Chemical Versatility in Catalysis and Inhibition of the Class IIb Histone Deacetylases.

The zinc-dependent histone deacetylases (HDACs 1-11) belong to the arginase-deacetylase superfamily of proteins, members of which share a common α/ß fold and catalytic metal binding site. While several HDACs play a role in epigenetic regulation by catalyzing acetyllysine hydrolysis in histone proteins, the biological activities of HDACs extend far beyond histones. HDACs also deacetylate nonhistone proteins in the nucleus as well as the cytosol to regulate myriad cellular processes. The substrate pool is even more diverse in that certain HDACs can hydrolyze other covalent modifications. For example, HDAC6 is also a lysine decrotonylase, and HDAC11 is a lysine-fatty acid deacylase. Surprisingly, HDAC10 is not a lysine deacetylase but instead is a polyamine deacetylase. Thus, the HDACs are biologically and chemically versatile catalysts as they regulate the function of diverse protein and nonprotein substrates throughout the cell.Owing to their critical regulatory functions, HDACs serve as prominent targets for drug design. At present, four HDAC inhibitors are FDA-approved for cancer chemotherapy. However, these inhibitors are active against multiple HDAC isozymes, and a lack of selectivity is thought to contribute to undesirable side effects. Current medicinal chemistry campaigns focus on the development of isozyme-selective inhibitors, and many such studies largely focus on HDAC6 and HDAC10. HDAC6 is a target for therapeutic intervention due to its cellular role as a tubulin deacetylase and tau deacetylase, and selective inhibitors are being studied in cancer chemotherapy and the treatment of peripheral neuropathy. Crystal structures of enzyme-inhibitor complexes reveal how various features of inhibitor design, such as zinc-coordinating groups, bifurcated capping groups, and aromatic fluorination patterns, contribute to affinity and isozyme selectivity. The polyamine deacetylase HDAC10 is also an emerging target for cancer chemotherapy. Crystal structures of intact substrates trapped in the HDAC10 active site reveal the molecular basis of strikingly narrow substrate specificity for N8-acetylspermidine hydrolysis. Active site features responsible for substrate specificity have been successfully exploited in the design of potent and selective inhibitors.In this Account, I review the structural chemistry and inhibition of HDACs, highlighting recent X-ray crystallographic and functional studies of HDAC6 and HDAC10 in my laboratory. These studies have yielded fascinating snapshots of catalysis as well as novel chemical transformations involving bound inhibitors. The zinc-bound water molecule in the HDAC active site is the catalytic nucleophile in the deacetylation reaction, but this activated water molecule can also react with inhibitor C═O or C═N groups to yield unanticipated reaction products that bind exceptionally tightly. Versatile active site chemistry unleashes the full inhibitory potential of such compounds, and X-ray crystallography allows us to view this chemistry in action.

Structure-Based Engineering of a Sesquiterpene Cyclase to Generate an Alcohol Product: Conversion of epi-Isozizaene Synthase into α-Bisabolol Synthase.

The sesquiterpene cyclase epi-isozizaene synthase (EIZS) from Streptomyces coelicolor catalyzes the metal-dependent conversion of farnesyl diphosphate (FPP) into the complex tricyclic product epi-isozizaene. This remarkable transformation is governed by an active site contour that serves as a template for catalysis, directing the conformations of multiple carbocation intermediates leading to the final product. Mutagenesis of residues defining the active site contour remolds its three-dimensional shape and reprograms the cyclization cascade to generate alternative cyclization products. In some cases, mutagenesis enables alternative chemistry to quench carbocation intermediates, e.g., through hydroxylation. Here, we combine structural and biochemical data from previously characterized EIZS mutants to design and prepare F95S-F198S EIZS, which converts EIZS into an α-bisabolol synthase with moderate fidelity (65% at 18 °C, 74% at 4 °C). We report the complete biochemical characterization of this double mutant as well as the 1.47 Å resolution X-ray crystal structure of its complex with three Mg2+ ions, inorganic pyrophosphate, and the benzyltriethylammonium cation, which partially mimics a carbocation intermediate. Most notably, the two mutations together create an active site contour that stabilizes the bisabolyl carbocation intermediate and positions a water molecule for the hydroxylation reaction. Structural comparison with a naturally occurring α-bisabolol synthase reveals common active site features that direct α-bisabolol generation. In showing that EIZS can be redesigned to generate a sesquiterpene alcohol product instead of a sesquiterpene hydrocarbon product, we have expanded the potential of EIZS as a platform for the development of designer cyclases that could be utilized in synthetic biology applications.

