Defining the metal binding pathways of human metallothionein 1a: balancing zinc availability and cadmium seclusion
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".