Three-dimensional Structure of a Kunitz-type Inhibitor in Complex with an Elastase-like Enzyme.

Elastase-like enzymes are involved in important diseases such as acute pancreatitis, chronic inflammatory lung diseases, and cancer. Structural insights into their interaction with specific inhibitors will contribute to the development of novel anti-elastase compounds that resist rapid oxidation and proteolysis. Proteinaceous Kunitz-type inhibitors homologous to the bovine pancreatic trypsin inhibitor (BPTI) provide a suitable scaffold, but the structural aspects of their interaction with elastase-like enzymes have not been elucidated. Here, we increased the selectivity of ShPI-1, a versatile serine protease inhibitor from the sea anemone Stichodactyla helianthus with high biomedical and biotechnological potential, toward elastase-like enzymes by substitution of the P1 residue (Lys(13)) with leucine. The variant (rShPI-1/K13L) exhibits a novel anti-porcine pancreatic elastase (PPE) activity together with a significantly improved inhibition of human neuthrophil elastase and chymotrypsin. The crystal structure of the PPE·rShPI-1/K13L complex determined at 2.0 Å resolution provided the first details of the canonical interaction between a BPTI-Kunitz-type domain and elastase-like enzymes. In addition to the essential impact of the variant P1 residue for complex stability, the interface is improved by increased contributions of the primary and secondary binding loop as compared with similar trypsin and chymotrypsin complexes. A comparison of the interaction network with elastase complexes of canonical inhibitors from the chelonian in family supports a key role of the P3 site in ShPI-1 in directing its selectivity against pancreatic and neutrophil elastases. Our results provide the structural basis for site-specific mutagenesis to further improve the binding affinity and/or direct the selectivity of BPTI-Kunitz-type inhibitors toward elastase-like enzymes.

Two variants of the major serine protease inhibitor from the sea anemone Stichodactyla helianthus, expressed in Pichia pastoris.

The major protease inhibitor from the sea anemone Stichodactyla helianthus (ShPI-1) is a non-specific inhibitor that binds trypsin and other trypsin-like enzymes, as well as chymotrypsin, and human neutrophil elastase. We performed site-directed mutagenesis of ShPI-1 to produce two variants (rShPI-1/K13L and rShPI/Y15S) that were expressed in Pichia pastoris, purified, and characterized. After a single purification step, 65 mg and 15 mg of protein per liter of culture supernatant were obtained for rShPI-1/K13L and rShPI/Y15S, respectively. Functional studies demonstrated a 100-fold decreased trypsin inhibitory activity as result of the K13L substitution at the reactive (P1) site. This protein variant has a novel tight-binding inhibitor activity of pancreatic elastase and increased activity toward neutrophil elastase in comparison to rShPI-1A. In contrast, the substitution Y15S at P2' site did not affect the Ki value against trypsin, but did reduce activity 10-fold against chymotrypsin and neutrophil elastase. Our results provide two new ShPI-1 variants with modified inhibitory activities, one of them with increased biomedical potential. This study also offers new insight into the functional impact of the P1 and P2' sites on ShPI-1 specificity.

N-glycosylations of human α1,3-fucosyltransferase IX are required for full enzyme activity.

Human α1,3-fucosyltransferase IX catalyzes the transfer of l-fucose from guanosine diphosphate-ß-L-fucose to N-acetyllactosamine, generating a Lewis X epitope, and is thereby involved in the synthesis of fucosylated cell surface glycoconjugates. It contains three putative N-glycosylation sites (Asn62, Asn101 and Asn153). The current study considers the functional role of these potential N-glycosylations within the enzyme. We produced truncated variants of human fucosyltransferase IX containing the soluble extracellular catalytic domain. To analyze the relevance of each N-glycosylation site, several genomic mutant DNAs encoding a glutamine (Gln/Q) instead of the asparagine residue were created prosperously using site-directed mutagenesis and subsequently expressed in Spodoptera frugiperda cells applying a baculovirus expression system. After production and purification of these variants of human FucT IX, the wild-type (wt) enzyme and the variants were characterized regarding their activity and kinetic properties. The variants showed lower activity than the wt FucT, whereas the individual N-glycosylation sites had different effects on the enzyme activity and kinetic parameters. While the single variant N62Q still showed ∼60% of wt activity and N101Q retained ∼30% activity, replacement of Asn153 by glutamine led to an almost complete loss of enzymatic activity. The same could be observed for variants missing two or more putative N-glycosylation sites, which indicated the importance of N-glycosylation for enzyme stability and activity.

A fluorescence correlation spectroscopy-based enzyme assay for human Dicer.

Here, we present an in vitro assay based on fluorescence correlation spectroscopy (FCS), which allows investigation of the kinetic behaviour of human Dicer. The assay is based on the different mobilities of substrate and product. The change of substrate mobility was independent of the choice of the fluorescence label, allowing exclusion of non-specific photophysical artefacts. Dicer and RNase III cleavage led to different product diffusion times. Single-stranded RNA did not change its mobility after cleavage by both double-strand-specific RNases. In agreement with the literature, the RNase activity of Dicer could be inhibited by substituting Ca²âº for Mg²âº. In a defined system of two diffusion species of similar label and mobility differences, such as substrate and product, the linearity of the assay could be proven. An FCS-based enzyme assay is proposed, which allows monitoring of Dicer activity with high specificity in vitro.