Structure of the prenyltransferase in bifunctional copalyl diphosphate synthase from Penicillium fellutanum reveals an open hexamer conformation.

Copalyl diphosphate synthase from Penicillium fellutanum (PfCPS) is an assembly-line terpene synthase that contains both prenyltransferase and class II cyclase activities. The prenyltransferase catalyzes processive chain elongation reactions using dimethylallyl diphosphate and three equivalents of isopentenyl diphosphate to yield geranylgeranyl diphosphate, which is then utilized as a substrate by the class II cyclase domain to generate copalyl diphosphate. Here, we report the 2.81 Å-resolution cryo-EM structure of the hexameric prenyltransferase of full-length PfCPS, which is surrounded by randomly splayed-out class II cyclase domains connected by disordered polypeptide linkers. The hexamer can be described as a trimer of dimers; surprisingly, one of the three dimer-dimer interfaces is separated to yield an open hexamer conformation, thus breaking the D3 symmetry typically observed in crystal structures of other prenyltransferase hexamers such as wild-type human GGPP synthase (hGGPPS). Interestingly, however, an open hexamer conformation was previously observed in the crystal structure of D188Y hGGPPS, apparently facilitated by hexamer-hexamer packing in the crystal lattice. The cryo-EM structure of the PfCPS prenyltransferase hexamer is the first to reveal that an open conformation can be achieved even in the absence of a point mutation or interaction with another hexamer. Even though PfCPS octamers are not detected, we suggest that the open hexamer conformation represents an intermediate in the hexamer-octamer equilibrium for those prenyltransferases that do exhibit oligomeric heterogeneity.

Crystal structure of histone deacetylase 6 complexed with (R)-lipoic acid, an essential cofactor in central carbon metabolism.

The enzyme cofactor (R)-lipoic acid plays a critical role in central carbon metabolism due to its catalytic function in the generation of acetyl-CoA, which links glycolysis with the tricarboxylic acid cycle. This cofactor is also essential for the generation of succinyl CoA within the tricarboxylic acid cycle. However, the biological functions of (R)-lipoic acid extend beyond metabolism owing to its facile redox chemistry. Most recently, the reduced form of (R)-lipoic acid, (R)-dihydrolipoic acid, has been shown to inhibit histone deacetylases (HDACs) with selectivity for the inhibition of HDAC6. Here, we report the 2.4 Å-resolution X-ray crystal structure of the complex between (R)-dihydrolipoic acid and HDAC6 catalytic domain 2 from Danio rerio, and we report a dissociation constant (KD) of 350 nM for this complex as determined by isothermal titration calorimetry. The crystal structure illuminates key affinity determinants in the enzyme active site, including thiolate-Zn2+ coordination and S-π interactions in the F583-F643 aromatic crevice. This study provides the first visualization of the connection between HDAC function and the biological response to oxidative stress: the dithiol moiety of (R)-dihydrolipoic acid can serve as a redox-regulated pharmacophore capable of simultaneously targeting the catalytic Zn2+ ion and the aromatic crevice in the active site of HDAC6.

Structure and Function of Kdac1, a Class II Deacetylase from the Multidrug-Resistant Pathogen Acinetobacter baumannii.

Proteomics studies indicate that 10% of proteins in the opportunistic pathogen Acinetobacter baumannii are acetylated, suggesting that lysine acetyltransferases and deacetylases function to maintain and regulate a robust bacterial acetylome. As the first step in exploring these fascinating prokaryotic enzymes, we now report the preparation and characterization of the lysine deacetylase Kdac1. We show that Kdac1 catalyzes the deacetylation of free acetyllysine and acetyllysine tetrapeptide assay substrates, and we also report the X-ray crystal structures of unliganded Kdac1 as well as its complex with the hydroxamate inhibitor Citarinostat. Kdac1 is a tetramer in solution and in the crystal; the crystal structure reveals that the L1 loop functions to stabilize quaternary structure, forming inter-subunit hydrogen bonds and salt bridges around a central arginine residue (R30). Surprisingly, the L1 loop partially blocks entry to the active site, but it is sufficiently flexible to allow for the binding of two Citarinostat molecules in the active site. The L12 loop is also important for maintaining quaternary structure; here, a conserved arginine (R278) accepts hydrogen bonds from the backbone carbonyl groups of residues in an adjacent monomer. Structural comparisons with two other prokaryotic lysine deacetylases reveal conserved residues in the L1 and L12 loops that similarly support tetramer assembly. These studies provide a structural foundation for understanding enzymes that regulate protein function in bacteria through reversible lysine acetylation, serving as a first step in the exploration of these enzymes as possible targets for the development of new antibiotics.

