The reactivity of pyridoxal-5'-phosphate (PLP) with cysteine and its derivatives has been of increasing interest because the corresponding product, a thiazolidine PLP-cysteine adduct, can be formed via PLP-dependent enzymatic and non-enzymatic reactions. Here, we report biochemical and X-ray crystallographic snapshots of thiazolidine formation in reaction of D-cysteine with PLP in SufS, a PLP-dependent L-cysteine desulfurase. By comparing L- and D-penicillamine-bound SufS showing no thiazolidine formation in the crystals with D-cysteine SufS, we proposed a thiazolidine formation mechanism with important factors: the polar environments provided by the carbonyl groups of Ala28-Ala29 and Lys224-mediated base catalysis for the nucleophilic thiolate of D-cysteine.