Reconstruction of a fatty acid synthesis cycle from acyl carrier protein and cofactor structural snapshots.

Fatty acids (FAs) play a central metabolic role in living cells as constituents of membranes, cellular energy reserves, and second messenger precursors. A 2.6 MDa FA synthase (FAS), where the enzymatic reactions and structures are known, is responsible for FA biosynthesis in yeast. Essential in the yeast FAS catalytic cycle is the acyl carrier protein (ACP) that actively shuttles substrates, biosynthetic intermediates, and products from one active site to another. We resolve the S. cerevisiae FAS structure at 1.9 Å, elucidating cofactors and water networks involved in their recognition. Structural snapshots of ACP domains bound to various enzymatic domains allow the reconstruction of a full yeast FA biosynthesis cycle. The structural information suggests that each FAS functional unit could accommodate exogenous proteins to incorporate various enzymatic activities, and we show proof-of-concept experiments where ectopic proteins are used to modulate FAS product profiles.

The Structural Enzymology of Iterative Aromatic Polyketide Synthases: A Critical Comparison with Fatty Acid Synthases.

Polyketides are a large family of structurally complex natural products including compounds with important bioactivities. Polyketides are biosynthesized by polyketide synthases (PKSs), multienzyme complexes derived evolutionarily from fatty acid synthases (FASs). The focus of this review is to critically compare the properties of FASs with iterative aromatic PKSs, including type II PKSs and fungal type I nonreducing PKSs whose chemical logic is distinct from that of modular PKSs. This review focuses on structural and enzymological studies that reveal both similarities and striking differences between FASs and aromatic PKSs. The potential application of FAS and aromatic PKS structures for bioengineering future drugs and biofuels is highlighted.

An ADP-ribosyltransferase toxin kills bacterial cells by modifying structured non-coding RNAs.

ADP-ribosyltransferases (ARTs) were among the first identified bacterial virulence factors. Canonical ART toxins are delivered into host cells where they modify essential proteins, thereby inactivating cellular processes and promoting pathogenesis. Our understanding of ARTs has since expanded beyond protein-targeting toxins to include antibiotic inactivation and DNA damage repair. Here, we report the discovery of RhsP2 as an ART toxin delivered between competing bacteria by a type VI secretion system of Pseudomonas aeruginosa. A structure of RhsP2 reveals that it resembles protein-targeting ARTs such as diphtheria toxin. Remarkably, however, RhsP2 ADP-ribosylates 2'-hydroxyl groups of double-stranded RNA, and thus, its activity is highly promiscuous with identified cellular targets including the tRNA pool and the RNA-processing ribozyme, ribonuclease P. Consequently, cell death arises from the inhibition of translation and disruption of tRNA processing. Overall, our data demonstrate a previously undescribed mechanism of bacterial antagonism and uncover an unprecedented activity catalyzed by ART enzymes.

Double-duty isomerases: a case study of isomerization-coupled enzymatic catalysis.

Enzymes can usually be unambiguously assigned to one of seven classes specifying the basic chemistry of their catalyzed reactions. Less frequently, two or more reaction classes are catalyzed by a single enzyme within one active site. Two examples are an isomerohydrolase and an isomero-oxygenase that catalyze isomerization-coupled reactions crucial for production of vision-supporting 11-cis-retinoids. In these enzymes, isomerization is obligately paired and mechanistically intertwined with a second reaction class. A handful of other enzymes carrying out similarly coupled isomerization reactions have been described, some of which have been subjected to detailed structure-function analyses. Herein we review these rarefied enzymes, focusing on the mechanistic and structural basis of their reaction coupling with the goal of revealing catalytic commonalities.

Nanopore tweezers show fractional-nucleotide translocation in sequence-dependent pausing by RNA polymerase.