Reprogramming the Cyclization Cascade of epi-Isozizaene Synthase to Generate Alternative Terpene Products.

The class I sesquiterpene cyclase epi-isozizaene synthase from Streptomyces coelicolor (EIZS) catalyzes the transformation of linear farnesyl diphosphate (FPP) into the tricyclic hydrocarbon epi-isozizaene in the biosynthesis of albaflavenone antibiotics. The active site cavity of EIZS is largely framed by four aromatic residues - F95, F96, F198, and W203 - that form a product-shaped contour, serving as a template to chaperone conformations of the flexible substrate and multiple carbocation intermediates leading to epi-isozizaene. Remolding the active site contour by mutagenesis can redirect the cyclization cascade away from epi-isozizaene biosynthesis to generate alternative sesquiterpene products. Here, we present the biochemical and structural characterization of four EIZS mutants in which aromatic residues have been substituted with polar residues (F95S, F96H, F198S, and F198T) to generate alternative cyclization products. Most notably, F95S EIZS generates a mixture of monocyclic sesquiterpene precursors of bisabolane, a D2 diesel fuel substitute. X-ray crystal structures of the characterized mutants reveal subtle changes in the active site contour showing how each aromatic residue influences the chemistry of a different carbocation intermediate in the cyclization cascade. We advance that EIZS may serve as a robust platform for the development of designer cyclases for the generation of high-value sesquiterpene products ranging from pharmaceuticals to biofuels in synthetic biology approaches.

Transient Prenyltransferase-Cyclase Association in Fusicoccadiene Synthase, an Assembly-Line Terpene Synthase.

Fusicoccadiene synthase from the fungus Phomopsis amygdali (PaFS) is an assembly-line terpene synthase that catalyzes the first two steps in the biosynthesis of Fusiccocin A, a diterpene glycoside. The C-terminal prenyltransferase domain of PaFS catalyzes the condensation of one molecule of C5 dimethylallyl diphosphate and three molecules of C5 isopentenyl diphosphate to form C20 geranylgeranyl diphosphate, which then transits to the cyclase domain for cyclization to form fusicoccadiene. Previous structural studies of PaFS using electron microscopy (EM) revealed a central octameric prenyltransferase core with eight cyclase domains tethered in random distal positions through flexible 70-residue linkers. However, proximal prenyltransferase-cyclase configurations could be captured by covalent cross-linking and observed by cryo-EM and mass spectrometry. Here, we use cryo-EM to show that proximally configured prenyltransferase-cyclase complexes are observable even in the absence of covalent cross-linking; moreover, such complexes can involve multiple cyclase domains. A conserved basic patch on the prenyltransferase domain comprises the primary touchpoint with the cyclase domain. These results support a model for transient prenyltransferase-cyclase association in which the cyclase domains of PaFS are in facile equilibrium between proximal associated and random distal positions relative to the central prenyltransferase octamer. The results of biophysical measurements using small-angle X-ray scattering, analytical ultracentrifugation, dynamic light scattering, and size-exclusion chromatography in-line with multi-angle light scattering are consistent with this model. This model accordingly provides a framework for understanding substrate transit between the prenyltransferase and cyclase domains as well as the cooperativity observed for geranylgeranyl diphosphate cyclization.

Structure of the Repurposed Fungal Terpene Cyclase FlvF Implicated in the C-N Bond-Forming Reaction of Flavunoidine Biosynthesis.

