular dichroism spectra. These unusual metalation properties of apo-CA in the presence of apo-MT3 are evidence of the effects of protein-protein interactions. With adjustment to take into account the interaction of both proteins, we report the complete Cd(II) and Zn(II) binding constants of MT3 under physiological conditions, as well as the pH dependence of these binding pathways.

Cu(I) binds to Zn7-MT2 via two parallel pathways

Metallothionein proteins are essential for Cu(I) and Zn(II) homeostasis as well as heavy metal detoxification. The metallation properties of MT2 are of great interest due to their wide patterns of expression and correlation with multiple diseases including cancers, neurological disorders, and respiratory diseases. Use of isotopically pure 63Cu(I) and 68Zn(II) eliminates the complexity of the Cu, Zn-MT2 mass spectral peaks due to significant overlap of naturally abundant isotopes. This allows for the resolution of the precise Cu(I) and Zn(II) stoichiometries when both Cu(I) and Zn(II) are bound to MT2 at physiological pH as expected in vivo. Exact Cu: Zn ratios were determined from mass spectral simulations carried out for every point in the titration. We report that Cu(I) metallation of Zn7-MT2 can only be understood in terms of two pathways occurring in parallel with pathway ① resulting in Cu5Zn5-MT2 and Cu9Zn3-MT2. Pathway ② results in Cu6Zn4-MT2 and Cu10Zn2-MT2, which are the major products of the reaction. From the electrospray ionization (ESI)-mass spectral data we report a series of formation constants (KF) for species starting from Zn7-MT2 up to Cu11Zn2-MT2. Room temperature phosphorescence and circular dichroism (CD) spectra were measured in parallel with the ESI-mass spectrometry data allowing for the assignment of specific species to specific spectral bands. Through analysis of the CD spectral bands, we propose that Cu(I) binds to the β domain first to form a Cu5Zn1 cluster or Cu6 cluster with emission at 670 and 750 nm, respectively, leaving the Zn4 cluster in the α domain.

Metallothionein-3: 63 Cu(I) binds to human 68 Zn7 -βα MT3 with no preference for Cu4 -β cluster formation

Human metallothioneins (MTs) are involved in binding the essential elements, Cu(I) and Zn(II), and the toxic element, Cd(II), in metal-thiolate clusters using 20 reduced cysteines. The brain-specific MT3 binds a mixture of Cu(I) and Zn(II) in vivo. Its metallation properties are critically important because of potential connections between Cu, Zn and neurodegenerative diseases. We report that the use of isotopically pure 63 Cu(I) and 68 Zn(II) greatly enhances the element resolution in the ESI-mass spectral data revealing species with differing Cu:Zn ratios but the same total number of metals. Room temperature phosphorescence and circular dichroism spectral data measured in parallel with ESI-mass spectral data identified the presence of specific Cu(I)-thiolate clusters in the presence of Zn(II). A series of Cu(I)-thiolate clusters form following Cu(I) addition to apo MT3: the two main clusters that form are a Cu6 cluster in the β domain followed by a Cu4 cluster in the α domain. 63 Cu(I) addition to 68 Zn7 -MT3 results in multiple species, including clustered Cu5 Zn5 -MT3 and Cu9 Zn3 -MT3. We assign the domain location of the metals for Cu5 Zn5 -MT3 as a Cu5 Zn1 -β cluster and a Zn4 -α cluster and for Cu9 Zn3 -MT3 as a Cu6 -β cluster and a Cu3 Zn3 -α cluster. While many reports of the average MT3 metal content exist, determining the exact Cu,Zn stoichiometry has proven very difficult even with native ESI-MS. The work in this paper solves the ambiguity introduced by the overlap of the naturally abundant Cu(I) and Zn(II) isotopes. Contrary to other reports, there is no indication of a major fraction of Cu4 -β-Znn -α-MT3 forming.

Xenobiotic Bi3+ Coordination by Cysteine-Rich Metallothionein-3 Reveals a Cooperatively Formed Thiolate-Sharing Bi2S5 Cluster

The field of designing artificial metalloproteins has yet to effectively tackle the incorporation of multimetal clusters, which is a key component of natural metalloproteins, such as metallothioneins (MTs) and calmodulin. MT is a physiological, essential, cysteine-rich metalloprotein that binds to a variety of metals but is only known to form metal-thiolate clusters with Cd2+, Zn2+, and Cu+. Bismuth is a xenobiotic metal and a component of metallodrugs used to treat gastric ulcers and cancer, as well as an emerging metal used in industrial practices. Electrospray ionization mass spectrometry, UV-visible spectroscopy, and extended X-ray absorption fine structure spectroscopy were used to probe the Bi3+ binding site structures in apo-MT3 (brain-located MT) at pH 7.4 and 2 and provide the complete set of binding affinities. We discovered the highly cooperative formation of a novel Bi3+ species, Bi2MT3, under physiological conditions, where each Bi3+ ion is coordinated by three cysteinyl thiolates, with one of the thiolates bridging between the two Bi3+ ions. This cluster structure was associated with a strong visible region absorption band, which was disrupted by the addition of Zn2+ and reversibly disrupted by acidification and increased temperature. This is the first reported presence of bridging cysteines for a xenobiotic metal in MT3 and the Bi2MT structure is the first Bi cluster found in a metalloprotein.