RNA polymerases (RNAPs) carry out the first step in the central dogma of molecular biology by transcribing DNA into RNA. Despite their importance, much about how RNAPs work remains unclear, in part because the small (3.4 Angstrom) and fast (~40 ms/nt) steps during transcription were difficult to resolve. Here, we used high-resolution nanopore tweezers to observe the motion of single Escherichia coli RNAP molecules as it transcribes DNA ~1,000 times improved temporal resolution, resolving single-nucleotide and fractional-nucleotide steps of individual RNAPs at saturating nucleoside triphosphate concentrations. We analyzed RNAP during processive transcription elongation and sequence-dependent pausing at the yrbL elemental pause sequence. Each time RNAP encounters the yrbL elemental pause sequence, it rapidly interconverts between five translocational states, residing predominantly in a half-translocated state. The kinetics and force-dependence of this half-translocated state indicate it is a functional intermediate between pre- and post-translocated states. Using structural and kinetics data, we show that, in the half-translocated and post-translocated states, sequence-specific protein-DNA interaction occurs between RNAP and a guanine base at the downstream end of the transcription bubble (core recognition element). Kinetic data show that this interaction stabilizes the half-translocated and post-translocated states relative to the pre-translocated state. We develop a kinetic model for RNAP at the yrbL pause and discuss this in the context of key structural features.

The Yeast INO80 Complex Operates as a Tunable DNA Length-Sensitive Switch to Regulate Nucleosome Sliding.

The yeast INO80 chromatin remodeling complex plays essential roles in regulating DNA damage repair, replication, and promoter architecture. INO80's role in these processes is likely related to its ability to slide nucleosomes, but the underlying mechanism is poorly understood. Here we use ensemble and single-molecule enzymology to study INO80-catalyzed nucleosome sliding. We find that the rate of nucleosome sliding by INO80 increases ∼100-fold when the flanking DNA length is increased from 40 to 60 bp. Furthermore, once sliding is initiated, INO80 moves the nucleosome rapidly at least 20 bp without pausing to re-assess flanking DNA length, and it can change the direction of nucleosome sliding without dissociation. Finally, we show that the Nhp10 module of INO80 plays an auto-inhibitory role, tuning INO80's switch-like response to flanking DNA. Our results indicate that INO80 is a highly processive remodeling motor that is tightly regulated by both substrate cues and non-catalytic subunits.

Biocatalytic control of site-selectivity and chain length-selectivity in radical amino acid halogenases.

Biocatalytic C-H activation has the potential to merge enzymatic and synthetic strategies for bond formation. FeII/αKG-dependent halogenases are particularly distinguished for their ability both to control selective C-H activation as well as to direct group transfer of a bound anion along a reaction axis separate from oxygen rebound, enabling the development of new transformations. In this context, we elucidate the basis for the selectivity of enzymes that perform selective halogenation to yield 4-Cl-lysine (BesD), 5-Cl-lysine (HalB), and 4-Cl-ornithine (HalD), allowing us to probe how site-selectivity and chain length selectivity are achieved. We now report the crystal structure of the HalB and HalD, revealing the key role of the substrate-binding lid in positioning the substrate for C4 vs C5 chlorination and recognition of lysine vs ornithine. Targeted engineering of the substrate-binding lid further demonstrates that these selectivities can be altered or switched, showcasing the potential to develop halogenases for biocatalytic applications.

The specificity landscape of bacterial ribonuclease P.

Developing quantitative models of substrate specificity for RNA processing enzymes is a key step toward understanding their biology and guiding applications in biotechnology and biomedicine. Optimally, models to predict relative rate constants for alternative substrates should integrate an understanding of structures of the enzyme bound to "fast" and "slow" substrates, large datasets of rate constants for alternative substrates, and transcriptomic data identifying in vivo processing sites. Such data are either available or emerging for bacterial ribonucleoprotein RNase P a widespread and essential tRNA 5' processing endonuclease, thus making it a valuable model system for investigating principles of biological specificity. Indeed, the well-established structure and kinetics of bacterial RNase P enabled the development of high throughput measurements of rate constants for tRNA variants and provided the necessary framework for quantitative specificity modeling. Several studies document the importance of conformational changes in the precursor tRNA substrate as well as the RNA and protein subunits of bacterial RNase P during binding, although the functional roles and dynamics are still being resolved. Recently, results from cryo-EM studies of E. coli RNase P with alternative precursor tRNAs are revealing prospective mechanistic relationships between conformational changes and substrate specificity. Yet, extensive uncharted territory remains, including leveraging these advances for drug discovery, achieving a complete accounting of RNase P substrates, and understanding how the cellular context contributes to RNA processing specificity in vivo.

The genomic and enzymatic basis for iridoid biosynthesis in cat thyme (Teucrium marum).