The fungal species Aspergillus flavus produces an alkaloid terpenoid, flavunoidine, through a hybrid biosynthetic pathway combining both terpene cyclase and nonribosomal peptide synthetase enzymes. Flavunoidine consists of a tetracyclic, oxygenated sesquiterpene core decorated with dimethyl cadaverine and 5,5-dimethyl-l-pipecolate moieties. Unique to the flavunoidine biosynthetic pathway is FlvF, a putative enzyme implicated in stereospecific C-N bond formation as dimethyl cadaverine is linked to the sesquiterpene core to generate pre-flavunoidine. Here, we report the 2.6 Å resolution crystal structure of FlvF, which adopts the α-helical fold of a class I terpene synthase. However, FlvF is not a terpene synthase, as indicated by its lack of enzymatic activity with farnesyl diphosphate and its lack of signature metal ion binding motifs that would coordinate to catalytic Mg2+ ions. Thus, FlvF is the first example of a protein that adopts a terpene synthase fold but is not a terpene synthase. Two Bis-Tris molecules bind in the active site of FlvF, and the binding of these ligands guided the docking of pre-flavunoidine to generate a model of the enzyme-product complex. Phylogenetic analysis of FlvF and related fungal homologues reveals conservation of residues that interact with the tetracyclic sesquiterpene in this model, but less conservation of residues interacting with the pendant amino moiety. This may hint toward the possibility that alternative amino substrates can be linked to a common sesquiterpene core by FlvF homologues to generate flavunoidine congeners, such as the phospholipase C inhibitor hispidospermidin.

Structural Basis of Substrate Promiscuity and Catalysis by the Reverse Prenyltransferase N-Dimethylallyl-l-tryptophan Synthase from Fusarium fujikuroi.

The regiospecific prenylation of an aromatic amino acid catalyzed by a dimethylallyl-l-tryptophan synthase (DMATS) is a key step in the biosynthesis of many fungal and bacterial natural products. DMATS enzymes share a common "ABBA" fold with divergent active site contours that direct alternative C-C, C-N, and C-O bond-forming trajectories. DMATS1 from Fusarium fujikuroi catalyzes the reverse N-prenylation of l-Trp by generating an allylic carbocation from dimethylallyl diphosphate (DMAPP) that then alkylates the indole nitrogen of l-Trp. DMATS1 stands out among the greater DMATS family because it exhibits unusually broad substrate specificity: it can utilize geranyl diphosphate (GPP) or l-Tyr as an alternative prenyl donor or acceptor, respectively; it can catalyze both forward and reverse prenylation, i.e., at C1 or C3 of DMAPP; and it can catalyze C-N and C-O bond-forming reactions. Here, we report the crystal structures of DMATS1 and its complexes with l-Trp or l-Tyr and unreactive thiolodiphosphate analogues of the prenyl donors DMAPP and GPP. Structures of ternary complexes mimic Michaelis complexes with actual substrates and illuminate active site features that govern prenylation regiochemistry. Comparison with CymD, a bacterial enzyme that catalyzes the reverse N-prenylation of l-Trp with DMAPP, indicates that bacterial and fungal DMATS enzymes share a conserved reaction mechanism. However, the narrower active site contour of CymD enforces narrower substrate specificity. Structure-function relationships established for DMATS enzymes will ultimately inform protein engineering experiments that will broaden the utility of these enzymes as useful tools for synthetic biology.

Aromatic Ring Fluorination Patterns Modulate Inhibitory Potency of Fluorophenylhydroxamates Complexed with Histone Deacetylase 6.

