Arsenic binding to human metallothionein-3

Area of Science:

• Biochemistry
• Toxicology
• Neuroscience

Background:

• Arsenic poisoning poses a significant neurological risk, particularly to children, often due to contaminated drinking water.
• Metallothioneins (MTs) are proteins involved in cellular responses to toxic metals.
• Metallothionein isoform 3 (MT3) is expressed in the central nervous system and is believed to offer neuroprotection.

Purpose of the Study:

• To investigate the metalation pathways of trivalent arsenic (As³⁺) binding to apo-metallothionein 3 (apo-MT3) under physiological conditions.
• To determine the binding constants and reaction rates for sequential As³⁺ binding events to MT3.
• To characterize the As³⁺ binding pathway to both apo-MT3 and zinc-bound MT3 (Zn-MT3).

Main Methods:

• Spectroscopic analysis to determine absolute binding constants (log K) for sequential As³⁺ binding to apo-MT3.
• Kinetic studies at pH 3.5 to determine rate constants (k) for each As³⁺ binding step.
• Characterization of As³⁺ interaction with fully metalated (Zn₇MT3) and partially metalated (Zn-MT3) forms.

Main Results:

• High absolute binding constants (log K: 10.20 down to 8.31 M⁻¹) were determined for sequential As³⁺ binding to apo-MT3.
• Rate constants (k: 116.9 down to 21.0 M⁻¹ s⁻¹) indicate rapid As³⁺ reaction with apo-MT3 at pH 3.5.
• As³⁺ binds rapidly and non-cooperatively to apo-MT3 and partially metalated Zn-MT3 but does not displace zinc from fully metalated Zn₇MT3.

Conclusions:

• Trivalent arsenic (As³⁺) exhibits rapid and efficient binding to apo-MT3 and partially metalated Zn-MT3 under physiological conditions.
• The binding of As³⁺ to MT3 occurs in a non-cooperative manner.
• MT3's inability to displace zinc from Zn₇MT3 suggests a protective mechanism against arsenic toxicity in the central nervous system.