Iridoids are non-canonical monoterpenoids produced by both insects and plants. An example is the cat-attracting and insect-repelling volatile iridoid nepetalactone, produced by Nepeta sp. (catmint) and aphids. Recently, both nepetalactone biosynthetic pathways were elucidated, showing a remarkable convergent evolution. The iridoid, dolichodial, produced by Teucrium marum (cat thyme) and multiple insect species, has highly similar properties to nepetalactone but its biosynthetic origin remains unknown. We set out to determine the genomic, enzymatic, and evolutionary basis of iridoid biosynthesis in T. marum. First, we generated a de novo chromosome-scale genome assembly for T. marum using Oxford Nanopore Technologies long reads and proximity-by-ligation Hi-C reads. The 610.3 Mb assembly spans 15 pseudomolecules with a 32.9 Mb N50 scaffold size. This enabled identification of iridoid biosynthetic genes, whose roles were verified via activity assays. Phylogenomic analysis revealed that the evolutionary history of T. marum iridoid synthase, the iridoid scaffold-forming enzyme, is not orthologous to typical iridoid synthases but is derived from its conserved paralog. We discovered an enzymatic route from nepetalactol to diverse iridoids through the coupled activity of an iridoid oxidase cytochrome P450 and acetyltransferases, via an inferred acylated intermediate. This work provides a genomic resource for specialized metabolite research in mints and demonstration of the role of acetylation in T. marum iridoid diversity. This work will enable future biocatalytic or biosynthetic production of potent insect repellents, as well as comparative studies into iridoid biosynthesis in insects.

Chemical diversity in angiosperms - monoterpene synthases control complex reactions that provide the precursors for ecologically and commercially important monoterpenoids.

Monoterpene synthases (MTSs) catalyze the first committed step in the biosynthesis of monoterpenoids, a class of specialized metabolites with particularly high chemical diversity in angiosperms. In addition to accomplishing a rate enhancement, these enzymes manage the formation and turnover of highly reactive carbocation intermediates formed from a prenyl diphosphate substrate. At each step along the reaction path, a cationic intermediate can be subject to cyclization, migration of a proton, hydride, or alkyl group, or quenching to terminate the sequence. However, enzymatic control of ligand folding, stabilization of specific intermediates, and defined quenching chemistry can maintain the specificity for forming a signature product. This review article will discuss our current understanding of how angiosperm MTSs control the reaction environment. Such knowledge allows inferences about the origin and regulation of chemical diversity, which is pertinent for appreciating the role of monoterpenoids in plant ecology but also for aiding commercial efforts that harness the accumulation of these specialized metabolites for the food, cosmetic, and pharmaceutical industries.

Structure, regulation, and mechanisms of nonmuscle myosin-2.

Members of the myosin superfamily of molecular motors are large mechanochemical ATPases that are implicated in an ever-expanding array of cellular functions. This review focuses on mammalian nonmuscle myosin-2 (NM2) paralogs, ubiquitous members of the myosin-2 family of filament-forming motors. Through the conversion of chemical energy into mechanical work, NM2 paralogs remodel and shape cells and tissues. This process is tightly controlled in time and space by numerous synergetic regulation mechanisms to meet cellular demands. We review how recent advances in structural biology together with elegant biophysical and cell biological approaches have contributed to our understanding of the shared and unique mechanisms of NM2 paralogs as they relate to their kinetics, regulation, assembly, and cellular function.

Key structural role of a conserved cis-proline revealed by the P285S variant of soybean serine hydroxymethyltransferase 8.

The enzyme serine hydroxymethyltransferase (SHMT) plays a key role in folate metabolism and is conserved in all kingdoms of life.  SHMT is a pyridoxal 5'-phosphate (PLP) - dependent enzyme that catalyzes the conversion of L-serine and (6S)-tetrahydrofolate to glycine and 5,10-methylene tetrahydrofolate. Crystal structures of multiple members of the SHMT family have shown that the enzyme has a single conserved cis proline, which is located near the active site.  Here, we have characterized a Pro to Ser amino acid variant (P285S) that affects this conserved cis proline in soybean SHMT8.  P285S was identified as one of a set of mutations that affect the resistance of soybean to the agricultural pathogen soybean cyst nematode.  We find that replacement of Pro285 by serine eliminates PLP-mediated catalytic activity of SHMT8, reduces folate binding, decreases enzyme stability, and affects the dimer-tetramer ratio of the enzyme in solution.  Crystal structures at 1.9 - 2.2 Å resolution reveal a local reordering of the polypeptide chain that extends an a-helix and shifts a turn region into the active site.  This results in a dramatically perturbed PLP-binding pose, where the ring of the cofactor is flipped by ~180° with concomitant loss of conserved enzyme-PLP interactions.  A nearby region of the polypeptide becomes disordered, evidenced by missing electron density for ~10 residues.  These structural perturbations are consistent with the loss of enzyme activity and folate binding and underscore the important role of the Pro285 cis-peptide in SHMT structure and function.

