Proteolysis targeting chimeras (PROTACs) are heterobifunctional small molecules that simultaneously bind to a target protein and an E3 ligase, thereby leading to ubiquitination and subsequent degradation of the target. They present an exciting opportunity to modulate proteins in a manner independent of enzymatic or signaling activity. As such, they have recently emerged as an attractive mechanism to explore previously "undruggable" targets. Despite this interest, fundamental questions remain regarding the parameters most critical for achieving potency and selectivity. Here we employ a series of biochemical and cellular techniques to investigate requirements for efficient knockdown of Bruton's tyrosine kinase (BTK), a nonreceptor tyrosine kinase essential for B cell maturation. Members of an 11-compound PROTAC library were investigated for their ability to form binary and ternary complexes with BTK and cereblon (CRBN, an E3 ligase component). Results were extended to measure effects on BTK-CRBN cooperative interactions as well as in vitro and in vivo BTK degradation. Our data show that alleviation of steric clashes between BTK and CRBN by modulating PROTAC linker length within this chemical series allows potent BTK degradation in the absence of thermodynamic cooperativity.

Herpesviruses are the second leading cause of human viral diseases. Herpes Simplex Virus types 1 and 2 and Varicella-zoster virus produce neurotropic infections such as cutaneous and genital herpes, chickenpox, and shingles. Infections of a lymphotropic nature are caused by cytomegalovirus, HSV-6, HSV-7, and Epstein-Barr virus producing lymphoma, carcinoma, and congenital abnormalities. Yet another series of serious health problems are posed by infections in immunocompromised individuals. Common therapies for herpes viral infections employ nucleoside analogs, such as Acyclovir, and target the viral DNA polymerase, essential for viral DNA replication. Although clinically useful, this class of drugs exhibits a narrow antiviral spectrum, and resistance to these agents is an emerging problem for disease management. A better understanding of herpes virus replication will help the development of new safe and effective broad spectrum anti-herpetic drugs that fill an unmet need. Here, we present the first crystal structure of a herpesvirus polymerase, the Herpes Simplex Virus type 1 DNA polymerase, at 2.7 Å resolution. The structural similarity of this polymerase to other ␣ polymerases has allowed us to construct high confidence models of a replication complex of the polymerase and of Acyclovir as a DNA chain terminator. We propose a novel inhibition mechanism in which a representative of a series of non-nucleosidic viral polymerase inhibitors, the 4-oxo-dihydroquinolines, binds at the polymerase active site interacting non-covalently with both the polymerase and the DNA duplex.

Carbonic anhydrase II (CAII) is a zinc metalloenzyme that catalyzes the hydration of CO 2 to yield bicarbonate and a proton. N-(4-Sulfamylbenzoyl)benzylamine (SBB) is a tight-binding inhibitor of human CAII with K d ) 2.1 nM. Previous X-ray crystallographic work shows that the benzyl ring of SBB makes an edge-to-face interaction with Phe-131 in the enzyme active site. We have manipulated the electrostatics of this interaction by systematically substituting electronegative fluorine atoms for the benzyl ring hydrogens of SBB. Crystal structures of 10 enzyme-inhibitor complexes have been determined to atomic resolution. Analysis of these structures reveals that the main contributions to enzyme-inhibitor affinity can be approximated by a combination of dipole-induced dipole, dipole-quadrupole, and quadrupole-quadrupole interactions. Surprisingly, different electrostatic components dominate affinity in different enzyme-inhibitor pairs.

Through fragment-based drug design focused on engaging the active site of IRAK4 and leveraging three-dimensional topology in a ligand-efficient manner, a micromolar hit identified from a screen of a Pfizer fragment library was optimized to afford IRAK4 inhibitors with nanomolar potency in cellular assays. The medicinal chemistry effort featured the judicious placement of lipophilicity, informed by co-crystal structures with IRAK4 and optimization of ADME properties to deliver clinical candidate PF-06650833 (compound 40). This compound displays a 5-unit increase in lipophilic efficiency from the fragment hit, excellent kinase selectivity, and pharmacokinetic properties suitable for oral administration.

Several protein engineering approaches were combined to optimize the selectivity and activity of Vibrio fluvialis aminotransferase (Vfat) for the synthesis of (3S,5R)-ethyl 3-amino-5-methyloctanoate; a key intermediate in the synthesis of imagabalin, an advanced candidate for the treatment of generalized anxiety disorder. Starting from wild-type Vfat, which had extremely low activity catalyzing the desired reaction, we engineered an improved enzyme with a 60-fold increase in initial reaction velocity for transamination of (R)-ethyl 5-methyl 3-oxooctanoate to (3S,5R)-ethyl 3-amino-5-methyloctanoate. To achieve this, <450 variants were screened, which allowed accurate assessment of enzyme performance using a low-throughput ultra performance liquid chromatography assay. During the course of this work, crystal structures of Vfat wild type and an improved variant (Vfat variant r414) were solved and they are reported here for the first time. This work also provides insight into the critical residues for substrate specificity for the transamination of (R)-ethyl 5-methyl 3-oxooctanoate and structurally related β-ketoesters.