Arsenic binding to human metallothionein-3

Arsenic poisoning is of great concern with respect to its neurological toxicity, which is especially significant for young children. Human exposure to arsenic occurs worldwide from contaminated drinking water. In human physiology, one response to toxic metals is through coordination with the metallochaperone metallothionein (MT). Central nervous system expression of MT isoform 3 (MT3) is thought to be neuroprotective. We report for the first time on the metalation pathways of As3+ binding to apo-MT3 under physiological conditions, yielding the absolute binding constants (log Kn, n = 1-6) for each sequential As3+ binding event: 10.20, 10.02, 9.79, 9.48, 9.06, and 8.31 M-1. We report on the rate of the reaction of As3+ with apo-MT3 at pH 3.5 with rate constants (kn, n = 1-6) determined for each sequential As3+ binding event: 116.9, 101.2, 85.6, 64.0, 43.9, and 21.0 M-1 s-1. We further characterize the As3+ binding pathway to fully metalated Zn7MT3 and partially metalated Zn-MT3. As3+ binds rapidly with high binding constants under physiological conditions in a noncooperative manner, but is unable to replace the Zn2+ in fully-metalated Zn-MT3. As3+ binding to partially metalated Zn-MT3 takes place with a rearrangement of the Zn-binding profile. Our work shows that As 3+ rapidly and efficiently binds to both apo-MT3 and partially metalated Zn-MT3 at physiological pH.

Structural Role of Cadmium and Zinc in Metallothionein Oxidation by Hydrogen Peroxide: The Resilience of Metal-Thiolate Clusters

Oxidative stress is a state involving an imbalance of reactive oxygen species in a cell and is linked to a variety of diseases. The metal-binding protein metallothionein (MT) may play a role in protection due to its high cysteine content. Many studies have shown that oxidative stress will cause MT to both form disulfide bonds and release bound metals. However, studies on the more biologically relevant partially metalated MTs have been largely neglected. Additionally, most studies to date have used spectroscopic methods that cannot detect specific intermediate species. In this paper, we describe the oxidation and the subsequent metal displacement pathway of fully and partially metalated MTs with hydrogen peroxide. The rates of the reactions were monitored using electrospray ionization mass spectrometry (ESI-MS) techniques, which resolved and characterized the individual intermediate M_x(SH)_yMT species. The rate constants were calculated for each species formation. Through ESI-MS and circular dichroism spectroscopy, it was found that the three metals in the β-domain were the first to be released from the fully metalated MTs. The Cd(II) in the partially metalated Cd(II)-bound MTs rearranged to form a protective Cd₄MT cluster structure upon exposure to oxidation. The partially metalated Zn(II)-bound MTs oxidized at a faster rate as the Zn(II) did not rearrange in response to oxidation. Additionally, density functional theory calculations showed that the terminally bound cysteines were more negative and thus more susceptible to oxidation than the bridging cysteines. The results of this study highlight the importance of metal-thiolate structures and metal identity in MT's response to oxidation.

63Cu(I) binding to human kidney 68Zn7-βα MT1A: determination of Cu(I)-thiolate cluster domain specificity from ESI-MS and room temperature phosphorescence spectroscopy

Mammalian metallothioneins (MTs) are important proteins in Zn(II) and Cu(I) homeostasis with the Zn(II) and Cu(I) binding to the 20 cysteines in metal-thiolate clusters. Previous electrospray ionization (ESI) mass spectrometric (MS) analyses of Cu(I) binding to Zn7-MT were complicated by significant overlap of the natural abundance isotopic patterns for Zn(II) and Cu(I) leading to impossibly ambiguous stoichiometries. In this paper, isotopically pure 63Cu(I) and 68Zn(II) allowed determination of the specific stoichiometries in the 68 Zn,63Cu-βα MT1A species formed following the stepwise addition of 63Cu(I) to 68Zn7-βα MT1A. These species were characterized by ESI-MS and room temperature emission spectroscopy. The key species that form and their emission band centres are Zn5Cu5-βα MT1A (λ = 684 nm), Zn4Cu6-βα MT1A (λ = 750 nm), Zn3Cu9-βα MT1A (λ = 750 nm), Zn2Cu10-βα MT1A (λ = 750 nm), and Zn1Cu14-βα MT1A (λ = 634 nm). The specific domain stoichiometry of each species was determined by assessing the species forming following 63Cu(I) addition to the 68Zn3-β MT1A and 68Zn4-α MT1A domain fragments. The domain fragment emission suggests that Zn5Cu5-βα MT1A contains a Zn1Cu5-β cluster and the Zn4Cu6-βα MT1A, Zn3Cu9-βα MT1A, and Zn2Cu10-βα MT1A each contain a Cu6-β cluster. The species forming with >10 mol. eq. of 63Cu(I) in βα-MT1A exhibit emission from the Cu6-β cluster and an α domain cluster. This high emission intensity is seen at the end of the titrations of 68Zn7-βα MT1A and the 68Zn4-α MT1A domain fragment suggesting that the initial presence of the Zn(II) results in clustered Cu(I) binding in the α domain.

