Structural characterization of HypX responsible for CO biosynthesis in the maturation of NiFe-hydrogenase

[Fe]-Hydrogenase, Cofactor Biosynthesis and Engineering

[Fe]-hydrogenase catalyzes the heterolytic cleavage of H2 and reversible hydride transfer to methenyl-tetrahydromethanopterin. The iron-guanylylpyridinol (FeGP) cofactor is the prosthetic group of this enzyme, in which mononuclear Fe(II) is ligated with a pyridinol and two CO ligands. The pyridinol ligand fixes the iron by an acyl carbon and a pyridinol nitrogen. Biosynthetic proteins for this cofactor are encoded in the hmd co-occurring (hcg) genes. The function of HcgB, HcgC, HcgD, HcgE, and HcgF was studied by using structure-to-function analysis, which is based on the crystal structure of the proteins and subsequent enzyme assays. Recently, we reported the catalytic properties of HcgA and HcgG, novel radical S-adenosyl methionine enzymes, by using an in vitro biosynthesis assay. Here, we review the properties of [Fe]-hydrogenase and the FeGP cofactor, and the biosynthesis of the FeGP cofactor. Finally, we discuss the expected engineering of [Fe]-hydrogenase and the FeGP cofactor.

Elucidating polymorphs of crystal structures by intensity-based hierarchical clustering analysis of multiple diffraction data sets

In macromolecular structure determination using X-ray diffraction from multiple crystals, the presence of different structures (structural polymorphs) necessitates the classification of the diffraction data for appropriate structural analysis. Hierarchical clustering analysis (HCA) is a promising technique that has so far been used to extract isomorphous data, mainly for single-structure determination. Although in principle the use of HCA can be extended to detect polymorphs, the absence of a reference to define the threshold used to group the isomorphous data sets (the `isomorphic threshold') poses a challenge. Here, unit-cell-based and intensity-based HCAs have been applied to data sets for apo trypsin and inhibitor-bound trypsin that were mixed post data acquisition to investigate the efficacy of HCA in classifying polymorphous data sets. Single-step intensity-based HCA successfully classified polymorphs with a certain `isomorphic threshold'. In data sets for several samples containing an unknown degree of structural heterogeneity, polymorphs could be identified by intensity-based HCA using the suggested `isomorphic threshold'. Polymorphs were also detected in single crystals using data collected using the continuous helical scheme. These findings are expected to facilitate the determination of multiple structural snapshots by exploiting automated data collection and analysis.

Enzymatic α-Ketothioester Decarbonylation Occurs in the Assembly Line of Barbamide for Skeleton Editing

The decarbonylation reaction has been developed significantly in organic chemistry as an effective approach to various synthetic applications, but enzymatic precedents for this reaction are rare. Based on investigations into the hybrid nonribosomal peptide synthetase (NRPS)-polyketide synthase (PKS) assembly line of barbamide, we report an on-line α-ketothioester decarbonylation reaction that leads to one-carbon truncation of the elongating skeleton. This enzymatic editing reaction occurs in the first round of lipopeptide extension and modification involving the multienzymes BarE and BarF, which successively house an NRPS module to initiate the biosynthesis and a PKS module to catalyze the first round of chain extension. Starting with processing a leucine-derived α-ketoacyl starter, the ketosynthase domain in BarE displays an unusual dual activity that results in net one-carbon chain elongation. It extrudes carbon monoxide from α-keto-isocaproyl thioester and then mediates decarboxylative condenses of the resultant isovaleryl thioester with malonyl thioester to form a diketide intermediate, followed by BarF-based O-methylation to stabilize the enol form of the β-carbonyl and afford an unusual E-double bond. Biochemical characterization, chemical synthesis, computational analysis, and the experimental outcome of site-directed mutagenesis illustrate the extraordinary catalytic capability of this ketosynthase domain. This work furthers the appreciation of assembly line chemistry and opens the door to new approaches for skeleton editing/engineering of related molecules using synthetic biology approaches.

