An enzyme that does it all

Proc. Natl Acad. Sci. USA 113, 5275–5280 (2016)

Biological systems use a variety of ways to synthesize inorganic materials from aqueous solution. Control over material composition and structure is typically achieved through a complex process involving the assembly of different proteins or other biomolecules. Now, Bryan Berger and colleagues at Lehigh University in the US show that biomineralization is possible using a single enzyme, offering a green route to manufacture functional nanomaterials.

The researchers combined a purified form of the cystathionine γ-lyase enzyme with cadmium acetate and L-cysteine. The enzyme, which is obtained from the aerobic bacteria Stenotrophomonas maltophilia, is known to catalyse the production of H₂S from L-cysteine to form CdS in cell cultures. Using this combination, the researchers obtained monodisperse and crystalline CdS nanocrystals (2–4 nm in diameter) with optoelectronic properties similar to those synthesized in cells or by chemical methods. When L-cysteine was substituted with glutathione — which is not a substrate for the enzyme and is a capping agent known to stabilize water-soluble CdS nanocrystals — no nanocrystals formed, confirming that L-cysteine is the sulfur source. Replacing L-cysteine with Na₂S as the sulfur source still led to nanocrystal formation, suggesting that the enzyme can regulate nanocrystal growth independent of H₂S production. These results show that this single enzyme is capable of both catalysing the reactive precursors for mineralization and templating the subsequent nanocrystal growth.