COVID-Related Stress and Work Intentions in a Sample of US Health Care Workers

OBJECTIVE

To evaluate relationships between coronavirus disease 2019 (COVID-19)-related stress and work intentions in a sample of US health care workers.

PATIENTS AND METHODS

Between July 1 and December 31, 2020, health care workers were surveyed for fear of viral exposure or transmission, COVID-19-related anxiety or depression, work overload, burnout, and intentions to reduce hours or leave their jobs.

RESULTS

Among 20,665 respondents at 124 institutions (median organizational response rate, 34%), intention to reduce hours was highest among nurses (33.7%; n=776), physicians (31.4%; n=2914), and advanced practice providers (APPs; 28.9%; n=608) while lowest among clerical staff (13.6%; n=242) and administrators (6.8%; n=50; all P<.001). Burnout (odds ratio [OR], 2.15; 95% CI, 1.93 to 2.38), fear of exposure, COVID-19-related anxiety/depression, and workload were independently related to intent to reduce work hours within 12 months (all P<.01). Intention to leave one's practice within 2 years was highest among nurses (40.0%; n=921), APPs (33.0%; n=694), other clinical staff (29.4%; n=718), and physicians (23.8%; n=2204) while lowest among administrators (12.6%; n=93; all P<.001). Burnout (OR, 2.57; 95% CI, 2.29 to 2.88), fear of exposure, COVID-19-related anxiety/depression, and workload were predictors of intent to leave. Feeling valued by one's organization was protective of reducing hours (OR, 0.65; 95% CI, 0.59 to 0.72) and intending to leave (OR, 0.40; 95% CI, 0.36 to 0.45; all P<.01).

CONCLUSION

Approximately 1 in 3 physicians, APPs, and nurses surveyed intend to reduce work hours. One in 5 physicians and 2 in 5 nurses intend to leave their practice altogether. Reducing burnout and improving a sense of feeling valued may allow health care organizations to better maintain their workforces postpandemic.

A di-Copper Peptidyl Complex Mimics the Activity of Catalase, a Key Antioxidant Metalloenzyme

Catalases (CAT) are antioxidant metalloenzymes necessary for life in oxygen-metabolizing cells to regulate H₂O₂ concentration by accelerating its dismutation. Many physiopathological situations are associated with oxidative stress resulting from H₂O₂ overproduction, during which antioxidant defenses are overwhelmed. We have used a combinatorial approach associated with an activity-based screening to discover a first peptidyl di-copper complex mimicking CAT. The complex was studied in detail and characterized for its CAT activity both in solutions and in cells using different analytical methods. The complex exhibited CAT activity in solutions and, more interestingly, on HyPer HeLa cells that possess a genetically encoded ratiometric fluorescent sensors of H₂O₂. These results highlight the efficiency of a combinatorial approach for the discovery of peptidyl complexes that exhibit catalytic activity.

Structurally restricted Bi(III) metallation of apo-βMT1a: metal-induced tangling

Non-toxic bismuth salts are used in anti-ulcer medications and to protect against nephrotoxicity from anticancer drugs. Bismuth salts also induce metallothionein (MT), a metal-binding protein that lacks a formal secondary structure. We report the impact on the metallation properties of Bi(III) to the 9-cysteine β fragment of MT as a function of cysteine accessibility using electrospray ionization mass spectrometry. At pH 7.4, Bi2βMT formed cooperatively. Cysteine modification shows that each Bi(III) was terminally bound to three cysteinyl thiolates. Non-cooperative Bi(III) binding was observed at pH 2.3, where cysteine accessibility is increased. However, competition from H4EDTA inhibited Bi(III) binding. When GdmCl, a well-known denaturing agent, was used to increase cysteine accessibility of the apoβMT at pH 7.4, a greater fraction of Bi3βMT formed using all nine cysteines. The change in binding profile and equilibrium of Bi2βMT was determined as a function of acidification, which changed as a result of competition with H4EDTA. There was no Bi(III) transfer between Bi2βMT, Cd3βMT, and Zn3βMT. This lack of metal exchange and the resistance towards binding the third Bi(III) suggest a rigidity in the Bi2βMT binding sites that inhibits Bi(III) mobility. These experiments emphasize the conformational control of metallation that results in substantially different metallated products: at pH 7.4 (many cysteines buried) Bi2βMT, whereas at pH 7.4 (all cysteines accessible) enhanced formation of Bi3βMT. These data suggest that the addition of the first two Bi(III) crosslinks the protein, blocking access to the remaining three cysteines for the third Bi(III), as a result of tangle formation.

Association of Physician Burnout With Suicidal Ideation and Medical Errors

IMPORTANCE

Addressing physician suicide requires understanding its association with possible risk factors such as burnout and depression.

OBJECTIVE

To assess the association between burnout and suicidal ideation after adjusting for depression and the association of burnout and depression with self-reported medical errors.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study was conducted from November 12, 2018, to February 15, 2019. Attending and postgraduate trainee physicians randomly sampled from the American Medical Association Physician Masterfile were emailed invitations to complete an online survey in waves until a convenience sample of more than 1200 practicing physicians agreed to participate.

