Spencer Knapp

Crystal structure of Human beta-hexosaminidase B: Understanding the molecular basis of Sandhoff and Tay-Sachs disease

In humans, two major beta-hexosaminidase isoenzymes exist: Hex A and Hex B. Hex A is a heterodimer of subunits alpha and beta (60% identity), whereas Hex B is a homodimer of beta-subunits. Interest in human beta-hexosaminidase stems from its association with Tay-Sachs and Sandhoff disease; these are prototypical lysosomal storage disorders resulting from the abnormal accumulation of G(M2)-ganglioside (G(M2)). Hex A degrades G(M2) by removing a terminal N-acetyl-D-galactosamine (beta-GalNAc) residue, and this activity requires the G(M2)-activator, a protein which solubilizes the ganglioside for presentation to Hex A. We present here the crystal structure of human Hex B, alone (2.4A) and in complex with the mechanistic inhibitors GalNAc-isofagomine (2.2A) or NAG-thiazoline (2.5A). From these, and the known X-ray structure of the G(M2)-activator, we have modeled Hex A in complex with the activator and ganglioside. Together, our crystallographic and modeling data demonstrate how alpha and beta-subunits dimerize to form either Hex A or Hex B, how these isoenzymes hydrolyze diverse substrates, and how many documented point mutations cause Sandhoff disease (beta-subunit mutations) and Tay-Sachs disease (alpha-subunit mutations).

A Mitotic GlcNAcylation/Phosphorylation Signaling Complex Alters the Posttranslational State of the Cytoskeletal Protein Vimentin

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic intracellular protein modification responsive to stress, hormones, nutrients, and cell cycle stage. Alterations in O-GlcNAc addition or removal (cycling) impair cell cycle progression and cytokinesis, but the mechanisms are not well understood. Here, we demonstrate that the enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are in a transient complex at M phase with the mitotic kinase Aurora B and protein phosphatase 1. OGT colocalized to the midbody during telophase with Aurora B. Furthermore, these proteins coprecipitated with each other in a late mitotic extract. The complex was stable under Aurora inhibition; however, the total cellular levels of O-GlcNAc were increased and the localization of OGT was decreased at the midbody after Aurora inhibition. Vimentin, an intermediate filament protein, is an M phase substrate for both Aurora B and OGT. Overexpression of OGT or OGA led to defects in mitotic phosphorylation on multiple sites, whereas OGT overexpression increased mitotic GlcNAcylation of vimentin. OGA inhibition caused a decrease in vimentin late mitotic phosphorylation but increased GlcNAcylation. Together, these data demonstrate that the O-GlcNAc cycling enzymes associate with kinases and phosphatases at M phase to regulate the posttranslational status of vimentin.

(+)-SJ733, a clinical candidate for malaria that acts through ATP4 to induce rapid host-mediated clearance of Plasmodium

Significance Useful antimalarial drugs must be rapidly acting, highly efficacious, and have low potential for developing resistance. (+)-SJ733 targets a Plasmodium cation-transporting ATPase, ATP4. (+)-SJ733 cleared parasites in vivo as quickly as artesunate by specifically inducing eryptosis/senescence in infected, treated erythrocytes. Although in vitro selection of pfatp4 mutants with (+)-SJ733 proceeded with moderate frequency, during in vivo selection of pbatp4 mutants, resistance emerged slowly and produced marginally resistant mutants with poor fitness. In addition, (+)-SJ733 met all other criteria for a clinical candidate, including high oral bioavailability, a high safety margin, and transmission blocking activity. These results demonstrate that targeting ATP4 has great potential to deliver useful drugs for malaria eradication. Drug discovery for malaria has been transformed in the last 5 years by the discovery of many new lead compounds identified by phenotypic screening. The process of developing these compounds as drug leads and studying the cellular responses they induce is revealing new targets that regulate key processes in the Plasmodium parasites that cause malaria. We disclose herein that the clinical candidate (+)-SJ733 acts upon one of these targets, ATP4. ATP4 is thought to be a cation-transporting ATPase responsible for maintaining low intracellular Na+ levels in the parasite. Treatment of parasitized erythrocytes with (+)-SJ733 in vitro caused a rapid perturbation of Na+ homeostasis in the parasite. This perturbation was followed by profound physical changes in the infected cells, including increased membrane rigidity and externalization of phosphatidylserine, consistent with eryptosis (erythrocyte suicide) or senescence. These changes are proposed to underpin the rapid (+)-SJ733-induced clearance of parasites seen in vivo. Plasmodium falciparum ATPase 4 (pfatp4) mutations that confer resistance to (+)-SJ733 carry a high fitness cost. The speed with which (+)-SJ733 kills parasites and the high fitness cost associated with resistance-conferring mutations appear to slow and suppress the selection of highly drug-resistant mutants in vivo. Together, our data suggest that inhibitors of PfATP4 have highly attractive features for fast-acting antimalarials to be used in the global eradication campaign.

Intramolecular amino delivery reactions for the synthesis of valienamine and analogues

Abstract Iodocyclization and [3,3] sigmatropic rearrangement reactions of N -substituted carbonimidothioates are used to prepare valienamine ( 1 ), 7- nor -valienamine ( 8 ), and the valienamine-based pseudo -disaccharide 12 .

Radical based annulations of iodo lactams

Abstract N-Alkylation of iodo lactams with reactive alkyl halides, or of the derived selenide lactams with less reactive alkyl halides, leads to substrates for free radical initiated cyclization to pyrrolizidinones and indolizidinones, e. g. 2 → 3 . The first examples of iodide / vinyl bromide, selenide / aldehyde, and selenide / vinyl chloride radical cyclizations are described.

STEREOSELECTIVE SYNTHESIS OF PENARESIDIN A AND RELATED AZETIDINE ALKALOIDS

Several sphingosine-like azetidine alkaloids have been synthesized from the Garner aldehyde 5, including the ATPase activator penaresidin A (1a), its side chain isomer 1b, and the 16-nor analogue (3b) of the protein kinase C inhibitor penazetidine A 3a.

A procedure for “iodolactamization”

Abstract Treatment of an unsaturated amide with TMS-OTf, then iodine in THF gives the iodolactam.

Biomimetic Seleninates and Selenonates

The synthesis of a variety of pyranose-, nucleoside-, (amino acid)-, and polyhydric-based seleninic and selenonic acids by DMDO oxidation of the corresponding selenoesters is reported, as well as some unusual coupling reactions of the seleninate and selenonate functionality with biological nucleophilic groups (sulfhydryl, indole, phenol, imidazole, carboxamide) that are found in proteins and enzyme active sites.

A [3+2] annulation procedure for methylenecyclopentanes

Abstract A two step sequence converts certain cyclic enones to fused methylenecyclopentanes using the annulation reagent 2-chloromethyl-3-trimethylsilylpropene.

The tethered nitrogen in natural products synthesis

A variety of nitrogen-containing natural products, including aminosugars and aminocyclitols, have been synthesized by routes that feature the intramolecular delivery of a temporarily-tethered nitrogen nucleophile to an electrophilic site. This general tactic for amino group introduction frequently provides entropic advantages, as well as improved site selectivity and stereoselectivity, compared with the corresponding intermolecular approach. An occasional additional benefit is that the resulting cyclized products can be more easily manipulated toward the desired target than the corresponding free amino compounds. These aspects are illustrated in a discussion of several natural products syntheses from the author’s laboratory.‘Help from without is often enfeebling in its effects, but help from within invariably invigorates’Samuel Smiles, Self-Help, 1859