Escherichia coli RclA is a highly active hypothiocyanite reductase.

Hypothiocyanite and hypothiocyanous acid (OSCN-/HOSCN) are pseudohypohalous acids released by the innate immune system which are capable of rapidly oxidizing sulfur-containing amino acids, causing significant protein aggregation and damage to invading bacteria. HOSCN is abundant in saliva and airway secretions and has long been considered a highly specific antimicrobial that is nearly harmless to mammalian cells. However, certain bacteria, commensal and pathogenic, are able to escape damage by HOSCN and other harmful antimicrobials during inflammation, which allows them to continue to grow and, in some cases, cause severe disease. The exact genes or mechanisms by which bacteria respond to HOSCN have not yet been elucidated. We have found, in Escherichia coli, that the flavoprotein RclA, previously implicated in reactive chlorine resistance, reduces HOSCN to thiocyanate with near-perfect catalytic efficiency and strongly protects E. coli against HOSCN toxicity. This is notable in E. coli because this species thrives in the chronically inflamed environment found in patients with inflammatory bowel disease and is able to compete with and outgrow other important commensal organisms, suggesting that HOSCN may be a relevant antimicrobial in the gut, which has not previously been explored. RclA is conserved in a variety of epithelium-colonizing bacteria, implicating its HOSCN reductase activity in a variety of host-microbe interactions. We show that an rclA mutant of the probiotic Limosilactobacillus reuteri is sensitive to HOSCN and that RclA homologs from Staphylococcus aureus, Streptococcus pneumoniae, and Bacteroides thetaiotaomicron all have potent protective activity against HOSCN when expressed in E. coli.

Regulation of protein function and degradation by heme, heme responsive motifs, and CO.

Heme is an essential biomolecule and cofactor involved in a myriad of biological processes. In this review, we focus on how heme binding to heme regulatory motifs (HRMs), catalytic sites, and gas signaling molecules as well as how changes in the heme redox state regulate protein structure, function, and degradation. We also relate these heme-dependent changes to the affected metabolic processes. We center our discussion on two HRM-containing proteins: human heme oxygenase-2, a protein that binds and degrades heme (releasing Fe2+ and CO) in its catalytic core and binds Fe3+-heme at HRMs located within an unstructured region of the enzyme, and the transcriptional regulator Rev-erbß, a protein that binds Fe3+-heme at an HRM and is involved in CO sensing. We will discuss these and other proteins as they relate to cellular heme composition, homeostasis, and trafficking. In addition, we will discuss the HRM-containing family of proteins and how the stability and activity of these proteins are regulated in a dependent manner through the HRMs. Then, after reviewing CO-mediated protein regulation of heme proteins, we turn our attention to the involvement of heme, HRMs, and CO in circadian rhythms. In sum, we stress the importance of understanding the various roles of heme and the distribution of the different heme pools as they relate to the heme redox state, CO, and heme binding affinities.

Murine HSD17ß13 does not control liver steatosis and modestly impacts fibrosis in a sex- and diet-specific manner.