Fluorophore binding aptamers as a tool for RNA visualization.

Fluorescence correlation spectroscopy (FCS) is suitable for the detection of fluorescent molecules in living cells. For the visualization of mRNA, we genetically fused a fluorophore-specific RNA aptamer to the coding mRNA of the green fluorescent protein, as well as to noncoding sequences. Using these constructs, we showed that the aptamer portion of the mRNA still binds the fluorophore in the nanomolar range as determined via FCS. Furthermore, the binding took place in the context of total RNA extract. A tandem construct of the RNA aptamer even exhibited a lower K(d) than the monomer. This FCS-based method establishes a tool for minimal invasive detection of RNA at the single molecule level in individual living cells.

High level expression of monomeric and dimeric human alpha1,3-fucosyltransferase V.

alpha3/4-Fucosyltransferases play a crucial role in inflammatory processes and tumor metastasis. While several human fucosyltransferases (FucTs) with different acceptor substrate specificities have been identified, the design of specific inhibitors for therapeutic approaches is hampered by the lack of structural information. In this study, we evaluated the expression of different constructs of human fucosyltransferase V to generate the large amounts required for structural studies. The truncated constructs lacking the transmembrane region and the cytosolic N-terminus, were expressed in baculovirus-infected Trichoplusia ni (Tn) insect cells and in two non-lytic expression systems, stably transfected human HEK 293 and T. ni cells. Since secretion of some glycosyltransferases is controlled by formation of dimeric molecules via disulfide bonds, one of the fucosyltransferase V constructs contained the N-terminal cysteine residue 64 for dimerization, whereas this residue was replaced in the other construct by serine. In both human and insect cells dimerization did not prove to be essential for efficient expression and secretion. On the basis of enzymatic activity, the yield of secreted fucosyltransferase V was approximately 10-fold higher in stably transfected insect cells than in HEK 293 cells. In particular the monomeric form of the enzyme provides a valuable tool for structural analyses to elucidate the fine specifity of fucosyltransferase V-mediated fucosylation of Lewis type glycans.

Chemoenzymatic synthesis of functional sialyl Lewis(x) mimetics with a heteroaromatic core.

Functional mimetics of the sialyl Lewis(X) tetrasaccharide were prepared by the enzymatic sialylation of a 1,3-diglycosylated indole and a glycosyl azide, which was subsequently transformed into a 1,4-diglycosylated 1,2,3-triazole, by using the trans-sialidase of Trypanosoma cruzi. These compounds inhibited the binding of E-, L-, and P-selectin-coated nanoparticles to polyacrylamide-bound sialyl-Lewis(X) -containing neighboring sulfated tyrosine residues (sTyr/sLe(X) -PAA) at low or sub-millimolar concentrations. Except for E-selectin, the mimetics showed higher activities than the natural tetrasaccharide.

Comparison of expression systems for human fucosyltransferase IX.

Human fucosyltransferase IX (hFucT-IX) is a highly conserved alpha1,3 fucosyltransferase with a distinct acceptor and site specificity. hFucT-IX catalyses the transfer of activated fucose to a sugar acceptor, thereby forming the Lewis x epitope. This epitope is responsible for recognition phenomena throughout the body e.g. in tumour growth. Detailed characterisation of hFucT-IX structure-function relationships by kinetic and X-ray structure analysis is prerequisite to the development of enzyme inhibitors for clinical applications such as the suppression of tumour metastasis. For these analyses substantial amounts of hFucT-IX are desirable. Since hFucT-IX is not present in considerable amounts in common cells an overproduction of recombinant hFucT-IX is appropriate. To evaluate the best system for this overproduction we compared different strategies employing prokaryotes (Escherichia coli), mammalian cells and insect cells. Insect cells were tested using stable and baculoviral expression strategies. Current results favour the use of the baculoviral expression system for further experiments.

Transsialidase from Trypanosoma cruzi for regio- and stereoselective synthesis of N-acyl-modified sialylated oligosaccharides and measurement of transfer rates.

Recombinant transsialidase from Trypanosoma cruzi (TcTS) was used for the sialylation with natural and non-natural derivatives of neuraminic acid. Neu5Ac-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe was prepared in 80 % yield. Correspondingly, the modified trisaccharide derivatives Neu5Prop-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe (32 %) and Neu5Gc-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe (Prop=propanoyl, Gc=glycolyl) were obtained in 60 % yield, respectively.

Preparation of sialylated oligosaccharides employing recombinant trans-sialidase from Trypanosoma cruzi.

Terminally sialylated oligosaccharides were synthesised employing recombinant trans-sialidase from Trypanosoma cruzi. Regio- and stereoselectively Sia-alpha(2-3)-Gal-betaR derivatives could be obtained in respectable yields, using combined chemical and enzymatic methodologies. An array of different disaccharide precursors such as Gal-beta(1-3)-GalNAc-alphaSer/Thr, lactosides and lactosamide derivatives were sialylated and successfully purified by facile isolation procedures. Depending on the acceptor structure isolated, yields for trans-sialylation products were between 20 and 60%.