sufS

SufS is a cysteine desulfurase enzyme that participates in the SUF (sulfur mobilization) system, a pathway used to assemble iron-sulfur (Fe-S) clusters in certain bacteria and plant chloroplasts. In many bacteria, including Escherichia coli, the sufS gene is part of the sufABCDSE operon, which encodes the core components of the SUF machinery.

SufS is a pyridoxal phosphate (PLP)-dependent enzyme that catalyzes the removal of sulfur from cysteine to form alanine and a persulfide on a catalytic cysteine residue within SufS. The persulfide sulfur is transferred to SufE, a small sulfurtransferase with its own reactive cysteine. SufE then delivers the sulfur to the SufBCD complex, which serves as the Fe-S cluster assembly scaffold. The assembled Fe-S cluster is subsequently transferred to recipient apoproteins, often via the carrier protein SufA.

Biological significance and regulation: the SUF system is particularly important under conditions that challenge Fe-S cluster biosynthesis, such as oxidative stress or iron limitation, when the ISC pathway is less effective. Expression of sufS and other SUF genes is upregulated in response to cellular stress and changes in Fe-S cluster status, ensuring continued Fe-S protein maturation when standard pathways are stressed.

Structure and interactions: SufS forms a functional dimer and relies on PLP as a cofactor. Its activity depends on a catalytic cysteine that forms a persulfide; it interacts closely with SufE to relay sulfur to the Fe-S cluster assembly machinery.

Distribution and significance: SufS is conserved in many bacteria and in the chloroplasts of plants, reflecting its essential role in maintaining Fe-S protein function under adverse conditions. It is a key component of the alternative Fe-S cluster biogenesis pathway that complements the ISC system.