Because the Vmax (kcat) of SeR is improved for SeO3 2-relative to GRLMR, we suggest that local concentrations of Se 0 enzymatic product are higher for SeR than for GRLMR. This makes the solutions produced by SeR locally more 'saturated', shrinking the critical nucleus size of the resulting SeNPs. The smaller SeNPs observed for SeR enzymatic products can therefore be rationalized by the higher SeR kcat resulting in a smaller critical nucleus size. Increased number of nuclei leads to increased competition for growth, leading to a larger number of smaller nanoparticles.