Fig. 5: Structural orientation of d-Cys-ketimine precludes sulfur transfer.
From: d-cysteine impairs tumour growth by inhibiting cysteine desulfurase NFS1
a, Crystal structure of (NIAU)2 after incubation with l-propargylglycine (PG; PDB 8TVT). The blow-up shows the NFS1 active centre harbouring an external PG-ketimine. This cofactor-substrate entity is no longer covalently bound to Lys258NFS1, and mimics the Cys-ketimine intermediate one step before actual persulfidation (Extended Data Fig. 7a). b, Models of bound l-Cys-ketimine (top) and d-Cys-ketimine (bottom) based on the crystal structure solved after addition of PG and validated using two bacterial SufS structures (PDB 7XEJ and PDB 7XEP). For l-Cys-ketimine both catalytic His156NFS1 and sulfur-accepting Cys381NFS1 are in close proximity to the l-Cys sulfur, while for d-Cys-ketimine these distances are much larger and the d-Cys sulfur is too far away for efficient persulfidation. For inspection of the model in 3D, a Chimera X file is available in Supplementary File 1. c, The (NIAUX)2 complex is not able to produce free sulfide from d-Cys using the DTT-dependent desulfurase activity assay. The rate of sulfide production was determined in n = 3 biological replicates, presented as the mean ± s.d., and compared by a paired two-tailed Student’s t-test, **P = 0.0039. d, A simplified mechanistic scheme for the reactions that l-Cys and d-Cys undergo with the NFS1-bound PLP (Extended Data Fig. 7a). When l-Cys or d-Cys are added to the enzyme, harbouring in its ground state the Lys258NFS1-bound PLP as an internal aldimine (Schiff base), an external (that is, non-enzyme-bound) Cys-ketimine is formed. Proton abstraction by His156NFS1 facilitates persulfidation of Cys381NFS1 for l-Cys but not d-Cys.
