HydG, the "dangler" iron, and catalytic production of free CO and CN-: implications for [FeFe]-hydrogenase maturation.

The organometallic H-cluster of the [FeFe]-hydrogenase consists of a [4Fe-4S] cubane bridged via a cysteinyl thiolate to a 2Fe subcluster ([2Fe]H) containing CO, CN-, and dithiomethylamine (DTMA) ligands. The H-cluster is synthesized by three dedicated maturation proteins: the radical SAM enzymes HydE and HydG synthesize the non-protein ligands, while the GTPase HydF serves as a scaffold for assembly of [2Fe]H prior to its delivery to the [FeFe]-hydrogenase containing the [4Fe-4S] cubane. HydG uses l-tyrosine as a substrate, cleaving it to produce p-cresol as well as the CO and CN- ligands to the H-cluster, although there is some question as to whether these are formed as free diatomics or as part of a [Fe(CO)2(CN)] synthon. Here we show that Clostridium acetobutylicum (C.a.) HydG catalyzes formation of multiple equivalents of free CO at rates comparable to those for CN- formation. Free CN- is also formed in excess molar equivalents over protein. A g = 8.9 EPR signal is observed for C.a. HydG reconstituted to load the 5th "dangler" iron of the auxiliary [4Fe-4S][FeCys] cluster and is assigned to this "dangler-loaded" cluster state. Free CO and CN- formation and the degree of activation of [FeFe]-hydrogenase all occur regardless of dangler loading, but are increased 10-35% in the dangler-loaded HydG; this indicates the dangler iron is not essential to this process but may affect relevant catalysis. During HydG turnover in the presence of myoglobin, the g = 8.9 signal remains unchanged, indicating that a [Fe(CO)2(CN)(Cys)] synthon is not formed at the dangler iron. Mutation of the only protein ligand to the dangler iron, H272, to alanine nearly completely abolishes both free CO formation and hydrogenase activation, however results show this is not due solely to the loss of the dangler iron. In experiments with wild type and H272A HydG, and with different degrees of dangler loading, we observe a consistent correlation between free CO/CN- formation and hydrogenase activation. Taken in full, our results point to free CO/CN-, but not an [Fe(CO)2(CN)(Cys)] synthon, as essential species in hydrogenase maturation.

Radical S-adenosylmethionine maquette chemistry: Cx3Cx2C peptide coordinated redox active [4Fe-4S] clusters.

The synthesis and characterization of short peptide-based maquettes of metalloprotein active sites facilitate an inquiry into their structure/function relationships and evolution. The [4Fe-4S]-maquettes of bacterial ferredoxin metalloproteins (Fd) have been used in the past to engineer redox active centers into artificial metalloenzymes. The novelty of our study is the application of maquettes to the superfamily of [4Fe-4S] cluster and S-adenosylmethionine-dependent radical metalloenzymes (radical SAM). The radical SAM superfamily enzymes contain site-differentiated, redox active [4Fe-4S] clusters coordinated to Cx3Cx2C or related motifs, which is in contrast to the Cx2Cx2C motif found in bacterial ferredoxins (Fd). Under an optimized set of experimental conditions, a high degree of reconstitution (80-100%) was achieved for both radical SAM- and Fd-maquettes. Negligible chemical speciation was observed for all sequences, with predominantly [4Fe-4S]2+ for the 'as-reconstituted' state. However, the reduction of [4Fe-4S]2+-maquettes provides low conversion (7-17%) to the paramagnetic [4Fe-4S]+ state, independent of either the spacing of the cysteine residues (Cx3Cx2C vs. Cx2Cx2C), the nature of intervening amino acids, or the length of the cluster binding motif. In the absence of the stabilizing protein environment, the reduction process is proposed to proceed via [4Fe-4S]2+ cluster disassembly and reassembly in a more reduced state. UV-Vis and EPR spectroscopic techniques are employed as analytical tools to quantitate the as-reconstituted (or oxidized) and one-electron reduced states of the [4Fe-4S] clusters, respectively. We demonstrate that short Fd and radical SAM derived 7- to 9-mer peptides containing appropriate cysteine motifs function equally well in coordinating redox active [4Fe-4S] clusters.

Paradigm Shift for Radical S-Adenosyl-l-methionine Reactions: The Organometallic Intermediate Ω Is Central to Catalysis.

Radical S-adenosyl-l-methionine (SAM) enzymes comprise a vast superfamily catalyzing diverse reactions essential to all life through homolytic SAM cleavage to liberate the highly reactive 5'-deoxyadenosyl radical (5'-dAdo·). Our recent observation of a catalytically competent organometallic intermediate Ω that forms during reaction of the radical SAM (RS) enzyme pyruvate formate-lyase activating-enzyme (PFL-AE) was therefore quite surprising, and led to the question of its broad relevance in the superfamily. We now show that Ω in PFL-AE forms as an intermediate under a variety of mixing order conditions, suggesting it is central to catalysis in this enzyme. We further demonstrate that Ω forms in a suite of RS enzymes chosen to span the totality of superfamily reaction types, implicating Ω as essential in catalysis across the RS superfamily. Finally, EPR and electron nuclear double resonance spectroscopy establish that Ω involves an Fe-C5' bond between 5'-dAdo· and the [4Fe-4S] cluster. An analogous organometallic bond is found in the well-known adenosylcobalamin (coenzyme B12) cofactor used to initiate radical reactions via a 5'-dAdo· intermediate. Liberation of a reactive 5'-dAdo· intermediate via homolytic metal-carbon bond cleavage thus appears to be similar for Ω and coenzyme B12. However, coenzyme B12 is involved in enzymes catalyzing only a small number (∼12) of distinct reactions, whereas the RS superfamily has more than 100 000 distinct sequences and over 80 reaction types characterized to date. The appearance of Ω across the RS superfamily therefore dramatically enlarges the sphere of bio-organometallic chemistry in Nature.