Essentiality of the Escherichia coli YgfZ Protein for the In Vivo Thiomethylation of Ribosomal Protein S12 by the RimO Enzyme

Enzymes carrying Iron-Sulfur (Fe-S) clusters perform many important cellular functions and their biogenesis require complex protein machinery. In mitochondria, the IBA57 protein is essential and promotes assembly of [4Fe-4S] clusters and their insertion into acceptor proteins. YgfZ is the bacterial homologue of IBA57 but its precise role in Fe-S cluster metabolism is uncharacterized. YgfZ is needed for activity of the radical S-adenosyl methionine [4Fe-4S] cluster enzyme MiaB which thiomethylates some tRNAs. The growth of cells lacking YgfZ is compromised especially at low temperature. The RimO enzyme is homologous to MiaB and thiomethylates a conserved aspartic acid in ribosomal protein S12. To quantitate thiomethylation by RimO, we developed a bottom-up LC-MS² analysis of total cell extracts. We show here that the in vivo activity of RimO is very low in the absence of YgfZ and independent of growth temperature. We discuss these results in relation to the hypotheses relating to the role of the auxiliary 4Fe-4S cluster in the Radical SAM enzymes that make Carbon-Sulfur bonds.

Purification and structural elucidation of a cobalamin-dependent radical SAM enzyme

The cobalamin (Cbl)-dependent radical S-adenosylmethionine (SAM) enzymes use a [4Fe-4S] cluster, SAM, and Cbl to carry out remarkable catalytic feats in a large number of biosynthetic pathways. However, despite the abundance of annotated Cbl-dependent radical SAM enzymes, relatively few molecular details exist regarding how these enzymes function. Traditionally, challenges associated with purifying and reconstituting Cbl-dependent radical SAM enzymes have hindered biochemical studies aimed at elucidating the structures and mechanisms of these enzymes. Herein, we describe a bottom-up approach that was used to crystallize OxsB, learn about the overall architecture of a Cbl-dependent radical SAM enzyme, and facilitate mechanistic studies. We report lessons learned from the crystallization of different states of OxsB, including the apo-, selenomethionine (SeMet)-labeled, and fully reconstituted form of OxsB that has a [4Fe-4S] cluster, SAM, and Cbl bound. Further, we suggest that, when appropriate, this bottom-up method can be used to facilitate studies on enzymes in this class for which there are challenges associated with purifying and reconstituting the active enzyme.

Multiple profile models extract features from protein sequence data and resolve functional diversity of very different protein families

Functional classification of proteins from sequences alone has become a critical bottleneck in understanding the myriad of protein sequences that accumulate in our databases. The great diversity of homologous sequences hides, in many cases, a variety of functional activities that cannot be anticipated. Their identification appears critical for a fundamental understanding of the evolution of living organisms and for biotechnological applications. ProfileView is a sequence-based computational method, designed to functionally classify sets of homologous sequences. It relies on two main ideas: the use of multiple profile models whose construction explores evolutionary information in available databases, and a novel definition of a representation space in which to analyse sequences with multiple profile models combined together. ProfileView classifies protein families by enriching known functional groups with new sequences and discovering new groups and subgroups. We validate ProfileView on seven classes of widespread proteins involved in the interaction with nucleic acids, amino acids and small molecules, and in a large variety of functions and enzymatic reactions. Profile-View agrees with the large set of functional data collected for these proteins from the literature regarding the organisation into functional subgroups and residues that characterise the functions. In addition, ProfileView resolves undefined functional classifications and extracts the molecular determinants underlying protein functional diversity, showing its potential to select sequences towards accurate experimental design and discovery of novel biological functions. On protein families with complex domain architecture, ProfileView functional classification reconciles domain combinations, unlike phylogenetic reconstruction. ProfileView proves to outperform the functional classification approach PANTHER, the two k-mer based methods CUPP and eCAMI and a neural network approach based on Restricted Boltzmann Machines. It overcomes time complexity limitations of the latter.