Bavarostat (EKZ-001) is a selective inhibitor of histone deacetylase 6 (HDAC6) that contains a meta-fluorophenylhydroxamate Zn2+-binding group. The recently determined crystal structure of its complex with HDAC6 from Danio rerio (zebrafish) revealed that the meta-fluoro substituent binds exclusively in an aromatic crevice defined by F583 and F643 rather than being oriented out toward solvent. To explore the binding of inhibitor C-F groups in this fluorophilic crevice, we now report a series of 10 simple fluorophenylhydroxamates bearing one or more fluorine atoms with different substitution patterns. Inhibitory potencies against human and zebrafish HDAC6 range widely from 121 to >30,000 nM. The best inhibitory potency is measured for meta-difluorophenylhydroxamate (5) with IC50 = 121 nM against human HDAC6; the worst inhibitory potencies are measured for ortho-fluorophenylhydroxamate (1) as well as fluorophenylhydroxamates 4, 7, 9, and 10, although there are some variations in activity trends against human and zebrafish HDAC6. These studies show that aromatic ring fluorination at the meta position(s) does not improve inhibitory activity against human HDAC6 relative to the nonfluorinated parent compound phenylhydroxamate (IC50 = 120 nM), but meta-fluorination does not seriously compromise inhibitory activity either. Crystal structures of selected zebrafish HDAC6-fluorophenylhydroxamate complexes reveal that the fluoroaromatic ring is uniformly accommodated in the F583-F643 aromatic crevice, so ring fluorination does not perturb the inhibitor binding conformation. However, hydroxamate-Zn2+ coordination is bidentate for some inhibitors and monodentate for others. These studies will inform design strategies underlying the design of 18F-labeled HDAC6 inhibitors intended for positron emission tomography.

Aza-SAHA Derivatives Are Selective Histone Deacetylase 10 Chemical Probes That Inhibit Polyamine Deacetylation and Phenocopy HDAC10 Knockout.

We report the first well-characterized selective chemical probe for histone deacetylase 10 (HDAC10) with unprecedented selectivity over other HDAC isozymes. HDAC10 deacetylates polyamines and has a distinct substrate specificity, making it unique among the 11 zinc-dependent HDAC hydrolases. Taking inspiration from HDAC10 polyamine substrates, we systematically inserted an amino group ("aza-scan") into the hexyl linker moiety of the approved drug Vorinostat (SAHA). This one-atom replacement (C→N) transformed SAHA from an unselective pan-HDAC inhibitor into a specific HDAC10 inhibitor. Optimization of the aza-SAHA structure yielded the HDAC10 chemical probe DKFZ-748, with potency and selectivity demonstrated by cellular and biochemical target engagement, as well as thermal shift assays. Cocrystal structures of our aza-SAHA derivatives with HDAC10 provide a structural rationale for potency, and chemoproteomic profiling confirmed exquisite cellular HDAC10-selectivity of DKFZ-748 across the target landscape of HDAC drugs. Treatment of cells with DKFZ-748, followed by quantification of selected polyamines, validated for the first time the suspected cellular function of HDAC10 as a polyamine deacetylase. Finally, in a polyamine-limiting in vitro tumor model, DKFZ-748 showed dose-dependent growth inhibition of HeLa cells. We expect DKFZ-748 and related probes to enable further studies on the enigmatic biology of HDAC10 and acetylated polyamines in both physiological and pathological settings.

First Fluorescent Acetylspermidine Deacetylation Assay for HDAC10 Identifies Selective Inhibitors with Cellular Target Engagement.

Histone deacetylases (HDACs) are important epigenetic regulators involved in many diseases, especially cancer. Five HDAC inhibitors have been approved for anticancer therapy and many are in clinical trials. Among the 11 zinc-dependent HDACs, HDAC10 has received relatively little attention by drug discovery campaigns, despite its involvement, e. g., in the pathogenesis of neuroblastoma. This is due in part to a lack of robust enzymatic conversion assays. In contrast to the protein lysine deacetylase and deacylase activity of most other HDAC subtypes, it has recently been shown that HDAC10 has strong preferences for deacetylation of oligoamine substrates like acetyl-putrescine or -spermidine. Hence, it is also termed a polyamine deacetylase (PDAC). Here, we present the first fluorescent enzymatic conversion assay for HDAC10 using an aminocoumarin-labelled acetyl-spermidine derivative to measure its PDAC activity, which is suitable for high-throughput screening. Using this assay, we identified potent inhibitors of HDAC10-mediated spermidine deacetylation in vitro. Based on the oligoamine preference of HDAC10, we also designed inhibitors with a basic moiety in appropriate distance to the zinc binding hydroxamate that showed potent inhibition of HDAC10 with high selectivity, and we solved a HDAC10-inhibitor structure using X-ray crystallography. We could demonstrate selective cellular target engagement for HDAC10 but a lysosomal phenotype in neuroblastoma cells that was previously associated with HDAC10 inhibition was not observed. Thus, we have developed new chemical probes for HDAC10 that allow further clarification of the biological role of this enzyme.