MAIN OUTCOMES AND MEASURES

The primary outcome was the association of burnout with suicidal ideation after adjustment for depression. The secondary outcome was the association of burnout and depression with self-reported medical errors. Burnout, depression, suicidal ideation, and medical errors were measured using subscales of the Stanford Professional Fulfillment Index, Maslach Burnout Inventory-Human Services Survey for Medical Personnel, and Mini-Z burnout survey and the Patient-Reported Outcomes Measurement Information System depression Short Form. Associations were evaluated using multivariable regression models.

RESULTS

Of the 1354 respondents, 893 (66.0%) were White, 1268 (93.6%) were non-Hispanic, 762 (56.3%) were men, 912 (67.4%) were non-primary care physicians, 934 (69.0%) were attending physicians, and 824 (60.9%) were younger than 45 years. Each SD-unit increase in burnout was associated with 85% increased odds of suicidal ideation (odds ratio [OR], 1.85; 95% CI, 1.47-2.31). After adjusting for depression, there was no longer an association (OR, 0.85; 95% CI, 0.63-1.17). In the adjusted model, each SD-unit increase in depression was associated with 202% increased odds of suicidal ideation (OR, 3.02; 95% CI, 2.30-3.95). In the adjusted model for self-reported medical errors, each SD-unit increase in burnout was associated with an increase in self-reported medical errors (OR, 1.48; 95% CI, 1.28-1.71), whereas depression was not associated with self-reported medical errors (OR, 1.01; 95% CI, 0.88-1.16).

CONCLUSIONS AND RELEVANCE

The results of this cross-sectional study suggest that depression but not physician burnout is directly associated with suicidal ideation. Burnout was associated with self-reported medical errors. Future investigation might examine whether burnout represents an upstream intervention target to prevent suicidal ideation by preventing depression.

Interplay between Carbonic Anhydrases and Metallothioneins: Structural Control of Metalation

Carbonic anhydrases (CAs) and metallothioneins (MTs) are both families of zinc metalloproteins central to life, however, they coordinate and interact with their Zn2+ ion cofactors in completely different ways. CAs and MTs are highly sensitive to the cellular environment and play key roles in maintaining cellular homeostasis. In addition, CAs and MTs have multiple isoforms with differentiated regulation. This review discusses current literature regarding these two families of metalloproteins in carcinogenesis, with a dialogue on the association of these two ubiquitous proteins in vitro in the context of metalation. Metalation of CA by Zn-MT and Cd-MT is described. Evidence for protein-protein interactions is introduced from changes in metalation profiles of MT from electrospray ionization mass spectrometry and the metalation rate from stopped-flow kinetics. The implications on cellular control of pH and metal donation is also discussed in the context of diseased states.

Competition between Al3+ and Fe3+ binding to human transferrin and toxicological implications: structural investigations using ultra-high resolution ESI MS and CD spectroscopy

Human serum transferrin (hTF) is an iron binding protein with the primary task of ensuring well-controlled transport of Fe3+-ions in the bloodstream. Furthermore, hTF has been identified as a key component in the trafficking of Al3+-ions from the serum to cells. It is clear that binding alone does not guarantee cellular uptake via the transferrin receptor, since this is determined by the structural properties of the metal-protein complex. The conformation of the metallated hTF is critically important for delivery of Fe3+ or any other metal into the cell. The combination of ultra-high resolution ESI mass spectrometry and CD spectroscopy together provide accurate species distribution of the Fe3+ during stepwise addition to apo-hTF and an indirect indication of the tertiary structure of the metallated protein. These two methods together are extremely fine probes of structural changes as a function of precise metal binding status at micromolar concentrations. Simulation of the precise domain distribution could be determined during the stepwise metallation from 0 to 2 Fe3+ added. Analysis of the ESI-MS data for the stepwise metallation of apo-hTF and Al1 or 2-hTF with Fe3+ was carried out and used to simulate the experimental speciation based on the reported KF values. There are six main conclusions: (1) Fe3+ binds predominantly, initially to the C-lobe. (2) The CD spectral properties indicate that the C-lobe metallation dominates the structural properties of both binding sites; N-lobe metallation modifies the C-lobe structure. (3) Fe3+ metallation of the mixed Al1-2-hTF results in the dominant form of Fe1Al1-hTF. (4) The first Fe3+ bound to Al1-hTF binds predominantly in the C-lobe domain. (5) The CD spectral properties when Fe3+ binds to Al1-2-hTF indicates that Al-N-lobe occupation mirrors the structural effects of N-lobe occupation by Fe3+. (6) With respect to how Al3+ might enter the cell, the formation of a hybrid form Al1Fe1-hTF might enable the Al3+ to enter the cell via receptor-mediated endocytosis due to the binding of Fe3+ in the C-lobe of the protein which is primarily responsible for the structure of the metal-protein complex.

pH dependence of the non-cooperative binding of Bi3+ to human apo-metallothionein 1A: kinetics, speciation, and stoichiometry

ESI-MS along with cysteine modification show that the binding of Bi³⁺ to apo-metallothionein is non-cooperative with a coordination of BiS(cys)₃ up to Bi₆MT. Stopped flow kinetics reveal that the rate of binding depends on the pH and the Bi³⁺ anion.

Metallothionein Cd4S11cluster formation dominates in the protection of carbonic anhydrase

Results from ESI-MS and stopped flow kinetics show that apo-MT protects from toxic metalation of apo-CA with Cd²⁺due to the protein–protein interactions in solution.