The B(12)-independent glycerol dehydratase activating enzyme from Clostridium butyricum cleaves SAM to produce 5'-deoxyadenosine and not 5'-deoxy-5'-(methylthio)adenosine

Glycerol dehydratase activating enzyme (GD-AE) is a radical S-adenosyl-l-methionine (SAM) enzyme that installs a catalytically essential amino acid backbone radical onto glycerol dehydratase in bacteria under anaerobic conditions. Although GD-AE is closely homologous to other radical SAM activases that have been shown to cleave the S-C(5') bond of SAM to produce 5'-deoxyadenosine (5'-dAdoH) and methionine, GD-AE from Clostridium butyricum has been reported to instead cleave the S-C(γ) bond of SAM to yield 5'-deoxy-5'-(methylthio)adenosine (MTA). Here we re-investigate the SAM cleavage reaction catalyzed by GD-AE and show that it produces the widely observed 5'-dAdoH, and not the less conventional product MTA.

Characterization of Paramagnetic Iron-Sulfur Clusters Using Electron Paramagnetic Resonance Spectroscopy

Continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy is a powerful ally in characterizing the multitude of redox-active iron-sulfur cluster-containing ([Fe-S]) species present in biological samples. The technique detects only those clusters that are paramagnetic-having a nonzero total electron spin (S > 0)-thus, it can discriminate between clusters in different oxidation states. The low-temperature CW-EPR spectrum of an [Fe-S] yields the three magnetic g-values that serve as a fingerprint of its electronic structure. This chapter briefly describes the underlying theory that defines this electronic structure and provides a recipe for the acquisition and analysis of EPR spectra of [Fe-S] proteins.

miCLIP-MaPseq Identifies Substrates of Radical SAM RNA-Methylating Enzyme Using Mechanistic Cross-Linking and Mismatch Profiling

The family of radical SAM RNA-methylating enzymes comprises a large group of proteins that contains only a few functionally characterized members. Several enzymes in this family have been implicated in the regulation of translation and antibiotic susceptibility, emphasizing their significance in bacterial physiology and their relevance to human health. While few characterized enzymes have been shown to modify diverse RNA substrates, highlighting potentially broad substrate scope within the family, many enzymes in this class have no known substrates. The precise knowledge of RNA substrates and modification sites for uncharacterized family members is important for unraveling their biological function. Here, we describe a strategy for substrate identification that takes advantage of mechanism-based cross-linking between the enzyme and its RNA substrates, which we named individual-nucleotide-resolution cross-linking and immunoprecipitation combined with mutational profiling with sequencing (miCLIP-MaPseq). Identification of the position of the modification site is achieved using thermostable group II intron reverse transcriptase (TGIRT), which introduces a mismatch at the site of the cross-link.

Sulfur amino acid metabolism and related metabotypes of autism spectrum disorder: A review of biochemical evidence for a hypothesis

There are multiple lines of evidence for an impaired sulfur amino acid (SAA) metabolism in autism spectrum disorder (ASD). For instance, the concentrations of methionine, cysteine and S-adenosylmethionine (SAM) in body fluids of individuals with ASD is significantly lower while the concentration of S-adenosylhomocysteine (SAH) is significantly higher as compared to healthy individuals. Reduced methionine and SAM may reflect impaired remethylation pathway whereas increased SAH may reflect reduced S-adenosylhomocysteine hydrolase activity in the catabolic direction. Reduced SAM/SAH ratio reflects an impaired methylation capacity. We hypothesize multiple mechanisms to explain how the interplay of oxidative stress, neuroinflammation, mercury exposure, maternal use of valproate, altered gut microbiome and certain genetic variants may lead to these SAA metabotypes. Furthermore, we also propose a number of mechanisms to explain the metabolic consequences of abnormal SAA metabotypes. For instance in the brain, reduced SAM/SAH ratio will result in melatonin deficiency and hypomethylation of a number of biomolecules such as DNA, RNA and histones. In addition to previously proposed mechanisms, we propose that impaired activity of "radical SAM" enzymes will result in reduced endogenous lipoic acid synthesis, reduced molybdenum cofactor synthesis and impaired porphyrin metabolism leading to mitochondrial dysfunction, porphyrinuria and impaired sulfation capacity. Furthermore depletion of SAM may also lead to the disturbed mTOR signaling pathway in a subgroup of ASD. The proposed "SAM-depletion hypothesis" is an inclusive model to explain the relationship between heterogeneous risk factors and metabotypes observed in a subset of children with ASD.

