Structure and Function of the Acetylpolyamine Amidohydrolase from the Deep Earth Halophile Marinobacter subterrani

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.

Properties and biotechnological applications of microbial deacetylase

Deacetylases, a class of enzymes that can catalyze the hydrolysis of acetylated substrates to remove the acetyl group, used in producing various products with high qualities, are one of the most influential industrial enzymes. These enzymes are highly specific, non-toxic, sustainable, and eco-friendly biocatalysts. Deacetylases and deacetylated compounds have been widely applicated in pharmaceuticals, medicine, food, and the environment. This review synthetically summarizes deacetylases' sources, characterizations, classifications, and applications. Moreover, the typical structural characteristics of deacetylases from different microbial sources are summarized. We also reviewed the deacetylase-catalyzed reactions for producing various deacetylated compounds, such as chitosan-oligosaccharide (COS), mycothiol, 7-aminocephalosporanic acid (7-ACA), glucosamines, amino acids, and polyamines. It is aimed to expound on the advantages and challenges of deacetylases in industrial applications. Moreover, it also serves perspectives on obtaining promising and innovative biocatalysts for enzymatic deacetylation. KEYPOINTS: • The fundamental properties of microbial deacetylases of various microorganisms are presented. • The biochemical characterizations, structures, and catalyzation mechanisms of microbial deacetylases are summarized. • The applications of microbial deacetylases in food, pharmaceutical, medicine, and the environment were discussed.

Functional polyamine metabolic enzymes and pathways encoded by the virosphere

Viruses produce more viruses by manipulating the metabolic and replication systems of their host cells. Many have acquired metabolic genes from ancestral hosts and use the encoded enzymes to subvert host metabolism. The polyamine spermidine is required for bacteriophage and eukaryotic virus replication, and herein, we have identified and functionally characterized diverse phage- and virus-encoded polyamine metabolic enzymes and pathways. These include pyridoxal 5'-phosphate (PLP)-dependent ornithine decarboxylase (ODC), pyruvoyl-dependent ODC and arginine decarboxylase (ADC), arginase, S-adenosylmethionine decarboxylase (AdoMetDC/speD), spermidine synthase, homospermidine synthase, spermidine N-acetyltransferase, and N-acetylspermidine amidohydrolase. We identified homologs of the spermidine-modified translation factor eIF5a encoded by giant viruses of the Imitervirales. Although AdoMetDC/speD is prevalent among marine phages, some homologs have lost AdoMetDC activity and have evolved into pyruvoyl-dependent ADC or ODC. The pelagiphages that encode the pyruvoyl-dependent ADCs infect the abundant ocean bacterium Candidatus Pelagibacter ubique, which we have found encodes a PLP-dependent ODC homolog that has evolved into an ADC, indicating that infected cells would contain both PLP- and pyruvoyl-dependent ADCs. Complete or partial spermidine or homospermidine biosynthetic pathways are found encoded in the giant viruses of the Algavirales and Imitervirales, and in addition, some viruses of the Imitervirales can release spermidine from the inactive N-acetylspermidine. In contrast, diverse phages encode spermidine N-acetyltransferase that can sequester spermidine into its inactive N-acetyl form. Together, the virome-encoded enzymes and pathways for biosynthesis and release or biochemical sequestration of spermidine or its structural analog homospermidine consolidate and expand evidence supporting an important and global role of spermidine in virus biology.

Selective Inhibitors of Histone Deacetylase 10 (HDAC-10)

Histone acetylation balance is one epigenetic mechanism controlling gene expression associated with disease progression. It has been observed that histone deacetylase 10 (HDAC-10) isozyme contributes to the chemotherapy resistance; in addition, the poor clinical outcome observed in patients with aggressive solid tumors, such as neuroblastoma, has been associated with its overexpression. Moreover, HDAC-10 selective inhibition suppresses the autophagic response, thus providing an improved risk-benefit profile compared to cytotoxic cancer chemotherapy drugs. On these bases, HDAC-10 is becoming an emerging target for drug design. Due to the rapid progress in the development of next-generation HDAC inhibitors, this review article aims to provide an overview on novel selective or dual HDAC-8/10 inhibitors, as new leads for cancer chemotherapy, able to avoid the severe side-effects of several actual approved "pan" HDAC inhibitors. A literature search was conducted in MedLine, PubMed, Caplus, SciFinder Scholar databases from 2015 to the present. Since the disclosure that the HDAC-6 inhibitor Tubastatin A was able to bind HDAC-10 efficiently, several related analogues were synthesized and tested. Both tricyclic (25-30) and bicyclic (31-42) derivatives were considered. The best pharmacological profile was shown by 36 (HDAC-10 pIC₅₀ = 8.4 and pIC₅₀ towards Class I HDACs from 5.2-6.4). In parallel, based on the evidence that high levels of HDAC-8 are a marker of poor prognosis in neuroblastoma treatment, dual HDAC-8/10 inhibitors were designed. The hydroxamic acid TH34 (HDAC-8 and 10 IC₅₀ = 1.9 µM and 7.7 µM, respectively) and the hybrid derivatives 46d, 46e and 46g were the most promising both in terms of potency and selectivity. Literature surveys indicate several structural requirements for inhibitory potency and selectivity towards HDAC-10, e.g., electrostatic and/or hydrogen bond interactions with E274 and complementarity to the P(E,A) CE motif helix.