In Vitro Biosynthesis of the [Fe]-Hydrogenase Cofactor Verifies the Proposed Biosynthetic Precursors

In the FeGP cofactor of [Fe]-hydrogenase, low-spin FeII is in complex with two CO ligands and a pyridinol derivative; the latter ligates the iron with a 6-acylmethyl substituent and the pyridinol nitrogen. A guanylylpyridinol derivative, 6-carboxymethyl-3,5-dimethyl-4-guanylyl-2-pyridinol (3), is produced by the decomposition of the FeGP cofactor under irradiation with UV-A/blue light and is also postulated to be a precursor of FeGP cofactor biosynthesis. HcgC and HcgB catalyze consecutive biosynthesis steps leading to 3. Here, we report an in vitro biosynthesis assay of the FeGP cofactor using the cell extract of the ΔhcgBΔhcgC strain of Methanococcus maripaludis, which does not biosynthesize 3. We chemically synthesized pyridinol precursors 1 and 2, and detected the production of the FeGP cofactor from 1, 2 and 3. These results indicated that 1, 2 and 3 are the precursors of the FeGP cofactor, and the carboxy group of 3 is converted to the acyl ligand.

Structural Insight into [NiFe] Hydrogenase Maturation by Transient Complexes between Hyp Proteins

[NiFe] hydrogenases catalyze reversible hydrogen production/consumption. The core unit of [NiFe] hydrogenase consists of a large and a small subunit. The active site of the large subunit of [NiFe] hydrogenases contains a NiFe(CN)₂CO cluster. The biosynthesis/maturation of these hydrogenases is a complex and dynamic process catalyzed primarily by six Hyp proteins (HypABCDEF), which play central roles in the maturation process. HypA and HypB are involved in the Ni insertion, whereas HypC, D, E, and F are required for the biosynthesis, assembly, and insertion of the Fe(CN)₂CO group. HypE and HypF catalyze the synthesis of the CN group through the carbamoylation and cyanation of the C-terminus cysteine of HypE. HypC and HypD form a scaffold for the assembly of the Fe(CN)₂CO moiety.Over the last decades, a large number of biochemical studies on maturation proteins have been performed, revealing basic functions of each Hyp protein and the overall framework of the maturation pathway. However, it is only in the last 10 years that structural insight has been gained, and our group has made significant contributions to the structural biology of hydrogenase maturation proteins.Since our first publication, where crystal structures of three Hyp proteins have been determined, we have performed a series of structural studies of all six Hyp proteins from a hyperthermophilic archaeon Thermococcus kodakarensis, providing molecular details of each Hyp protein. We have also determined the crystal structures of transient complexes between Hyp proteins that are formed during the maturation process to sequentially incorporate the components of the NiFe(CN)₂CO cluster to immature large subunits of [NiFe] hydrogenases. Such complexes, whose crystal structures are determined, include HypA-HypB, HypA-HyhL (hydrogenase large subunit), HypC-HypD, and HypC-HypD-HypE. The structures of the HypC-HypD, and HypCDE complexes reveal a sophisticated process of transient formation of the HypCDE complex, providing insight into the molecular basis of Fe atom cyanation. The high-resolution structures of the carbamoylated and cyanated forms of HypE reveal a structural basis for the biological conversion of primary amide to nitrile. The structure of the HypA-HypB complex elucidates nucleotide-dependent transient complex formation between these two proteins and the molecular basis of acquisition and release of labile Ni. Furthermore, our recent structure analysis of a complex between HypA and immature HyhL reveals that spatial rearrangement of both the N- and C-terminal tails of HyhL will occur upon the [NiFe] cluster insertion, which function as a key checkpoint for the maturation completion. This Account will focus on recent advances in structural studies of the Hyp proteins and on mechanistic insights into the [NiFe] hydrogenase maturation.

Biophysical research in Okazaki, Japan

This commentary summarizes the recent biophysical research conducted at the National Institute for Basic Biology, the National Institute for Physiological Sciences, and the Institute for Molecular Science in Okazaki, Japan.