Jozsef Aszodi

The catalytic, glycosyl transferase and acyl transferase modules of the cell wall peptidoglycan‐polymerizing penicillin‐binding protein 1b of Escherichia coli

The penicillin‐binding protein (PBP) 1b of Escherichia coli catalyses the assembly of lipid‐transported N‐acetyl glucosaminyl‐β‐1,4‐N‐acetylmuramoyl‐l‐alanyl‐γ‐d‐glutamyl‐(l)‐meso‐diaminopimelyl‐(l)‐d‐alanyl‐d‐alanine disaccharide pentapeptide units into polymeric peptidoglycan. These units are phosphodiester linked, at C1 of muramic acid, to a C55 undecaprenyl carrier. PBP1b has been purified in the form of His tag (M46‐N844) PBP1bγ. This derivative provides the host cell in which it is produced with a functional wall peptidoglycan. His tag (M46‐N844) PBP1bγ possesses an amino‐terminal hydrophobic segment, which serves as transmembrane spanner of the native PBP. This segment is linked, via an ≅ 100‐amino‐acid insert, to a D198‐G435 glycosyl transferase module that possesses the five motifs characteristic of the PBPs of class A. In in vitro assays, the glycosyl transferase of the PBP catalyses the synthesis of linear glycan chains from the lipid carrier with an efficiency of ≅ 39 000 M−1 s−1. Glu‐233, of motif 1, is central to the catalysed reaction. It is proposed that the Glu‐233 γ‐COOH donates its proton to the oxygen atom of the scissile phosphoester bond of the lipid carrier, leading to the formation of an oxocarbonium cation, which then undergoes attack by the 4‐OH group of a nucleophile N‐acetylglucosamine. Asp‐234 of motif 1 or Glu‐290 of motif 3 could be involved in the stabilization of the oxocarbonium cation and the activation of the 4‐OH group of the N‐acetylglucosamine. In turn, Tyr‐310 of motif 4 is an important component of the amino acid sequence‐folding information. The glycosyl transferase module of PBP1b, the lysozymes and the lytic transglycosylase Slt70 have much the same catalytic machinery. They might be members of the same superfamily. The glycosyl transferase module is linked, via a short junction site, to the amino end of a Q447‐N844 acyl transferase module, which possesses the catalytic centre‐defining motifs of the penicilloyl serine transferases superfamily. In in vitro assays with the lipid precursor and in the presence of penicillin at concentrations sufficient to derivatize the active‐site serine 510 of the acyl transferase, the rate of glycan chain synthesis is unmodified, showing that the functioning of the glycosyl transferase is acyl transferase independent. In the absence of penicillin, the products of the Ser‐510‐assisted double‐proton shuttle are glycan strands substituted by cross‐linked tetrapeptide–pentapeptide and tetrapeptide–tetrapeptide dimers and uncross‐linked pentapeptide and tetrapeptide monomers. The acyl transferase of the PBP also catalyses aminolysis and hydrolysis of properly structured thiolesters, but it lacks activity on d‐alanyl‐d‐alanine‐terminated peptides. This substrate specificity suggests that carbonyl donor activity requires the attachment of the pentapeptides to the glycan chains made by the glycosyl transferase, and it implies that one and the same PBP molecule catalyses transglycosylation and peptide cross‐linking in a sequential manner. Attempts to produce truncated forms of the PBP lead to the conclusion that the multimodular polypeptide chain behaves as an integrated folding entity during PBP1b biogenesis.

Synthesis of analogues of the O-β-d-Ribofuranosyl Nucleoside Moiety of Liposidomycins. Part 1: contribution of the amino group and the Uracil Moiety upon the inhibition of MraY

The O-beta-D-ribofuranosyl nucleoside I is the minimal structural entity of liposidomycins maintaining enzyme inhibitory activity. Modifications performed on both the primary amine and the uracil moieties clearly demonstrate their major contribution to the inhibition of the bacterial translocase (MraY).

A colored dendrimer as a new soluble support in organic synthesis. Part 1: Suzuki reaction

A new strategy using a colored dendrimer as visible soluble support for organic synthesis has been developed. The efficiency of this new system has been demonstrated by the use of DRHMPA9–CH2OH as the support in a Suzuki coupling reaction. Due to the visibility of the support, following of the reaction has been rendered easier and the purification time of the crude product has been considerably shortened.

Synthese of 13-oxa- and 13-thia-PGI2: metabolically stable and biologically potent PGI2 analogues

Abstract 13-Oxa-13,14-dihydro-PGI 2 ( 6 ) and 13-thia-13,14-dihydro-PGI 2 ( 9 ) were synthesized from the corresponding PGF 2α analogues via iodo ether formation and subsequent elimination of HI.

Multi gram synthesis of UDP-N-acetylmuramic acid

As part of an effort to discover novel antibacterial agents, a new and efficient synthesis was established in order to provide a large amount of UDP-N-acetylmuramic acid (UDP-MurNAc).

Synthesis of new echinocandin derivatives via a diol-keto transposition

Abstract A new diol-carbonyl transposition reaction has been discovered in echinocandin type structures. An α-hydroxy hemiaminal moiety has been shown to undergo a pinacol-type rearrangement in the presence of trimethylsilyl iodide to afford ketone derivatives. Applied to deoxymulundocandin, this transposition led to a useful intermediate for further chemical modification.