The pathways and domain specificity of Cu(i) binding to human metallothionein 1A

We describe the sequential formation of 3 key Cu(i)–thiolate clusters in human metallothionein 1A using a combination of ESI-MS and phosphorescence lifetime methods.

Altering the optoelectronic properties of boron difluoride formazanate dyes via conjugation with platinum(ii)-acetylides

The absorption, emission, and electrochemical properties of conjugates of boron difluoride formazanate dyes and Pt(ii)-acetylides are systematically studied.

The heme-sensitive regulator SbnI has a bifunctional role in staphyloferrin B production by Staphylococcus aureus

Staphylococcus aureus infection relies on iron acquisition from its host. S. aureus takes up iron through heme uptake by the iron-responsive surface determinant (Isd) system and by the production of iron-scavenging siderophores. Staphyloferrin B (SB) is a siderophore produced by the 9-gene sbn gene cluster for SB biosynthesis and efflux. Recently, the ninth gene product, SbnI, was determined to be a free l-serine kinase that produces O-phospho-l-serine (OPS), a substrate for SB biosynthesis. Previous studies have also characterized SbnI as a DNA-binding regulatory protein that senses heme to control sbn gene expression for SB synthesis. Here, we present crystal structures at 1.9-2.1 Å resolution of a SbnI homolog from Staphylococcus pseudintermedius (SpSbnI) in both apo form and in complex with ADP, a product of the kinase reaction; the latter confirmed the active-site location. The structures revealed that SpSbnI forms a dimer through C-terminal domain swapping and a dimer of dimers through intermolecular disulfide formation. Heme binding had only a modest effect on SbnI enzymatic activity, suggesting that its two functions are independent and structurally distinct. We identified a heme-binding site and observed catalytic heme transfer between a heme-degrading protein of the Isd system, IsdI, and SbnI. These findings support the notion that SbnI has a bifunctional role contributing precursor OPS to SB synthesis and directly sensing heme to control expression of the sbn locus. We propose that heme transfer from IsdI to SbnI enables S. aureus to control iron source preference according to the sources available in the environment.

Kinetics of competitive Cd2+ binding pathways: the realistic structure of intrinsically disordered, partially metallated metallothioneins

The metallation of metallothionein can proceed via two different intermediate structures: a beaded structure that forms quickly (top) and a slow-forming cluster structure (bottom) before forming the fully metallated two-domain protein.

Differential quenching of the angular momentum of the B and Q bands of a porphyrin as a result of extended ring π-conjugation

A novel porphyrin, whose [Formula: see text]-system has been extended via the presence of two additional carbon–carbon triple bonds on opposite meso-positions and by fusion of a single naphthalene unit simultaneously bridging the third meso-position and the [Formula: see text]-carbon of one of the pyrroles, has been synthesized in good yield. Absorption, magnetic circular dichroism, emission, and theoretical spectra are reported for the fused and unfused trans-naphthalene free base and zinc porphyrins. The fusing of one of the naphthalene moieties results in significant changes to the absorption spectrum and, very unusually, the bridged meso-[Formula: see text]-pyrrole fusion results in quenching of the MCD Faraday pseudo-A term in the porphyrin’s B band (S2). This unique effect was interpreted as resulting from the origin of the electronic structure of the second excited state (the B state). The [Formula: see text] and [Formula: see text] polarizations are completely mixed by the electronic effects of the non-symmetric extended conjugation of the [Formula: see text] ring. Analysis of the origin of the MCD signal indicates that the presence of this novel mixed polarization leads to negligible angular momentum in the important B state. To our knowledge, this is the first report in which the magnetic moment in a porphyrin’s intensely absorbing B band has been quenched while the angular momentum in the Q band, the first excited state, remains as normal. This implies that the photophysical properties of the B state are likely very different than those of the Q state, which has novel and significant implications for applications, especially in non-linear spectroscopy.

Metallothionein: An Aggressive Scavenger-The Metabolism of Rhodium(II) Tetraacetate (Rh2(CH3CO2)4)

Anthropogenic sources of xenobiotic metals with no physiological benefit are increasingly prevalent in the environment. The platinum group metals (Pd, Pt, Rh, Ru, Os, and Ir) are found in marine and plant species near urban sources, and are known to bioaccumulate, introducing these metals into the human food chain. Many of these metals are also being used in innovative cancer therapy, which leads to a direct source of exposure for humans. This paper aims to further our understanding of nontraditional metal metabolism via metallothionein, a protein involved in physiologically important metal homeostasis. The aggressive reaction of metallothionein and dirhodium(II) tetraacetate, a common synthetic catalyst known for its cytotoxicity, was studied in detail in vitro. Optical spectroscopic and equilibrium and time-dependent mass spectral data were used to define binding constants for this robust reaction, and molecular dynamics calculations were conducted to explain the observed results.