Strategies To Design Selective Histone Deacetylase Inhibitors

This review classifies drug-design strategies successfully implemented in the development of histone deacetylase (HDAC) inhibitors, which have many applications including cancer treatment. Our focus is on especially demanded selective HDAC inhibitors and their structure-activity relationships in relation to corresponding protein structures. The main part of the paper is divided into six subsections each narrating how optimization of one of six structural features can influence inhibitor selectivity. It starts with the impact of the zinc binding group on selectivity, continues with the optimization of the linker placed in the substrate binding tunnel as well as the adjustment of the cap group interacting with the surface of the protein, and ends with the addition of groups targeting class-specific sub-pockets: the side-pocket-, lower-pocket- and foot-pocket-targeting groups. The review is rounded off with a conclusion and an outlook on the future of HDAC inhibitor design.

Structural Basis for the Selective Inhibition of HDAC10, the Cytosolic Polyamine Deacetylase

The cytosolic class IIb histone deacetylase HDAC10 is an emerging target for drug design. As an inducer of autophagy, its selective inhibition suppresses the autophagic response that otherwise attenuates the efficacy of cytotoxic cancer chemotherapy drugs. HDAC10 is a zinc-dependent polyamine deacetylase exhibiting maximal catalytic activity against N⁸-acetylspermidine. As revealed in the structure of Danio rerio (zebrafish) HDAC10, two conserved structural motifs direct this narrow substrate specificity: a 3₁₀ helix containing the P(E,A)CE motif that sterically constricts the active site and an electrostatic "gatekeeper," E274, that confers selectivity for cationic polyamine substrates. To accelerate drug design efforts targeting human HDAC10, we now report the preparation of "humanized" zebrafish HDAC10 in which two amino acid substitutions, A24E and D94A, yield an active site contour more similar to that of human HDAC10. X-ray crystal structures of this HDAC10 variant complexed with Tubastatin A and indole analogues bearing pendant tertiary amines reveal that inhibitors capable of hydrogen bonding with gatekeeper E274 exhibit high affinity and selectivity for HDAC10 over HDAC6 (the other class IIb isozyme). Moreover, these structures reveal that the P(E,A)CE motif helix can shift by up to 2 Å to accommodate the binding of bulky inhibitors. Thus, slender polyamine-like inhibitor structures are not exclusively required for selective, high affinity binding to HDAC10. Indeed, the flexibility of the P(E,A)CE motif helix could conceivably enable the binding of certain protein substrates.

Binding of N8-Acetylspermidine Analogues to Histone Deacetylase 10 Reveals Molecular Strategies for Blocking Polyamine Deacetylation

Eukaryotic histone deacetylase 10 (HDAC10) is a Zn²⁺-dependent hydrolase that exhibits catalytic specificity for the hydrolysis of the polyamine N⁸-acetylspermidine. The recently determined crystal structure of HDAC10 from Danio rerio (zebrafish) reveals a narrow active site cleft and a negatively charged "gatekeeper" (E274) that favors the binding of the slender cationic substrate. Because HDAC10 expression is upregulated in advanced-stage neuroblastoma and induces autophagy, the selective inhibition of HDAC10 suppresses the autophagic response and renders cancer cells more susceptible to cytotoxic chemotherapeutic drugs. Here, we describe X-ray crystal structures of zebrafish HDAC10 complexed with eight different analogues of N⁸-acetylspermidine. These analogues contain different Zn²⁺-binding groups, such as hydroxamate, thiolate, and the tetrahedral gem-diolate resulting from the addition of a Zn²⁺-bound water molecule to a ketone carbonyl group. Notably, the chemistry that accompanies the binding of ketonic substrate analogues is identical to the chemistry involved in the first step of catalysis, i.e., nucleophilic attack of a Zn²⁺-bound water molecule at the scissile carbonyl group of N⁸-acetylspermidine. The most potent inhibitor studied contains a thiolate Zn²⁺-binding group. These structures reveal interesting geometric changes in the metal coordination polyhedron that accommodate inhibitor binding. Additional interactions in the active site highlight features contributing to substrate specificity. These interactions are likely to contribute to inhibitor binding selectivity and will inform the future design of compounds selective for HDAC10 inhibition.