Assembly-Line Catalysis in Bifunctional Terpene Synthases.

The magnificent chemodiversity of more than 95 000 terpenoid natural products identified to date largely originates from catalysis by two types of terpene synthases, prenyltransferases and cyclases. Prenyltransferases utilize 5-carbon building blocks in processive chain elongation reactions to generate linear C5n isoprenoid diphosphates (n ≥ 2), which in turn serve as substrates for terpene cyclases that convert these linear precursors into structurally complex hydrocarbon products containing multiple rings and stereocenters. Terpene cyclization reactions are the most complex organic transformations found in nature in that more than half of the substrate carbon atoms undergo changes in chemical bonding during a multistep reaction sequence proceeding through several carbocation intermediates. Two general classes of cyclases are established on the basis of the chemistry of initial carbocation formation, and structural studies from our laboratory and others show that three fundamental protein folds designated α, ß, and γ govern this chemistry. Catalysis by a class I cyclase occurs in an α domain, where a trinuclear metal cluster activates the substrate diphosphate leaving group to generate an allylic cation. Catalysis by a class II cyclase occurs in a ß domain or at the interface of ß and γ domains, where an aspartic acid protonates the terminal π bond of the substrate to yield a tertiary carbocation. Crystal structures reveal domain architectures of α, αß, αßγ, ßγ, and ß.In some terpene synthases, these domains are combined to yield bifunctional enzymes that catalyze successive biosynthetic steps in assembly line fashion. Structurally characterized examples include bacterial geosmin synthase, an αα domain enzyme that catalyzes a class I cyclization reaction of C15 farnesyl diphosphate in one active site and a transannulation-fragmentation reaction in the other to yield C12 geosmin and C3 acetone products. In comparison, plant abietadiene synthase is an αßγ domain enzyme in which C20 geranylgeranyl diphosphate undergoes tandem class II-class I cyclization reactions to yield the tricyclic product. Recent structural studies from our laboratory show that bifunctional fungal cyclases form oligomeric complexes for assembly line catalysis. Bifunctional (+)-copalyl diphosphate synthase adopts (αßγ)6 architecture in which the α domain generates geranylgeranyl diphosphate, which then undergoes class II cyclization in the ßγ domains to yield the bicyclic product. Bifunctional fusicoccadiene synthase adopts (αα)6 or (αα)8 architecture in which one α domain generates geranylgeranyl diphosphate, which then undergoes class I cyclization in the other α domain to yield the tricyclic product. The prenyltransferase α domain mediates oligomerization in these systems. Attached by flexible polypeptide linkers, cyclase domains splay out from oligomeric prenyltransferase cores.In this Account, we review structure-function relationships for these bifunctional terpene synthases, with a focus on the oligomeric systems studied in our laboratory. The observation of substrate channeling for fusicoccadiene synthase suggests a model for dynamic cluster channeling in catalysis by oligomeric assembly line terpenoid synthases. Resulting efficiencies in carbon management suggest that such systems could be particularly attractive for use in synthetic biology approaches to generate high-value terpenoid natural products.

X-ray Crystallographic Snapshots of Substrate Binding in the Active Site of Histone Deacetylase 10.

Histone deacetylase 10 (HDAC10) is a zinc-dependent polyamine deacetylase enriched in the cytosol of eukaryotic cells. The active site of HDAC10 contains catalytic residues conserved in other HDAC isozymes that function as lysine deacetylases: Y307 assists the zinc ion in polarizing the substrate carbonyl for nucleophilic attack, and the H136-H137 dyad serves general base-general acid functions. As an inducer of autophagy, HDAC10 is an attractive target for the design of selective inhibitors that may be useful in cancer chemotherapy. Because detailed structural information regarding the catalytic mechanism of HDAC10 may inform new approaches to inhibitor design, we now report X-ray crystal structures of HDAC10 in which reaction intermediates with substrates N8-acetylspermidine and N-acetylputrescine are trapped in the active site. The Y307F substitution prevents activation of the substrate carbonyl for nucleophilic attack by the zinc-bound water molecule, thereby enabling crystallographic isolation of intact enzyme-substrate complexes. The H137A substitution removes the catalytically obligatory general acid, thereby enabling crystallographic isolation of oxyanionic tetrahedral intermediates. Finally, the acetate complex with the wild-type enzyme represents a product complex after dissociation of the polyamine coproduct. Taken together, these structures provide snapshots of the reaction coordinate of acetylpolyamine hydrolysis and are consistent with a mechanism in which tandem histidine residues H136 and H137 serve as general base and general acid catalysts, respectively. The function of the histidine dyad in the HDAC10 mechanism appears to be similar to that in HDAC6, but not HDAC8 in which both functions are served by the second histidine of the tandem pair.