Unravelling the mechanistic details of metal binding to mammalian metallothioneins from stoichiometric, kinetic, and binding affinity data

Metallothioneins (MTs) are small, cysteine-rich proteins, found throughout Nature. Their ability to bind a number of different metals with a range of stoichiometric ratios means that this protein family is critically important for essential metal (Zn²⁺ and Cu⁺) homeostasis, metal storage, metal donation to nascent metalloenzymes as well as heavy metal detoxification. With its 20 cysteines, metallothionein is also considered to protect cells against oxidative stress. MT has been studied by a large number of researchers over the last 6 decades using a variety of spectroscopic techniques. The lack of distinguishing chromophores for the multitude of binding sites has made the evaluation of stoichiometric properties for different metals challenging. Initially, only ¹¹³Cd-NMR spectroscopy could provide strong evidence for the proposed cluster formation of Cd-MT. The extraordinary development of electrospray ionization mass spectrometry (ESI-MS), where all coexisting species in solution are observed, revolutionized MT research. Prior to the use of ESI-MS data, a range of "magic numbers" representing metal-to-MT molar ratios were reported from optical spectroscopic studies. The availability of ESI mass spectral data led to (i) the confirmation of cluster formation, (ii) a conceptual understanding of the cooperativity involved in multiple metal binding events, (iii) the presence of domain specificity between regions of the protein and (iv) mechanistic details involving both binding affinities and rate constants. The kinetic experiments identified the presence of multiple individual binding sites, each with a unique rate constant and an analogous binding affinity. The almost linear trend in rate constants as a function of bound As³⁺ provided a unique insight that became a critical step in the complete understanding of the mechanistic details of the metalation of MT. To fully define the biological function of this sulfur-rich protein it is necessary to determine kinetic rate constants and binding affinities for the essential metals. Recently, Zn²⁺ competition experiments between both of the isolated fragments (α and β) and the full-length protein (βα-MT 1a) as well as Zn²⁺ competition between βα-MT 1a and carbonic anhydrase were reported. From these data, the trend in binding affinities and the values of the K_f of the 7 bimolecular reactions involved in metalation were determined. From the analysis of ESI-MS data for Cu⁺ binding to βα-MT 1a at different pH-values, a trend in the 20 binding affinities for the complete metalation mechanism was reported. This review details a personal view of the historical development of the determination of stoichiometry for metal binding, the structure of the binding sites, the rates of the metalation reactions and the underlying binding affinities for each metalation step. We have attempted to summarize the experimental developments that led to the publication in May 2017 of the experimental determination of the 20 binding constants for the 20 sequential bimolecular reactions for Cu⁺ binding to the 20 Cys of apoMT as a function of pH that show the appearance and disappearance of clusters. We report both published data and in a series of tables an assembly of stoichiometries, and equilibrium constants for Zn²⁺ and Cu⁺ for many different metallothioneins.

Glutathione binding to dirhodium tetraacetate: a spectroscopic, mass spectral and computational study of an anti-tumour compound

Glutathione (γ-l-glutamyl-l-cysteinyl-glycine) is a ubiquitous tripeptide found in all plants and animals. Glutathione has key roles as a metallochaperone and as a cellular thiol involved in metabolism. Little is known about how glutathione interacts with organometallic compounds in vivo. Here, we report the reactions of glutathione in vitro with dirhodium(ii) tetraacetate (tetrakis(μ-acetato)dirhodium(ii), Rh₂(OAc)₄), a compound with anti-tumour properties. Electrospray ionization mass spectrometry, UV-Visible absorption and circular dichroism spectroscopic methods were used to determine the stoichiometries and optical properties of the final conjugate. Computational analyses were used to predict the binding modes of glutathione to the Rh₂(OAc)₄, and report on the orbital assignments for the resulting products. We explored the competition by GSH for methionine-bound axial sites on Rh₂(OAc)₄ to investigate the use of weak thioether to protect its cellular-based anti-cancer activity. Our study highlights the important role that axial ligation would play in deactivating or significantly decreasing the efficacy of this bimetallic anti-tumor drug. The computational data explain the stability of the mono-adduct and the appearance of new absorption bands in the UV region including retention of the Rh-Rh single bond. Additionally, these data show that glutathione can effectively disable the potency of these metallo-drugs through orbital overlap of the entire Rh-Rh core as a result of the strong binding. Electronic absorption spectroscopy, mass spectrometry and computational analysis are a powerful combination in understanding possible chemical reactions in vivo and this information can be used to synthetically tune dirhodium complexes for use in the fight against cancer.

Capturing platinum in cisplatin: kinetic reactions with recombinant human apo-metallothionein 1a

Detailed mass spectra data show the stepwise sequence of cisplatin deconstruction by apo-metallothionein.

Exploring function activated chlorins using MCD spectroscopy and DFT methods: design of a chlorin with a remarkably intense, red Q band

Fictive β-substituted fluorene-thiophene Zn chlorins exhibit exceptionally intense, red Q bands with unusually small ΔHOMO.

Formation of oxidative and non-oxidative dimers in metallothioneins: Implications for charge-state analysis for structural determination

RATIONALE

Metallothioneins (MTs) are a class of dynamic proteins that have been investigated extensively using mass spectrometric methods due to their amenability to ionization. Here we detect the formation of oxidative and non-oxidative MT dimers using high-resolution mass spectrometry (HRMS) which has previously been overlooked with lower-resolution techniques.

METHODS

Recombinant human MT1a and its isolated domain fragments were analyzed by high-resolution Thermo Q-Exactive and Bruker time-of-flight (TOF) mass spectrometers. Covalent Cys modification was performed using N-ethylmalemide to probe the effect of Cys oxidation on dimer formation.