Human genetic studies show that loss of function mutations in 17-Beta hydroxysteroid dehydrogenase (HSD17ß13) are associated with protection from non-alcoholic steatohepatitis (NASH). As a result, therapies that reduce HSD17ß13 are being pursued for the treatment of NASH. However, inconsistent effects on steatosis, inflammation, and fibrosis pathogenesis have been reported in murine Hsd17b13 knockdown or knockout models. To clarify whether murine Hsd17b13 loss regulates liver damage and fibrosis, we characterized Hsd17b13 knockout mice subjected to pro-NASH diets or pro-inflammatory chemical-induced liver injury. There were no effects of Hsd17b13 loss on liver injury, inflammation, fibrosis, or lipids after 28 weeks on the Gubra-Amylin NASH (GAN) diet or 12 weeks on a 45% choline-deficient high-fat diet (CDAHFD). However, AAV-mediated re-expression of murine Hsd17b13 in KO mice increased liver macrophage abundance in both sexes fed the 45% CDAHFD. In contrast, there was a modest reduction in liver fibrosis, but not lipids or inflammation within Hsd17b13 null female, but not male, mice after 12 weeks of a 60% CDAHFD compared to WT littermates. Fibrosis and the abundance of liver macrophages were increased in Hsd17b13 KO females upon adenoviral re-expression of mouse HSD17ß13, but this was not reflected in inflammatory markers. Additionally, we found minimal differences in liver injury, lipids, or inflammatory and fibrotic markers 48 h after acute CCl4 exposure. In summary, murine Hsd17b13 loss has modest diet- and sex-specific effects on liver fibrosis which contrasts with human genetic studies. This suggests a disconnect between the biological function of HSD17ß13 in mice and humans.

6-O-Alkyl 4-methylumbelliferyl-ß-D-glucosides as selective substrates for GBA1 in the discovery of glycosylated sterols.

Gaucher disease (GD) is a lysosomal storage disorder (LSD) resulting from inherited glucocerebrosidase (GBA1) deficiency. GD diagnosis relies on GBA1 activity assays, typically employing 4-methylumbelliferyl-ß-D-glucopyranoside (4MU-ß-Glc) as fluorogenic substrate. However, these assays suffer from background 4MU release by the non-lysosomal GBA2 and cytosolic GBA3 enzymes. Here we developed GBA1-selective fluorogenic substrates by synthesizing a series of 6-O-acyl-4MU-ß-Glc substrates with diverse fatty acid tails. Because of chemical and enzymatic instability of the ester bonds, analogues of 6-O-palmitoyl-4MU-ß-Glc (3) with different chemical linkages were synthesized. 6-O-alkyl-4MU-ß-Glc 9, featuring an ether linkage, emerged as the most optimal GBA1 substrate, exhibiting both a low Km and compared to substrate 3 a high Vmax. Importantly, substrate 9 is not hydrolyzed by GBA2 and GBA3, and therefore acts as superior substrate for GD diagnosis. Plants contain glycosyl phytosterols (campesterol, ß-sitosterol and sigmasterol) that may also be acylated at C-6. LC-MS/MS analysis revealed that 6-O-acylated and regular glycosylcholesterol (HexChol) tend to be increased in GD patient spleens. Moreover, significant increases in 6-O-acyl-glycosyl-phytosterols were detected in GD spleens. Our findings suggest uptake of (6-O-acyl)-glycosyl-phytosterols from plant food and subsequent lysosomal processing by GBA1, and comprise the first example of accumulation of an exogenous class of glycolipids in GD. Excessive exposure of rodents to glycosylated phytosterols has been reported to induce manifestations of Parkinson's disease (PD). Further investigation is warranted to determine whether (6-O-acyl)-glycosyl-phytosterols could contribute to the enigmatic link between inherited defects in GBA1 and the risk for PD.

A fluorogenic substrate for the detection of lipid amidases in intact cells.

Lipid amidases of therapeutic relevance include acid ceramidase (AC), N-acylethanolamine-hydrolyzing acid amidase, and fatty acid amide hydrolase (FAAH). Although fluorogenic substrates have been developed for the three enzymes and high-throughput methods for screening have been reported, a platform for the specific detection of these enzyme activities in intact cells is lacking. In this article, we report on the coumarinic 1-deoxydihydroceramide RBM1-151, a 1-deoxy derivative and vinilog of RBM14-C12, as a novel substrate of amidases. This compound is hydrolyzed by AC (appKm = 7.0 µM; appVmax = 99.3 nM/min), N-acylethanolamine-hydrolyzing acid amidase (appKm = 0.73 µM; appVmax = 0.24 nM/min), and FAAH (appKm = 3.6 µM; appVmax = 7.6 nM/min) but not by other ceramidases. We provide proof of concept that the use of RBM1-151 in combination with reported irreversible inhibitors of AC and FAAH allows the determination in parallel of the three amidase activities in single experiments in intact cells.

Opposing regulation of the Nα-trimethylase METTL11A by its family members METTL11B and METTL13.

