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 K, 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 (k, 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.