RESULTS

Dimerization was detected in the analysis of select charge states of Zn₇ MT and apo-βMT. Specifically, high resolution (140 k) revealed the +6 dimer peaks overlapping with the +3 charge state, but not with the other charge states (+4, +5, +6). The proteins with covalently modified Cys did not show dimer formation in any of their charge states. Apo-α and apo-βαMT also did not form dimers under the conditions tested.

CONCLUSIONS

Dimerization of MT was detected for zinc metalated and certain apo-MT forms with HRMS, which was not seen with lower-resolution techniques. These dimers appear overlapped only with certain charge states, confounding their analysis for structural characterization of MTs. The Zn-MT dimers appeared to be non-oxidative; however, the formation of dimers in the apo-protein is likely dependent on Cys oxidation.

Zinc binds non-cooperatively to human liver metallothionein 2a at physiological pH

Maintenance of the homeostasis of zinc is very important in regulating bodily functions. There are over 300 Zn-dependent enzymes identified where Zn(II) plays a structural or catalytic role. However, an excess of Zn(II) in a cell is toxic and free Zn(II) is tightly controlled. Metallothioneins (MTs) are small cysteine rich proteins that can bind up to seven Zn(II) and act as a Zn(II) reservoir. The MT2a isoform is predominantly found in the liver. This study focused on designing an MT2a construct of recombinant human MT2a to determine the Zn(II) binding profile of MT2a in vitro. We analyzed the pH dependence of Zn-MT2a speciation from electrospray ionization mass spectral data. At physiological pH, Zn(II) is terminally bound to the cysteine thiols of MT2a, making bead-like structures (non-cooperative metal binding), while at low pH, Zn(II) formed Zn₄S₁₁-MT2a clusters involving bridged cysteinyl thiols to the Zn(II) (cooperative metal binding). The Zn(II) binding profile of MT2a was compared to Zn(II) binding profile of human kidney MT1a, which was reported in literature, and found that the Zn(II) binding profile of MT2a is similar to that of MT1a. The facility of forming bead-like structures at physiological pH for Zn₅-MT2a means that Zn₇-MT2a can donate up to two Zn(II) to Zn-dependent enzymes.

Stabilization of protein structure through π-π interaction in the second coordination sphere of pseudoazurin

Noncovalent, weak interactions in the second coordination sphere of the copper active site of Pseudoazurin (PAz) from Achromobacter cycloclastes were examined using a series of Met16X variants. In this study, the differences in protein stability due to the changes in the nature of the 16th amino acid (Met, Phe, Val, Ile) were investigated by electrospray ionization mass spectrometry (ESI-MS) and far-UV circular dichroism (CD) as a result of acid denaturation. The percentage of native states (folded holo forms) of Met16Phe variants was estimated to be 75% at pH 2.9 although the wild-type (WT), Met16Val and Met16Ile PAz, became completely unfolded. The high stability under acidic conditions is correlated with the result of the active site being stabilized by the aromatic substitution of the Met16 residue. The π-π interaction in the second coordination sphere makes a significant contribution to the stability of active site and the protein matrix.

Residue Modification and Mass Spectrometry for the Investigation of Structural and Metalation Properties of Metallothionein and Cysteine-Rich Proteins

Structural information regarding metallothioneins (MTs) has been hard to come by due to its highly dynamic nature in the absence of metal-thiolate cluster formation and crystallization difficulties. Thus, typical spectroscopic methods for structural determination are limited in their usefulness when applied to MTs. Mass spectrometric methods have revolutionized our understanding of protein dynamics, structure, and folding. Recently, advances have been made in residue modification mass spectrometry in order to probe the hard-to-characterize structure of apo- and partially metalated MTs. By using different cysteine specific alkylation reagents, time dependent electrospray ionization mass spectrometry (ESI-MS), and step-wise “snapshot” ESI-MS, we are beginning to understand the dynamics of the conformers of apo-MT and related species. In this review we highlight recent papers that use these and similar techniques for structure elucidation and attempt to explain in a concise manner the data interpretations of these complex methods. We expect increasing resolution in our picture of the structural conformations of metal-free MTs as these techniques are more widely adopted and combined with other promising tools for structural elucidation.

Lead(II) Binding in Metallothioneins

Heavy metal exposure has long been associated with metallothionein (MT) regulation and its functions. MT is a ubiquitous, cysteine-rich protein that is involved in homeostatic metal response for the essential metals zinc and copper, as well as detoxification of heavy metals; the most commonly proposed being cadmium. MT binds in vivo to a number of metals in addition to zinc, cadmium and copper, such as bismuth. In vitro, metallation with a wide range of metals (especially mercury, arsenic, and lead) has been reported using a variety of analytical methods. To fully understand MT and its role with lead metabolism, we will describe how MT interacts with a wide variety of metals that bind in vitro. In general, affinity to the metal-binding cysteine residues of MT follows that of metal binding to thiols: Zn(II) < Pb(II) < Cd (II) < Cu(I) < Ag(I) < Hg(II) < Bi(III). To introduce the metal binding properties that we feel directly relate to the metallation of metallothionein by Pb(II), we will explore MT's interactions with metals long known as toxic, particularly, Cd(II), Hg(II), and As(III), along with xenobiotic metals, and how these metal-binding studies complement those of lead binding. Lead's effects on an organism's physiological functions are not fully understood, but it is known that chronic exposure inflicts amongst other factors pernicious anemia and developmental issues in the brain, especially in children who are more vulnerable to its toxic effects. Understanding the interaction of lead with metallothioneins throughout the biosphere, from bacteria, to algae, to fish, to humans, is important in determining pathways for lead to enter and damage physiologically significant protein function, and thereby its toxicity.

