Reinhard Neier

Structure of Porphobilinogen Synthase from Pseudomonas aeruginosa in Complex with 5-Fluorolevulinic Acid Suggests a Double Schiff Base Mechanism

All natural tetrapyrroles, including hemes, chlorophylls and vitamin B12, share porphobilinogen (PBG) as a common precursor. Porphobilinogen synthase (PBGS) synthesizes PBG through the asymmetric condensation of two molecules of aminolevulinic acid (ALA). Crystal structures of PBGS from various sources confirm the presence of two distinct binding sites for each ALA molecule, termed A and P. We have solved the structure of the active-site variant D139N of the Mg2+-dependent PBGS from Pseudomonas aeruginosa in complex with the inhibitor 5-fluorolevulinic acid at high resolution. Uniquely, full occupancy of both substrate binding sites each by a single substrate-like molecule was observed. Both inhibitor molecules are covalently bound to two conserved, active-site lysine residues, Lys205 and Lys260, through Schiff bases. The active site now also contains a monovalent cation that may critically enhance enzymatic activity. Based on these structural data, we postulate a catalytic mechanism for P. aeruginosa PBGS initiated by a C-C bond formation between A and P-side ALA, followed by the formation of the intersubstrate Schiff base yielding the product PBG.

Synthesis, structure, and complexation properties of partially and completely reduced meso-octamethylporphyrinogens (calix[4]pyrroles).

New tricks for an old dog: Calixpyrroles bind anions efficiently and can be transformed into transition-metal complexes only under forcing conditions. Reducing the macrocycle creates a ligand that easily forms classical Werner complexes with copper, nickel, and palladium ions. The metal complexes present an array of four directed hydrogen bonds, which specifically bind the counterions (see picture; blue N, white H, green Cl, red Cu, Ni, or Pd).

The X-ray structure of yeast 5-aminolaevulinic acid dehydratase complexed with two diacid inhibitors

The structures of 5‐aminolaevulinic acid dehydratase complexed with two irreversible inhibitors (4‐oxosebacic acid and 4,7‐dioxosebacic acid) have been solved at high resolution. Both inhibitors bind by forming a Schiff base link with Lys 263 at the active site. Previous inhibitor binding studies have defined the interactions made by only one of the two substrate moieties (P‐side substrate) which bind to the enzyme during catalysis. The structures reported here provide an improved definition of the interactions made by both of the substrate molecules (A‐ and P‐side substrates). The most intriguing result is the novel finding that 4,7‐dioxosebacic acid forms a second Schiff base with the enzyme involving Lys 210. It has been known for many years that P‐side substrate forms a Schiff base (with Lys 263) but until now there has been no evidence that binding of A‐side substrate involves formation of a Schiff base with the enzyme. A catalytic mechanism involving substrate linked to the enzyme through Schiff bases at both the A‐ and P‐sites is proposed.

Evaluation of dipeptide-derivatives of 5-aminolevulinic acid as precursors for photosensitizers in photodynamic therapy

N-terminal-blocked and N-terminal-free pseudotripeptide Gly-Gly and Gly-Pro derivatives of 5-aminolevulinic acid (ALA) esters were synthesized as potential specific substrates for cellular peptidases and precursors for the production of the photosensitizer protoporphyrin IX (PpIX). These precursors were evaluated using human cell lines of either carcinoma or endothelial origin. N-blocked or N-free dipeptides-ALA-ethyl esters, but not tripeptides-ALA-ethyl esters (or dipeptides-ALA-ethyleneglycols,) were substrates for cellular peptidases and were metabolized to ALA. The precursors were hydrolyzed intracellularly involving serine-proteases and metalloproteases. Cell selectivity for human endothelial or carcinoma cells was observed for some of these dipeptides-ALA. Thus drugs coupled to Gly-Gly-/Gly-Pro-derivatives may selectively target defined cells in human cancer, depending on specific cellular activating pathways expressed by the cells.

Inhibition of Escherichia coli porphobilinogen synthase using analogs of postulated intermediates

BACKGROUND Porphobilinogen synthase is the second enzyme involved in the biosynthesis of natural tetrapyrrolic compounds, and condenses two molecules of 5-aminolevulinic acid (ALA) through a nonsymmetrical pathway to form porphobilinogen. Each substrate is recognized individually at two different active site positions to be regioselectively introduced into the product. According to pulse-labeling experiments, the substrate forming the propionic acid sidechain of porphobilinogen is recognized first. Two different mechanisms for the first bond-forming step between the two substrates have been proposed. The first involves carbon-carbon bond formation (an aldol-type reaction) and the second carbon-nitrogen bond formation, leading to an iminium ion. RESULTS With the help of kinetic studies, we determined the Michaelis constants for each substrate recognition site. These results explain the Michaelis-Menten behavior of substrate analog inhibitors - they act as competitive inhibitors. Under standard conditions, however, another set of inhibitors demonstrates uncompetitive, mixed, pure irreversible, slow-binding or even quasi-irreversible inhibition behavior. CONCLUSIONS Analysis of the different classes of inhibition behavior allowed us to make a correlation between the type of inhibition and a specific site of interaction. Analyzing the inhibition behavior of analogs of postulated intermediates strongly suggests that carbon-nitrogen bond formation occurs first.

Glycoside esters of 5-aminolevulinic acid for photodynamic therapy of cancer.

