Mercury(II) binds to both of chymotrypsin’s histidines, causing inhibition followed by irreversible denaturation/aggregation Original paper

What was studied?

This laboratory study tested whether mercury(II) can harm proteins by binding to histidine residues, not only to cysteine thiols. The researchers used chymotrypsin as a model enzyme because it has no free cysteine thiols (its cysteines are locked in disulfide bonds), so mercury cannot act through the usual cysteine-based pathway. They measured two outcomes side by side: loss of enzyme activity and visible aggregation/precipitation, then used small-molecule competitors and targeted histidine modification to pinpoint which amino-acid sites drove each effect.

Who was studied?

The study did not involve patients or living participants. It studied purified chymotrypsin in solution and exposed it to defined concentrations of HgCl2 under controlled pH and buffer conditions. The team also used mechanistic probes, including free imidazole and acetate as competing ligands and diethylpyrocarbonate (DEPC) to block histidine side chains, so they could connect mercury binding to specific residues and distinct outcomes: inhibition versus irreversible denaturation and aggregation.

What were the most important findings?

Mercury(II) inhibited chymotrypsin activity at low concentrations even when most enzyme remained soluble, then drove denaturation and aggregation at higher concentrations. Enzyme kinetics showed that mercury lowered catalytic turnover (kcat) while leaving substrate affinity (Km) largely unchanged, which fits a noncompetitive inhibition pattern consistent with mercury binding to a catalytic residue rather than blocking substrate binding. Aggregation strongly increased above pH ~6.5, matching histidine deprotonation behavior, and free imidazole strongly prevented precipitation, far more than acetate, supporting a dominant mercury–histidine interaction. DEPC produced a biphasic effect: at lower levels it increased mercury-driven precipitation, but at higher levels it fully protected the enzyme, consistent with selective blocking of different histidines.

What are the greatest implications of this study/ review?

This work expands the clinical logic of mercury toxicity by showing that histidine binding can be sufficient to shut down enzymes and trigger protein unfolding and aggregation, even when cysteine thiols are unavailable. The authors propose a two-site model with distinct consequences: mercury binding to His57 primarily inhibits catalysis, while binding to His40 triggers irreversible denaturation and aggregation, a pattern that may help explain heavy-metal–driven protein aggregation more broadly. For microbiome-relevant exposure framing, the study supports focusing on inorganic Hg(II) chemistry after demethylation, because Hg(II) can damage proteins through histidine interactions that are not captured by cysteine-only toxicity models.