Histidin im aktiven Zentrum von Enzymen

Selective Fluorescence Imaging of Cancer Cells Based on ROS-Triggered Intracellular Cross-Linking of Artificial Enzyme

Inside living cells, regulation of catalytic activity of artificial enzymes remains challenging due to issues such as biocompatibility, efficiency, and stability of the catalyst, by which the practical applications of artificial enzymes have been severely hindered. Here, an artificial enzyme, PTT-SGH, with responsiveness to reactive oxygen species (ROS), was obtained by introducing a catalytic histidine residue to pentaerythritol tetra(3-mercaptopropionate) (PTT). The artificial enzyme formed large aggregates in cells via the intracellular ROS-mediated oxidation of thiol groups. The process was significantly facilitated in tumor cells because of the higher ROS concentration in the tumor microenvironment. The catalytic activity of this artificial enzyme was intensively enhanced through deprotonation of cross-linked PTT-SGH, which showed typical esterase activities. Selective fluorescence imaging of tumor cells was achieved using the artificial enzyme to trigger the cleavage of the ester bond of the caged fluorophore inside living cells.

Creating Dynamic Nanospaces in Solution by Cationic Cages as Multirole Catalytic Platform for Unconventional C(sp)-H Activation Beyond Enzyme Mimics

Herein we demonstrate that, based on the creation of dynamic nanospaces in solution by highly charged positive coordination cage of [Pd₆ (RuL₃ )₈ ]²⁸⁺ , multirole and multi-way cage-confined catalysis is accomplishable for versatile functions and anomalous reactivities with the aid of the biomimetic cage effect. The high cationic-host charges drive partial deprotonation of 24 imidazole-NHs on cage sphere alike imidazole-residuals in proteins, generating amphoteric heterogeneity in solution to enforce effective cavity-basicity against solution-acidity. Synergistic actions arisen from cage hydrophobicity, host-guest electrostatic interactions and imidazole-N coordination facilitate C(sp)-H activation and carbanionic intermediate stabilization of terminal alkynes to achieve unusual H/D-exchange and Glaser coupling under acidic conditions, and enable phase transfers of water-insoluble substrates/products/co-catalysts to make immiscible-phase and bi-phase catalysis feasible, thus providing a useful catalytic protocol to combine merits from homogeneous, heterogeneous, enzymatic and phase transfer catalysis.

Organocatalytic Control over a Fuel-Driven Transient-Esterification Network*

Signal transduction in living systems is the conversion of information into a chemical change, and is the principal process by which cells communicate. In nature, these functions are encoded in non-equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. However, man-made catalytically controlled networks are rare. We incorporated catalysis into an artificial fuel-driven out-of-equilibrium CRN, where the forward (ester formation) and backward (ester hydrolysis) reactions are controlled by varying the ratio of two organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. This fuel-driven strategy was expanded to a responsive polymer system, where transient polymer conformation and aggregation are controlled through fuel and catalyst levels. Altogether, we show that organocatalysis can be used to control a man-made fuel-driven system and induce a change in a macromolecular superstructure, as in natural non-equilibrium systems.

Catalytically Active Peptide-Gold Nanoparticle Conjugates: Prospecting for Artificial Enzymes

The self-assembly of peptides onto the surface of gold nanoparticles has emerged as a promising strategy towards the creation of artificial enzymes. The resulting high local peptide density surrounding the nanoparticle leads to cooperative and synergistic effects, which result in rate accelerations and distinct catalytic properties compared to the unconjugated peptide. This Minireview summarizes contributions to and progress made in the field of catalytically active peptide-gold nanoparticle conjugates. The origin of distinct properties, as well as potential applications, are also discussed.

Taxol biosynthesis: Identification and characterization of two acetyl CoA:taxoid-O-acetyl transferases that divert pathway flux away from Taxol production

Two cDNAs encoding taxoid-O-acetyl transferases (TAX 9 and TAX 14) were obtained from a previously isolated family of Taxus acyl/aroyl transferase cDNA clones. The recombinant enzymes catalyze the acetylation of taxadien-5alpha,13alpha-diacetoxy-9alpha,10beta-diol to generate taxadien-5alpha,10beta,13alpha-tri-acetoxy-9alpha-ol and taxadien-5alpha,9alpha,13alpha-triacetoxy-10beta-ol, respectively, both of which then serve as substrates for a final acetylation step to yield taxusin, a prominent side-route metabolite of Taxus. Neither enzyme acetylate the 5alpha- or the 13alpha-hydroxyls of taxoid polyols, indicating that prior acylations is required for efficient peracetylation to taxusin. Both enzymes were kinetically characterized, and the regioselectivity of acetylation was shown to vary with pH. Sequence comparison with other taxoid acyl transferases confirmed that primary structure of this enzyme type reveals little about function in taxoid metabolism. Unlike previously identified acetyl transferases involved in Taxol production, these two enzymes appear to act exclusively on partially acetylated taxoid polyols to divert the Taxol pathway to side-route metabolites.