N-terminal protein methylation (Nα-methylation) is a posttranslational modification that influences numerous biological processes by regulating protein stability, protein-DNA interactions, and protein-protein interactions. Although significant progress has been made in understanding the biological roles of Nα-methylation, we still do not completely understand how the modifying methyltransferases are regulated. A common mode of methyltransferase regulation is through complex formation with close family members, and we have previously shown that the Nα-trimethylase METTL11A (NRMT1/NTMT1) is activated through binding of its close homolog METTL11B (NRMT2/NTMT2). Other recent reports indicate that METTL11A co-fractionates with a third METTL family member METTL13, which methylates both the N-terminus and lysine 55 (K55) of eukaryotic elongation factor 1 alpha. Here, using co-immunoprecipitations, mass spectrometry, and in vitro methylation assays, we confirm a regulatory interaction between METTL11A and METTL13 and show that while METTL11B is an activator of METTL11A, METTL13 inhibits METTL11A activity. This is the first example of a methyltransferase being opposingly regulated by different family members. Similarly, we find that METTL11A promotes the K55 methylation activity of METTL13 but inhibits its Nα-methylation activity. We also find that catalytic activity is not needed for these regulatory effects, demonstrating new, noncatalytic functions for METTL11A and METTL13. Finally, we show METTL11A, METTL11B, and METTL13 can complex together, and when all three are present, the regulatory effects of METTL13 take precedence over those of METTL11B. These findings provide a better understanding of Nα-methylation regulation and suggest a model where these methyltransferases can serve in both catalytic and noncatalytic roles.

An engineered N-acyltransferase-LOV2 domain fusion protein enables light-inducible allosteric control of enzymatic activity.

Transferases are ubiquitous across all known life. While much work has been done to understand and describe these essential enzymes, there have been minimal efforts to exert tight and reversible control over their activity for various biotechnological applications. Here, we apply a rational, computation-guided methodology to design and test a transferase-class enzyme allosterically regulated by light-oxygen-voltage 2 sensing domain. We utilize computational techniques to determine the intrinsic allosteric networks within N-acyltransferase (Orf11/∗Dbv8) and identify potential allosteric sites on the protein's surface. We insert light-oxygen-voltage 2 sensing domain at the predicted allosteric site, exerting reversible control over enzymatic activity. We demonstrate blue-light regulation of N-acyltransferase (Orf11/∗Dbv8) function. Our study for the first time demonstrates optogenetic regulation of a transferase-class enzyme as a proof-of-concept for controllable transferase design. This successful design opens the door for many future applications in metabolic engineering and cellular programming.

Kinetic analysis of RNA cleavage by coronavirus Nsp15 endonuclease: Evidence for acid-base catalysis and substrate-dependent metal ion activation.

Understanding the functional properties of severe acute respiratory syndrome coronavirus 2 nonstructural proteins is essential for defining their roles in the viral life cycle, developing improved therapeutics and diagnostics, and countering future variants. Coronavirus nonstructural protein Nsp15 is a hexameric U-specific endonuclease whose functions, substrate specificity, mechanism, and dynamics are not fully defined. Previous studies report that Nsp15 requires Mn2+ ions for optimal activity; however, the effects of divalent ions on Nsp15 reaction kinetics have not been investigated in detail. Here, we analyzed the single- and multiple-turnover kinetics for model ssRNA substrates. Our data confirm that divalent ions are dispensable for catalysis and show that Mn2+ activates Nsp15 cleavage of two different ssRNA oligonucleotide substrates but not a dinucleotide. Biphasic kinetics of ssRNA substrates demonstrates that Mn2+ stabilizes alternative enzyme states that have faster substrate cleavage on the enzyme. However, we did not detect Mn2+-induced conformational changes using CD and fluorescence spectroscopy. The pH-rate profiles in the presence and absence of Mn2+ reveal active-site ionizable groups with similar pKas of ca. 4.8 to 5.2. An Rp stereoisomer phosphorothioate modification at the scissile phosphate had minimal effect on catalysis supporting a mechanism involving an anionic transition state. However, the Sp stereoisomer is inactive because of weak binding, consistent with models that position the nonbridging phosphoryl oxygen deep in the active site. Together, these data demonstrate that Nsp15 employs a conventional acid-base catalytic mechanism passing through an anionic transition state, and that divalent ion activation is substrate dependent.