A Simple Metallothionein-Based Biosensor for Enhanced Detection of Arsenic and Mercury

Metallothioneins (MTs) are a family of cysteine-rich proteins whose biological roles include the regulation of essential metal ions and protection against the harmful effects of toxic metals. Due to its high affinity for many toxic, soft metals, recombinant human MT isoform 1a was incorporated into an electrochemical-based biosensor for the detection of As³⁺ and Hg²⁺. A simple design was chosen to maximize its potential in environmental monitoring and MT was physically adsorbed onto paper discs placed on screen-printed carbon electrodes (SPCEs). This system was tested with concentrations of arsenic and mercury typical of contaminated water sources ranging from 5 to 1000 ppb. The analytical performance of the MT-adsorbed paper discs on SPCEs demonstrated a greater than three-fold signal enhancement and a lower detection limit compared to blank SPCEs, 13 ppb for As³⁺ and 45 ppb for Hg²⁺. While not being as low as some of the recommended drinking water limits, the sensitivity of the simple MT-biosensor would be potentially useful in monitoring of areas of concern with a known contamination problem. This paper describes the ability of the metal binding protein metallothionein to enhance the effectiveness of a simple, low-cost electrochemical sensor.

Selective cysteine modification of metal-free human metallothionein 1a and its isolated domain fragments: Solution structural properties revealed via ESI-MS

Human metallothionein 1a, a protein with two cysteine-rich metal-binding domains (α with 11 Cys and β with 9), was analyzed in its metal-free form by selective, covalent Cys modification coupled with ESI-MS. The modification profiles of the isolated β- and α-fragments reacted with p-benzoquinone (Bq), N-ethylmalemide (NEM) and iodoacetamide (IAM) were compared with the full length protein using ESI-mass spectral data to follow the reaction pathway. Under denaturing conditions at low pH, the reaction profile with each modifier followed pathways that resulted in stochastic, Normal distributions of species whose maxima was equal to the mol. eq. of modifier added. Our interpretation of modification at this pH is that reaction with the cysteines is unimpeded when the full protein or those of its isolated domains are denatured. At neutral pH, where the protein is expected to be folded in a more compact structure, there is a difference in the larger Bq and NEM modification, whose reaction profiles indicate a cooperative pattern. The reaction profile with IAM under native conditions follows a similar stochastic distribution as at low pH, suggesting that this modifier is small enough to access the cysteines unimpeded by the compact structure. The data emphasize the utility of residue modification coupled with electrospray ionization mass spectrometry for the study of protein structure.

The spectroscopic impact of interactions with the four Gouterman orbitals from peripheral decoration of porphyrins with simple electron withdrawing and donating groups

Porphyrin β-substitution with strong electron withdrawing groups splits the LUMO, red-shifts the Q band, and introduces a dipole moment.

Defining the metal binding pathways of human metallothionein 1a: balancing zinc availability and cadmium seclusion

Metallothioneins (MTs) are cysteine-rich, metal-binding proteins that are found throughout Nature. This ubiquity highlights their importance in essential metal regulation, heavy metal detoxification and cellular redox chemistry. Missing from the current description of MT function is the underlying mechanism by which MTs achieve their proposed biological functions. To date, there have been conflicting reports on the mechanism of metal binding and the structures of the metal binding intermediates formed during metalation of apoMTs. The form of the metal-bound intermediates dictates the metal sequestering and metal-donating properties of the protein. Through a detailed analysis of spectral data from electrospray ionization mass spectromeric and circular dichroism methods we report that Zn(ii) and Cd(ii) metalation of the human MT1a takes place through two distinct pathways. The first pathway involves formation of beaded structures with up to five metals bound terminally to the 20 cysteines of the protein via a noncooperative mechanism. The second pathway is dominated by the formation of the four-metal domain cluster structure M4SCYS11via a cooperative mechanism. We report that there are different pathway preferences for Zn(ii) and Cd(ii) metalation of apo-hMT1a. Cd(ii) binding follows the beaded pathway above pH 7.1 but beginning below pH 7.1 the clustered (Cd4Scys11) pathway begins to dominate. In contrast, Zn(ii) binding follows the terminal, "beaded", pathway at all physiologically relevant pH (pH ≥ 5.2) only following the clustered pathway below pH 5.1. The results presented here allow us to reconcile the conflicting reports concerning the presence of different metalation intermediates of MTs. The conflict regarding cooperative versus noncooperative binding mechanisms is also reconciled with the experimental results described here. These two metal-specific pathways and the presence of radically different intermediate structures provide insight into the multi-functional nature of MT: binding Zn(ii) terminally for donation to metalloenzymes and sequestering toxic Cd(ii) in a cluster structure.