Arylsulfatases require a maturating enzyme to perform a co-or posttranslational modification to form a catalytically essential formylglycine (FGly) residue. In organisms that live aerobically, molecular oxygen is used enzymatically to oxidize cysteine to FGly. Under anaerobic conditions, S-adenosylmethionine (AdoMet) radical chemistry is used. Here we present the structures of an anaerobic sulfatase maturating enzyme (anSME), both with and without peptidyl-substrates, at 1.6-1.8 Å resolution. We find that anSMEs differ from their aerobic counterparts in using backbone-based hydrogen-bonding patterns to interact with their peptidylsubstrates, leading to decreased sequence specificity. These anSME structures from Clostridium perfringens are also the first of an AdoMet radical enzyme that performs dehydrogenase chemistry. Together with accompanying mutagenesis data, a mechanistic proposal is put forth for how AdoMet radical chemistry is coopted to perform a dehydrogenation reaction. In the oxidation of cysteine or serine to FGly by anSME, we identify D277 and an auxiliary [4Fe-4S] cluster as the likely acceptor of the final proton and electron, respectively. D277 and both auxiliary clusters are housed in a cysteinerich C-terminal domain, termed SPASM domain, that contains homology to ∼1,400 other unique AdoMet radical enzymes proposed to use [4Fe-4S] clusters to ligate peptidyl-substrates for subsequent modification. In contrast to this proposal, we find that neither auxiliary cluster in anSME bind substrate, and both are fully ligated by cysteine residues. Instead, our structural data suggest that the placement of these auxiliary clusters creates a conduit for electrons to travel from the buried substrate to the protein surface.iron-sulfur cluster fold | radical SAM dehydrogenase

The radical SAM (RS) enzymes RlmN and Cfr methylate 23S ribosomal RNA, modifying the C2 or C8 position of adenosine 2503. The methyl groups are installed by a two-step sequence involving initial methylation of a conserved Cys residue (RlmN Cys 355) by SAM. Methyl transfer to the substrate requires reductive cleavage of a second equivalent of SAM. Crystal structures of RlmN and RlmN with SAM show that a single molecule of SAM coordinates the [4Fe-4S] cluster. Residue Cys 355 is S-methylated and located proximal to the SAM methyl group, suggesting that SAM involved in the initial methyl transfer binds at the same site. Thus, RlmN accomplishes its complex reaction with structural economy, harnessing the two most important reactivities of SAM within a single site.

The secretion of peptides and proteins is essential for survival and ecological adaptation of bacteria. Dual-functional ATP-binding cassette transporters export antimicrobial or quorum signaling peptides in Gram-positive bacteria. Their substrates contain a leader sequence that is excised by an N-terminal peptidase C39 domain at a double Gly motif. We characterized the protease domain (LahT150) of a transporter from a lanthipeptide biosynthetic operon in Lachnospiraceae and demonstrate that this protease can remove the leader peptide from a diverse set of peptides. The 2.0 Å resolution crystal structure of the protease domain in complex with a covalently bound leader peptide demonstrates the basis for substrate recognition across the entire class of such transporters. The structural data also provide a model for understanding the role of leader peptide recognition in the translocation cycle, and the function of degenerate, non-functional C39-like domains (CLD) in substrate recruitment in toxin exporters in Gram-negative bacteria.

Lipoyl synthase (LipA) catalyzes the insertion of two sulfur atoms at the unactivated C6 and C8 positions of a protein-bound octanoyl chain to produce the lipoyl cofactor. To activate its substrate for sulfur insertion, LipA uses a [4Fe-4S] cluster and S-adenosylmethionine (AdoMet) radical chemistry; the remainder of the reaction mechanism, especially the source of the sulfur, has been less clear. One controversial proposal involves the removal of sulfur from a second (auxiliary) [4Fe-4S] cluster on the enzyme, resulting in destruction of the cluster during each round of catalysis. Here, we present two high-resolution crystal structures of LipA from Mycobacterium tuberculosis: one in its resting state and one at an intermediate state during turnover. In the resting state, an auxiliary [4Fe-4S] cluster has an unusual serine ligation to one of the irons. After reaction with an octanoyllysine-containing 8-mer peptide substrate and 1 eq AdoMet, conditions that allow for the first sulfur insertion but not the second insertion, the serine ligand dissociates from the cluster, the iron ion is lost, and a sulfur atom that is still part of the cluster becomes covalently attached to C6 of the octanoyl substrate. This intermediate structure provides a clear picture of iron-sulfur cluster destruction in action, supporting the role of the auxiliary cluster as the sulfur source in the LipA reaction and describing a radical strategy for sulfur incorporation into completely unactivated substrates.iron-sulfur cluster | radical SAM enzyme | lipoic acid T he functionalization of aliphatic carbon centers is widely regarded as one of the most kinetically challenging reactions in nature, a striking example of which is found in the biosynthesis of the lipoyl cofactor, famous for its central role as the "swinging arm" of the pyruvate dehydrogenase enzyme complex. Lipoyl synthase (LipA) generates the lipoyl cofactor by insertion of two sulfur atoms at C6 and C8 of a protein-bound n-octanoyl chain, sites distal from the nearest functionality (1-4). LipA and the closely related biotin synthase (BioB) (Scheme 1) are founding members of the ever-expanding S-adenosyl-L-methionine (AdoMet) radical enzyme superfamily that uses a [4Fe-4S] cluster to reductively cleave the C5′-S bond of AdoMet, generating methionine and a 5′-deoxyadenosyl radical (5′-dA•), a powerful oxidant (5, 6). LipA requires 2 eq AdoMet-one per sulfur insertion-and two sulfur atoms to produce 1 eq lipoyl product through radical-based chemistry (4). One of the most controversial aspects of the LipA and BioB reactions is the source of the sulfur. AdoMet radical enzymes that catalyze sulfur insertion always seem to have an additional iron-sulfur (Fe/S) cluster: a [2Fe-2S] cluster in BioB (7), a [4Fe-4S] cluster in LipA (8), and a [4Fe-4S] cluster in the methylthiotransferases RimO and MiaB (9, 10). These auxiliary clusters in LipA and BioB have been proposed to be cannibalized during turnover to supply the inserted sulfur atom(s) (8, 11-13), a proposal that has not enjoyed univ...