Aliphatic and ethylene glycol esters of 5-aminolevulinic acid (ALA) are very efficient precursors of the photosensitizer protoporphyrin IX (PpIX) for photodynamic therapy; however, they diffuse passively across the cell membrane and thus lack cell selectivity. We evaluated whether alpha-glucose, alpha-mannose, or beta-galactose esters of ALA would present improved properties as precursors of PpIX. Esterification was performed either at the position O-1 or O-6 of the sugars with or without an ethylene glycol linker, and these glycoside esters of ALA were evaluated in human cells. The results demonstrated that glycoside esters of ALA are efficient precursors of PpIX in human cancer and angiogenic endothelial cells, comparable to free ALA, but not in normal human fibroblasts. PpIX production was confirmed by fluorescence microscopy and photodynamic treatment of cells. The O-1 or O-6 positions of functionalization and the nature of the sugar moiety did not influence PpIX production. The presence of the ethylene glycol linker generally resulted in decreased PpIX production. The uptake of these glycoside esters of ALA by cells was not decreased in the presence of high concentrations of the related sugars. Inhibitors of alpha-glucosidases or alpha-mannosidases did not decrease PpIX production. These results suggest the involvement of active non-glycoside-specific membrane transporter(s) for uptake and of esterases rather than glycosidases in the release of ALA from the glycoside esters of ALA.

Real-time amyloid aggregation monitoring with a photonic crystal-based approach

We propose the application of a new label-free optical technique based on photonic nanostructures to real-time monitor the amyloid-beta 1-42 (Aβ(1-42)) fibrillization, including the early stages of the aggregation process, which are related to the onset of the Alzheimers Disease (AD). The aggregation of Aβ peptides into amyloid fibrils has commonly been associated with neuronal death, which culminates in the clinical features of the incurable degenerative AD. Recent studies revealed that cell toxicity is determined by the formation of soluble oligomeric forms of Aβ peptides in the early stages of aggregation. At this phase, classical amyloid detection techniques lack in sensitivity. Upon a chemical passivation of the sensing surface by means of polyethylene glycol, the proposed approach allows an accurate, real-time monitoring of the refractive index variation of the solution, wherein Aβ(1-42) peptides are aggregating. This measurement is directly related to the aggregation state of the peptide throughout oligomerization and subsequent fibrillization. Our findings open new perspectives in the understanding of the dynamics of amyloid formation, and validate this approach as a new and powerful method to screen aggregation at early stages.

Tandem Nucleophilic Addition/Diels-Alder Reaction of N-Butadienyl N,O-Ketene Silyl Acetals with C60: Stereoselective Formation of Bicyclic Octahydroquinolino-1,2,3,4-Tetrahydrobuckminsterfullerenes and Combined NMR Spectroscopic and Computational Evaluation of the Functionalization Reactions

We have studied the reactiv- ity of the N,O-ketene N-1,3-butadienyl- N-alkyl-O-silyl acetals 1a - e with C60 proceeding through a tandem process to give the adducts 2a - e. The addition order of these tandem reactions has been evaluated. The initial nucleophilic Michael-like addition of the electron- rich N,O-ketene acetal moiety proceeds unusually fast at 258C, followed by an intramolecularly accelerated Diels - Alder step that is highly diastereoselec- tive. The structures of compounds 2a - e were determined from the 1 H and 13 C NMR shifts and from H - H coupling patterns, while their stereochemistry was deduced from 2D T-ROESY NMR experiments. The proposed mechanism for the nucleophilic addition involves single electron transfer followed by radical anion - radical cation recombi- nation. Computational investigations of the reaction pathways, transition states, and conformational energies have been carried out to corroborate the experi- mental data.

Lewis-acid catalysed tandem reaction Diels–Alder–[3,3] sigmatropic shift between buta-1,3-dienyl thiocyanic acid ester and acryloyl chloride: application in the synthesis of 2-azabicyclo[2.2.2]oct-5-ene derivatives

Buta-1,3-dienyl thiocyanic acid ester reacts with acryloyl chloride in presence of Lewis acid catalysts to produce directly the rearranged product of type 3bvia a combination of a Diels-Alder reaction with a [3,3] sigmatropic shift; the 1,4-substituted cyclohexene is easily transformed into the 2-azabicyclo[2.2.2]oct-5-ene derivative 6b, which has been used as precursor for the synthesis of the Ibogamin skeleton.

Unimolecular dissociations of excited C3H6O+: A photoelectron—photoion coincidence study of cyclopropanol and allyl alcohol

Abstract The photoelectron spectrum of cyclopropanol has been determined and the first few bands have been assigned to the respective ionization processes. The dissociative photoionization of cyclopropanol and allyl alcohol has been studied by means of HeIα photoelectron—photoion coincidence spectroscopy. The breakdown diagrams of the two corresponding radical cations are practically indistinguishable within the entire energy range investigated (= 5 eV). This implies that extensive isomerizations to a single or to a mixture of common precursor structure(s) precede the dissociative processes. The currently available information on the structure of this reactant and, in particular, the role of other C 3 H 6 O + isomers is discussed. The kinetic and thermodynamic aspects of the present results are outlined. Although three generations of daughter ions are involved, the relative importance of the different fragmentation pathways could be determined. The RRKM analysis of the coincidence data suggests that the formation of the ethylene cation by loss of a CO molecule from the parent ion hardly competes with the other four primary fragmentation reactions in the sense of the statistical theory of unimolecular reactions.