H-cluster assembly intermediates built on HydF by the radical SAM enzymes HydE and HydG

[FeFe]-hydrogenase catalyzes the reversible reduction of protons to H₂ at a complex metallocofactor site, the H-cluster. Biosynthesis of this active-site H-cluster requires three maturation enzymes: the radical S-adenosylmethionine enzymes HydE and HydG synthesize the nonprotein ligands, while the GTPase HydF provides a scaffold for assembly of the 2Fe subcluster of the H-cluster ([2Fe]_H) prior to its transfer to hydrogenase. To delineate the assembly and delivery steps for the 2Fe precursor cluster coordinated to HydF ([2Fe]_F), we have heterologously expressed HydF in the presence of HydE alone (HydF^E) or HydG alone (HydF^G), and characterized the resulting purified HydF^E and HydF^G using UV-visible, EPR, and FTIR spectroscopies and biochemical assays. The iron-sulfur clusters on HydF are modified by co-expression with HydE or HydG, as evidenced by the changes in the visible, EPR, and FTIR spectral features. Further, biochemical assays show that HydF^E is capable of activating HydA^ΔEFG to a limited extent (~ 1% of WT) even though the normal source of CO and CN^- ligands of [2Fe]_H (HydG) was absent. Activation assays performed with HydF^G, in contrast, exhibit no ability to mature HydA^ΔEFG. It appears that in the case of HydF^E, trace diatomics from the cellular environment are incorporated into a [2Fe]_F-like precursor on HydF in the absence of HydG. We conclude that the product of HydE, presumably the dithiomethylamine ligand of [2Fe]_H, is absolutely essential to the activation process, while the diatomic products of HydG can be provided from alternate sources.

Methods for Expression, Purification, and Characterization of PqqE, a Radical SAM Enzyme in the PQQ Biosynthetic Pathway

PqqE is the first enzyme in the biosynthetic pathway of the redox cofactor pyrroloquinoline quinone (PQQ), catalyzing the formation of a carbon-carbon bond in the precursor peptide PqqA. PqqE is a radical S-adenosyl-l-methionine (SAM) (RS) enzyme, a family of enzymes that use the reductive cleavage of a [4Fe-4S] cluster-bound SAM molecule to generate a 5'-deoxyadenosyl radical. This radical is then used to initiate an array of reactions that otherwise would be unlikely to occur. PqqE is a founding member of a subset family of RS enzymes that, additionally to the SAM [4Fe-4S] cluster, have a SPASM domain containing additional, auxiliary Fe-S clusters. Most radical SAM enzymes are highly sensitive to oxygen, which destroys their Fe-S clusters. This can pose several limitations when working with these enzymes, since most of the work has to be done under anaerobic conditions. Here, we summarize the methods developed in our lab for the expression and purification of PqqE. We also highlight the several methods we have used for the characterization of the enzyme.

QueE: A Radical SAM Enzyme Involved in the Biosynthesis of 7-Deazapurine Containing Natural Products

7-Carboxy-7-deazaguanine (CDG) is a common intermediate in the biosynthesis of 7-deazapurine-containing natural products. The biosynthesis of CDG from GTP requires three enzymes: GTP cyclohydrolase I, 6-carboxy-5,6,7,8-tetrahydropterin (CPH4) synthase, and CDG synthase (QueE). QueE is a member of the radical S-adenosyl-l-methionine (SAM) superfamily and catalyzes the SAM-dependent radical-mediated ring contraction of CPH4 to generate CDG. This chapter focuses on methods to reconstitute the activity of QueE in vitro.

TYW1: A Radical SAM Enzyme Involved in the Biosynthesis of Wybutosine Bases

Transfer RNA is extensively modified by the actions of a variety of enzymes. The radical S-adenosyl-l-methionine enzyme TYW1 modifies tRNA^Phe forming the characteristic tricyclic ring via the condensation of carbons 2 and 3 of pyruvate. This chapter details methods that are required for studies of TYW1.

