The diverse functional repertoire of proteins promises to yield new materials with unprecedented capabilities, so long as versatile chemical methods are available to integrate biomolecules with synthetic components. As a demonstration of this potential, we have used site-selective strategies to cross-link polymer chains using the N- and C-termini of a metallothionein derived from a pea plant. This arrangement directly relates the swelling volume of the polymer to the folded state of the protein. The material retains the protein's ability to remove heavy metal ions from contaminated water samples, and can be regenerated through the subsequent addition of inexpensive chelators. The change in hydrogel volume that occurs as metal ions are bound allows the detection of contaminants through simple visual inspection. The utility of this bulk property change is demonstrated in the construction of a low-cost device that can report heavy metal contamination with no external power requirements. Most importantly, the generality of the protein modification chemistry allows the immediate generation of new hybrid materials from a wide range of protein sequences.