Sulfur mobilization represents one of the key steps in ubiquitous Fe-S clusters assembly and is performed by a recently characterized set of
proteins encompassing
cysteine desulfurases, assembly factors, and shuttle
proteins. Despite the evolutionary conservation of these
proteins, some degree of variability among organisms was observed, which might reflect functional specialization. L-
Cyst(e)ine
lyase (C-DES), a
pyridoxal 5'-phosphatedependent
enzyme identified in the cyanobacterium Synechocystis, was reported to use preferentially
cystine over
cysteine with production of
cysteine persulfide,
pyruvate, and
ammonia. In this study, we demonstrate that C-DES sequences are present in all cyanobacterial genomes and constitute a new family of
sulfur-mobilizing
enzymes, distinct from
cysteine desulfurases. The functional properties of C-DES from Synechocystis sp. PCC 6714 were investigated under pre-steady-state and steady-state conditions. Single wavelength and rapid scanning stopped-flow kinetic data indicate that the internal aldimine reacts with
cystine forming an external aldimine that rapidly decays to a transient quinonoid species and stable tautomers of the
alpha-aminoacrylate Schiff base. In the presence of
cysteine, the transient formation of a dipolar species precedes the selective and stable accumulation of the enolimine tautomer of the external aldimine, with no formation of the
alpha-aminoacrylate Schiff base under reducing conditions. Effective
sulfur mobilization from
cystine might represent a mechanism that allows adaptation of cyanobacteria to different environmental conditions and to light-dark cycles.