Science 358, 373–377 (2017)

Lipoic acid is a redox-active cofactor in various multienzyme complexes. During the biosynthesis of lipoic acid, lipoyl synthase (LipA) catalyzes the introduction of two sulfur atoms on the aliphatic chain by sacrificing one of its own two [4Fe–4S] clusters. Without a means to replace the damaged cluster, LipA catalyzes only a single turnover in vitro, and the mechanism that restores the cluster in vivo is not understood. Now, McCarthy and Booker have identified the Escherichia coli iron–sulfur cluster carrier protein NfuA to be capable of reconstituting this auxiliary cluster of LipA and restoring its catalytic activity. LipA and NfuA form a complex in vitro, enabling multiple turnovers by LipA. Isotopic tracing with 34S also indicated that NfuA directly transfers a new intact [4Fe–4S] cluster to LipA, rather than repairing the damaged one, and that LipA can donate not only two, but all four, of the sulfur atoms from its auxiliary cluster to the lipoic acid product. In E. coli, the absence of NfuA can be compensated by a secondary iron–sulfur supply pathway involving IscU, and IscU can likewise restore LipA activity in vitro. However, mammals lacking the homolog of NfuA exhibit lipoyl cofactor deficiency, which may now be explained by this protein's role in reconstituting LipA activity.