Dihydrofolate reductase (DHFR) is the enzyme responsible for the NADPH-dependent reduction of 5,6-dihydrofolate to 5,6,7,8-tetrahydrofolate, an essential cofactor in the synthesis of purines, thymidylate, methionine, and other key metabolites. Because of its importance in multiple cellular functions, DHFR has been the subject of much research targeting the enzyme with anticancer, antibacterial, and antimicrobial agents. Clinically used compounds targeting DHFR include methotrexate for the treatment of cancer and diaminopyrimidines (DAPs) such as trimethoprim (TMP) for the treatment of bacterial infections. DAP inhibitors of DHFR have been used clinically for >30 years and resistance to these agents has become widespread. Methicillin-resistant Staphylococcus aureus (MRSA), the causative agent of many serious nosocomial and community acquired infections, and other gram-positive organisms can show resistance to DAPs through mutation of the chromosomal gene or acquisition of an alternative DHFR termed ''S1 DHFR.'' To develop new therapies for health threats such as MRSA, it is important to understand the molecular basis of DAP resistance. Here, we report the crystal structure of the wild-type chromosomal DHFR from S. aureus in complex with NADPH and TMP. We have also solved the structure of the exogenous, TMP resistant S1 DHFR, apo and in complex with TMP. The structural and thermodynamic data point to important molecular differences between the two enzymes that lead to dramatically reduced affinity of DAPs to S1 DHFR. These differences in enzyme binding affinity translate into reduced antibacterial activity against strains of S. aureus that express S1 DHFR. Proteins 2009; 76:706-717. V V C 2009 Wiley-Liss, Inc.

antibiotic ͉ crystallography ͉ inhibition mechanism ͉ kinetics ͉ cell wall synthesis

Proline-rich tyrosine kinase 2 (PYK2) is a cytoplasmic, nonreceptor tyrosine kinase implicated in multiple signaling pathways. It is a negative regulator of osteogenesis and considered a viable drug target for osteoporosis treatment. The high-resolution structures of the human PYK2 kinase domain with different inhibitor complexes establish the conventional bilobal kinase architecture and show the conformational variability of the DFG loop. The basis for the lack of selectivity for the classical kinase inhibitor, PF-431396, within the FAK family is explained by our structural analyses. Importantly, the novel DFG-out conformation with two diarylurea inhibitors (BIRB796, PF-4618433) reveals a distinct subclass of non-receptor tyrosine kinases identifiable by the gatekeeper Met-502 and the unique hinge loop conformation of Leu-504. This is the first example of a leucine residue in the hinge loop that blocks the ATP binding site in the DFG-out conformation. Our structural, biophysical, and pharmacological studies suggest that the unique features of the DFG motif, including Leu-504 hinge-loop variability, can be exploited for the development of selective protein kinase inhibitors.Proline-rich tyrosine kinase 2 (PYK2) 2 and focal adhesion kinase (FAK) comprise the focal adhesion kinase subfamily of non-receptor tyrosine kinases. PYK2 and FAK are large multidomain proteins containing an N-terminal FERM domain, a central catalytic domain, and a C-terminal segment containing dual proline rich (PR) subdomains and a focal adhesion targeting (FAT) region (1, 2). While FAK is widely expressed, PYK2 expression is relatively restricted with highest levels in brain and the hematopoeitic system. Unlike FAK, optimal PYK2 activation is dependent on Ca 2ϩ mobilization. PYK2 (Ϫ/Ϫ) animals have been described previously, and develop normally (3, 4). Characterization of the immune system of PYK2(Ϫ/Ϫ) animals revealed the absence of marginal zone B-cells along with abnormal T-cell independent type II responses (4), and altered macrophage morphology, migration and signaling in response to cell attachment or chemokine treatment (3). These studies strengthen the link between PYK2 and signaling through chemokine and integrin receptors. In addition, PYK2(Ϫ/Ϫ) mice were shown to have increased susceptibility to diet-induced obesity and diabetes (5).Recently, the characterization of PYK2(Ϫ/Ϫ) mice showed a high bone mass phenotype resulting from increased osteogenesis and osteoblast activity. Using PYK2(Ϫ/Ϫ) mouse bone marrow cultures and hMSCs expressing a PYK2 shRNA, elimination or reduction of PYK2 protein levels resulted in significantly enhanced osteogeogenesis. Importantly, the daily administration of a pyrimidine-based PYK2 inhibitor, PF-431396, increased bone formation, and protected against bone loss in ovariectomized rats (6). PYK2(Ϫ/Ϫ) mice showed mild osteopetrosis which was attributed to the impairment in osteoclast function (7). Therefore, the high bone mass phenotype may result from both enhanced osteoblast and impaired osteocla...