Phosphoenolpyruvate-dependent protein kinase enzyme I of Streptococcus faecalis: purification and properties of the enzyme and characterization of its active center

Enzyme I, the phosphoenolpyruvate:protein phosphotransferase (EC 2.7.3.9), which is part of the bacterial phosphoenolpyruvate- (PEP) dependent phosphotransferase system, has been purified from Streptococcus faecalis by using a large-scale preparation. Size exclusion chromatography revealed a molecular weight of 140 000. On sodium dodecyl sulfate gels, enzyme I gave one band with a molecular weight of 70 000, indicating that enzyme I consists of two identical subunits. The first 59 amino acids of the amino-terminal part of the protein have been sequenced. It showed some similarities with enzyme I of Salmonella typhimurium. The active center of enzyme I has also been determined. After phosphorylation with [32P]PEP, the enzyme was cleaved by using different proteases. Labeled peptides were isolated by high-performance liquid chromatography on a reversed-phase column. The amino acid composition or amino acid sequence of the peptides has been determined. The largest labeled peptide was obtained with Lys-C protease and had the following sequence: -Ala-Phe-Val-Thr-Asp-Ile-Gly- Gly-Arg-Thr-Ser-His*-Ser-Ala-Ile-Met-Ala-Arg-Ser-Leu-Glu-Ile-Pro-Ala- Ile-Val-Gly-Thr-Lys-. It has previously been shown that the phosphoryl group is bound to the N-3 position of a histidyl residue in phosphorylated enzyme I. The single His in position 12 of the above peptide must therefore carry the phosphoryl group.

Modification of rat liver iodothyronine 5'-deiodinase activity with diethylpyrocarbonate and rose bengal; evidence for an active site histidine residue

Iodothyronine 5'-deiodinase activity of rat liver microsomes was rapidly and completely lost by treatment with diethylpyrocarbonate (DEP) and by photo-oxidation with Rose Bengal (RB). In both cases inactivation followed pseudo first order reaction kinetics. Inactivation by DEP was diminished in the presence of substrate or competitive inhibitors, and was reversed by hydroxylamine treatment. In addition to photo-oxidation, deiodinase activity was also inhibited by RB in the dark. This inhibition was reversible and competitive with substrate (Ki 60 nM). These results suggest the location of an essential histidine residue at or near the active site of rat liver iodothyronine deiodinase.

Phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus aureus: factor IIIlac, a trimeric phospho-carrier protein that also acts as a phase transfer catalyst

Factor IIIlac (FIII) consists of three identical subunits. It could be shown that each of the subunits carries a phosphoryl group upon phosphorylation (P-FIII) with phosphoenolpyruvate (PEP), enzyme I, and histidine-containing phospho-carrier protein (HPr). The phosphoryl group is bound to a histidyl residue in P-FIII. Each subunit of FIII contains four histidyl residues. After tryptic cleavage a peptide was isolated that contained one other histidyl residue besides the active center histidine. By further cleavage of the peptide T-2 with V-8 Staphylococcus aureus protease it could be shown that His-19 in the sequence of the peptide T-2 is the active center histidine. Another peptide (1-38), caused by incomplete tryptic cleavage, could be isolated. It inhibited the phospho-transfer reaction from PEP to the sugar molecule at the step of factor III-enzyme II recognition. It competes with factor III for the binding site of enzyme II, the membrane component. It is a very hydrophobic peptide. This hydrophobic region is buried in factor III. But upon phosphorylation of factor III it is turned out. Thus P-FIII binds to Triton X-100 micelles whereas factor III does not. This conformational change caused by phosphorylation could be shown by proton nuclear magnetic resonance methods [Kalbitzer, H.R., Deutscher, J., Hengstenberg, W., & Rösch, P. (1981) Biochemistry 20, 6178-6185], by circular dichroism spectroscopy, and by the Ouchterlony double-diffusion method. Antibodies against FIII do not precipitate P-FIII.