On the abundance of zinc in the evolutionarily old protein domains.

A recent paper in PNAS (1) analyzed the evolution of metalloproteins and confirmed our earlier observation on the abundance of Zn-binding proteins among the most widespread – and evolutionarily oldest – protein folds (2). As that observation appeared to contradict the extremely low (<10 M) estimates of Zn levels in the primeval anoxic ocean (3), we had suggested that the first cells developed in continental Zn-rich environments (2, 4). Specifically, we proposed that hydrothermal precipitates of zinc sulfide (ZnS), similar to those seen about modern deep-sea vents, catalyzed abiogenic photosynthesis of organic compounds under the solar light (2, 4). This led to the release of Zn ions, yielding a Zn-rich milieu. In the course of evolution, cells have maintained the initial metal homeostasis and preserved high levels of Zn in the cytoplasm (2).