Cobalamin-Dependent C-Methyltransferases From Marine Microbes: Accessibility via Rhizobia Expression

Cobalamin-dependent radical S-adenosylmethionine (rSAM) methyltransferases catalyze chemically challenging methylation reactions on diverse natural products at unactivated carbon centers. In vivo reconstitution and biosynthetic studies of natural product gene clusters encoding these enzymes are often severely limited by ineffective heterologous expression hosts, including the otherwise versatile Escherichia coli. In this chapter, we describe the use of rhizobia bacteria as effective expression hosts for cobalamin-dependent rSAM C-methyltransferases. We chose the natural product pathway encoding the heavily modified cytotoxic peptides, the polytheonamides, as our model pathway due to the presence of two methyltransferases responsible for a total of 17 C-methylations. Detailed protocols are given for vector construction, transformation, and heterologous expression in Rhizobium leguminosarum bv. viciae 3841. Additional methods pertaining to analytical separation and mass spectrometric analysis of modified peptides are also entailed. As genomics continues to uncover new enzymes and pathways from unknown and uncultivated microbes, use of metabolically distinct heterologous expression hosts like rhizobia will be a necessary tool to unravel the catalytic and metabolic diversity of marine microbial life.

Theory and Application of the Relationship Between Steady-State Isotope Effects on Enzyme Intermediate Concentrations and Net Rate Constants

Steady-state kinetic isotope effects on enzyme-catalyzed reactions are often interpreted in terms of the microscopic rate constants associated with the elementary reactions of interest. Unfortunately, this approach can lead to confusion, especially when more than one elementary reaction is isotopically sensitive, because it forces one to consider the full catalytic cycle one step at a time rather than as a complete whole. Herein we argue that shifting focus from intrinsic effects to net rate constants and enzyme intermediate concentrations provides a more natural and holistic interpretation by which the effects of partial rate limitation are more easily understood. In doing so, we demonstrate how the experimental determination of isotope effects on enzyme intermediate concentrations allows a direct determination of isotope effects on net rate constants. The chapter is divided into three main sections. The first outlines the basic theory and its interpretation. The second discusses an application of the theory in the study of the radical SAM enzyme DesII. The final section then provides the complete mathematical treatment.

Auxiliary iron-sulfur cofactors in radical SAM enzymes

A vast number of enzymes are now known to belong to a superfamily known as radical SAM, which all contain a [4Fe-4S] cluster ligated by three cysteine residues. The remaining, unligated, iron ion of the cluster binds in contact with the α-amino and α-carboxylate groups of S-adenosyl-l-methionine (SAM). This binding mode facilitates inner-sphere electron transfer from the reduced form of the cluster into the sulfur atom of SAM, resulting in a reductive cleavage of SAM to methionine and a 5'-deoxyadenosyl radical. The 5'-deoxyadenosyl radical then abstracts a target substrate hydrogen atom, initiating a wide variety of radical-based transformations. A subset of radical SAM enzymes contains one or more additional iron-sulfur clusters that are required for the reactions they catalyze. However, outside of a subset of sulfur insertion reactions, very little is known about the roles of these additional clusters. This review will highlight the most recent advances in the identification and characterization of radical SAM enzymes that harbor auxiliary iron-sulfur clusters. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Solution phase dynamics of the DNA repair enzyme spore photoproduct lyase as probed by H/D exchange

Spore photoproduct lyase (SPL) catalyzes the repair of the UV lesion spore photoproduct (SP) in a reaction dependent on S-adenosyl-L-methionine (SAM). We have utilized H/D exchange to show that in the presence of SAM, a significant reduction in H/D exchange is observed upon binding SPTpT or undamaged oligonucleotide, indicating a shift of 20 or 10 amide protons, respectively, from a rapidly-exchangable state to a fully-protected conformation. In the absence of SAM, neither the oligonucleotide nor the SPTpT produce a significant perturbation in H/D exchange, indicating SAM is a requisite binding partner. Performing the same experiments in aerobic conditions reduced the magnitude of ligand-induced structural changes, consistent with the importance of the oxygen-sensitive iron-sulfur cluster for SAM and substrate binding.