Biomineralization is an intriguing route towards the low temperature, aqueous phase, green synthesis of inorganic functional nanomaterials. Herein we describe the biomineralization and optical properties of Zn_xCd_1−xS and Zn_xCd_1−xS–ZnS quantum confined nanocrystals that have potential application in optoelectronics. The reported biomineralization process is reduced to perhaps its simplest form wherein a single recombinant cystathionine γ-lyase (CSE) enzyme is responsible for catalyzing mineralization within an otherwise inert solution and plays a role in controlling the alloy composition. The biomineralized nanocrystals are sphalerite structured with average diameter below 3 nm. Biomineralization of a passivating ZnS shell on Zn_0.73Cd_0.27S core nanocrystals is achieved through subsequent addition of Zn precursor. This shell growth increases the photoluminescence quantum yield to 7% and increases the radiative decay time to 97.6 ns compared with 40.9 ns for the core materials.