Engineering the Prenyltransferase Domain of a Bifunctional Assembly-Line Terpene Synthase.

Copalyl diphosphate (CPP) synthase from Penicillium verruculosum (PvCPS) is a bifunctional diterpene synthase with both prenyltransferase and class II cyclase activities. The prenyltransferase α domain catalyzes the condensation of C5 dimethylallyl diphosphate with three successively added C5 isopentenyl diphosphates (IPPs) to form C20 geranylgeranyl diphosphate (GGPP), which then undergoes a class II cyclization reaction at the ßγ domain interface to generate CPP. The prenyltransferase α domain mediates oligomerization to form a 648-kD (αßγ)6 hexamer. In the current study, we explore prenyltransferase structure-function relationships in this oligomeric assembly-line platform with the goal of generating alternative linear isoprenoid products. Specifically, we report steady-state enzyme kinetics, product analysis, and crystal structures of various site-specific variants of the prenyltransferase α domain. Crystal structures of the H786A, F760A, S723Y, S723F, and S723T variants have been determined at resolutions of 2.80, 3.10, 3.15, 2.65, and 2.00 Å, respectively. The substitution of S723 with bulky aromatic amino acids in the S723Y and S723F variants constricts the active site, thereby directing the formation of the shorter C15 isoprenoid, farnesyl diphosphate. While the S723T substitution only subtly alters enzyme kinetics and does not compromise GGPP biosynthesis, the crystal structure of this variant reveals a nonproductive binding mode for IPP that likely accounts for substrate inhibition at high concentrations. Finally, mutagenesis of the catalytic general acid in the class II cyclase domain, D313A, significantly compromises prenyltransferase activity. This result suggests molecular communication between the prenyltransferase and cyclase domains despite their distant connection by a flexible polypeptide linker.

Visualizing transiently associated catalytic domains in assembly-line biosynthesis using cryo-electron microscopy.

While cryo-electron microscopy (cryo-EM) has revolutionized the structure determination of supramolecular protein complexes that are refractory to structure determination by X-ray crystallography, structure determination by cryo-EM can nonetheless be complicated by excessive conformational flexibility or structural heterogeneity resulting from weak or transient protein-protein association. Since such transient complexes are often critical for function, specialized approaches must be employed for the determination of meaningful structure-function relationships. Here, we outline examples in which transient protein-protein interactions have been visualized successfully by cryo-EM in the biosynthesis of fatty acids, polyketides, and terpenes. These studies demonstrate the utility of chemical crosslinking to stabilize transient protein-protein complexes for cryo-EM structural analysis, as well as the use of partial signal subtraction and localized reconstruction to extract useful structural information out of cryo-EM data collected from inherently dynamic systems. While these approaches do not always yield atomic resolution insights on protein-protein interactions, they nonetheless enable direct experimental observation of complexes in assembly-line biosynthesis that would otherwise be too fleeting for structural analysis.

Structural analysis of histone deacetylase 8 mutants associated with Cornelia de Lange Syndrome spectrum disorders.

Cornelia de Lange Syndrome (CdLS) and associated spectrum disorders are characterized by one or more congenital anomalies including distinctive facial features, upper limb abnormalities, intellectual disability, and other symptoms. The molecular genetic basis of CdLS is linked to defects in cohesin, a protein complex that functions in sister chromatid cohesion, chromatin organization, and transcriptional regulation. Histone deacetylase 8 (HDAC8) plays an important role in cohesin function by catalyzing the deacetylation of SMC3, which is required for efficient recycling of the cohesin complex. Missense mutations in HDAC8 have been identified in children diagnosed with CdLS spectrum disorders, and here we outline structure-function relationships for four of these mutations. Specifically, we report the 1.50 Å-resolution structure of the I45T HDAC8-suberoylanilide hydroxamic acid complex, the 1.84 Å-resolution structure of E66D/Y306F HDAC8 complexed with a peptide assay substrate, and the 2.40 Å-resolution structure of G320R HDAC8 complexed with the inhibitor M344. Additionally, we present a computationally generated model of D176G HDAC8. These structures illuminate new structure-function relationships for HDAC8 and highlight the importance of long-range interactions in the protein scaffold that can influence catalytic function.

An Aromatic Cluster in the Active Site of epi-Isozizaene Synthase Is an Electrostatic Toggle for Divergent Terpene Cyclization Pathways.

The sesquiterpene cyclase epi-isozizaene synthase (EIZS) catalyzes the cyclization of farnesyl diphosphate to form the tricyclic precursor of the antibiotic albaflavenone. The hydrophobic active site is largely defined by aromatic residues that direct a multistep reaction sequence through multiple carbocation intermediates. The previous substitution of polar residues for a key aromatic residue, F96, converts EIZS into a high-fidelity sesquisabinene synthase: the F96S, F96M, and F96Q variants generate 78%, 91%, and 97% sesquisabinene A, respectively. Here, we report high-resolution X-ray crystal structures of two of these reprogrammed cyclases. The structures of the F96M EIZS-Mg2+3-risedronate and F96M EIZS-Mg2+3-inorganic pyrophosphate-benzyltriethylammonium cation complexes reveal structural changes in the F96 aromatic cluster that redirect the cyclization pathway leading from the bisabolyl carbocation intermediate in catalysis. The structure of the F96S EIZS-Mg2+3-neridronate complex reveals a partially occupied inhibitor and an enzyme active site caught in transition between open and closed states. Finally, three structures of wild-type EIZS complexed with the bisphosphonate inhibitors neridronate, pamidronate, and risedronate provide a foundation for understanding binding differences between wild-type and variant enzymes. These structures provide new insight regarding active site flexibility, particularly with regard to the potential for subtle expansion and contraction to accommodate ligands of varying sizes as well as bound water molecules. Additionally, these structures highlight the importance of conformational changes in the F96 aromatic cluster that could influence cation-π interactions with carbocation intermediates in catalysis.

Higher-order oligomerization of a chimeric αßγ bifunctional diterpene synthase with prenyltransferase and class II cyclase activities is concentration-dependent.

The unusual diterpene (C20) synthase copalyl diphosphate synthase from Penicillium verruculosum (PvCPS) is the first bifunctional terpene synthase identified with both prenyltransferase and class II cyclase activities in a single polypeptide chain with αßγ domain architecture. The C-terminal prenyltransferase α domain generates geranylgeranyl diphosphate which is then cyclized to form copalyl diphosphate at the N-terminal ßγ domain interface. We now demonstrate that PvCPS exists as a hexamer at high concentrations - a unique quaternary structure for known αßγ terpene synthases. Hexamer assembly is corroborated by a 2.41 Å-resolution crystal structure of the α domain prenyltransferase obtained from limited proteolysis of full-length PvCPS, as well as the ab initio model of full-length PvCPS derived from small-angle X-ray scattering data. Hexamerization of the prenyltransferase α domain appears to drive the hexamerization of full-length PvCPS. The PvCPS hexamer dissociates into lower-order species at lower concentrations, as evidenced by size-exclusion chromatography in-line with multiangle light scattering, sedimentation velocity analytical ultracentrifugation, and native polyacrylamide gel electrophoresis experiments, suggesting that oligomerization is concentration dependent. Even so, PvCPS oligomer assembly does not affect prenyltransferase activity in vitro.

Structural determinants of affinity and selectivity in the binding of inhibitors to histone deacetylase 6.

Histone deacetylase 6 (HDAC6) is associated with multiple neurological disorders as well as aggressive cancers, making its selective inhibition highly desirable for therapeutic purposes. The basic molecular design of an effective HDAC6 inhibitor consists of a zinc-binding group, a linker, and a capping group capable of making interactions at the mouth of the active site. To date, more than 50 high-resolution X-ray crystal structures of HDAC6-inhibitor complexes have been reported, many of which reveal intermolecular interactions that contribute to isozyme affinity and selectivity. Here, we review the key features of HDAC6 inhibitor design illuminated by these structural studies.