Catalysis of DNA cleavage and nucleoside triphosphate synthesis by NM23-H2/NDP kinase share an active site that implies a DNA repair function.

NM23/NDP kinases play an important role in development and cancer but their biological function is unknown, despite an intriguing collection of biochemical properties including nucleoside-diphosphate kinase (NDP kinase), DNA binding and transcription, a mutator function, and cleavage of unusually structured DNA by means of a covalent enzyme-DNA complex. To assess the role of the nuclease in human NM23-H2, we sought to identify the amino acid responsible for covalent catalysis. By sequencing a DNA-linked peptide and by site-directed mutagenesis, we identified lysine-12, a phylogenetically conserved residue, as the amino acid forming the covalent complex with DNA. In particular, the epsilon-amino group acts as the critical nucleophile, because substitution with glutamine but not arginine completely abrogated covalent adduct formation and DNA cleavage, whereas the DNA-binding properties remained intact. These findings and chemical modification data suggest that phosphodiester-bond cleavage occurs by a DNA glycosylase/lyase-like mechanism known as the signature of base excision DNA repair nucleases. Involvement of NM23/NDP kinase in a DNA repair pathway would be consistent with its role in normal and tumor cell development. Additionally, lysine-12, which is known in the x-ray crystallographic structure to lie in the catalytic pocket involved in the NDP kinase phosphorylation reaction, was found essential also for the NDP kinase activity of NM23-H2, suggesting that the two catalytic activities of NM23-H2 are fundamentally connected.

Mechanism of CheA protein kinase activation in receptor signaling complexes.

The chemotaxis receptor for aspartate, Tar, generates responses by regulating the activity of an associated histidine kinase, CheA. Tar is composed of an extracellular sensory domain connected by a transmembrane sequence to a cytoplasmic signaling domain. The cytoplasmic domain fused to a leucine zipper dimerization domain forms soluble active ternary complexes with CheA and an adapter protein, CheW. The kinetics of kinase activity within these complexes compared to CheA alone indicate approximately a 50% decrease in the KM for ATP and a 100-fold increase in the Vmax. A truncated CheA construct that lacks the phosphoaccepting H-domain and the CheY/CheB-binding domain forms an activated ternary complex that is similar to the one formed by the full-length CheA protein. The Vmax of H-domain phosphorylation by this complex is enhanced approximately 60-fold, the KM for ATP decreased to 50%, and the KM for H-domain decreased to 20% of the values obtained with the same CheA construct in the absence of receptor and CheW. The kinetic data support a mechanism of CheA regulation that involves perturbation of an equilibrium between an inactive form where the H-domain is loosely bound and an active form where the H-domain is tightly associated with the CheA active site and properly positioned for phosphotransfer. The data are consistent with an asymmetric mechanism of CheA activation [Levit, M., Liu, I., Surette, M. G., and Stock, J. B. (1996) J. Biol. Chem. 271, 32057-32063] wherein only one phosphoaccepting domain of CheA at a time can interact with an active center within a CheA dimer.

pH sensing in bacterial chemotaxis.

Bacteria are able to sense a broad range of chemical and energetic stimuli and modulate their swimming behaviour to migrate to more favourable environments. Signal transduction in bacterial chemotaxis is mediated by a two-component system composed of a protein histidine kinase, CheA, and a response regulator, CheY. The phosphorylated response regulator, P approximately CheY, binds to a protein at the flagellar motor, FliM, to cause reversals in flagellar motor rotation. The level of P approximately CheY is controlled by the activity of the kinase CheA, which is in turn regulated by membrane receptors at the cell surface. Membrane receptors such as the aspartate receptor, Tar, are composed of two distinct regions: an extracellular sensing domain that binds stimulatory ligands, aspartate in the case of Tar; and an intracellular signalling domain that forms a complex with the protein kinase CheA. What is the mechanism of transmembrane signalling? How does aspartate binding to the sensing domain at the outside surface of the membrane translate into a change in kinase activity at the membrane cytosol interface? Recent results suggest that the mechanism depends on perturbations in lateral packing within an extensive array of receptors localized to patches at the cell poles. Receptor patching appears to depend on higher-order associations with the kinase CheA as well as an adaptor protein, CheW. It is difficult to assess the locus of pH effects within the context of even a simple signal transduction system like that involved in bacterial chemotaxis. Previous results with mutant strains have indicated that the serine receptor, Tsr, is critical for pH sensing, but in vitro results do not support such a straightforward interpretation of the genetic data.

Stimulus response coupling in bacterial chemotaxis: receptor dimers in signalling arrays.

In the Escherichia coli chemotaxis system, a family of chemoreceptors in the cytoplasmic membrane binds stimulatory ligands and regulates the activity of an associated histidine kinase CheA to modulate swimming behaviour and thereby cause a net migration towards attractants and away from repellents. The chemoreceptors themselves have been shown to be predominantly dimeric, but in the presence of the kinase CheA plus an adapter protein, CheW, much higher order structures have been observed. Recent results indicate that transmembrane signalling occurs within receptor clusters rather than through isolated dimers. We propose that the mechanism involves receptor arrays where binding of ligands at the outside surface of the membrane affects lateral packing interactions that cause perturbations in the organization of the signalling array at the opposing surface of the membrane. Results with receptor chimeras as well as findings with tyrosine kinase receptors suggest that this mechanism may represent a common theme in membrane receptor function.

[Lignin and ligninase]. / Lignin i ligninaza.

Ligninases (lignin peroxidases) are heme-containing peroxidases excreted by some white-rot fungi as components of their lignolytic multienzyme complexes. These peroxidases functioning at rather acidic media catalyze oxidative cleavage of both synthetic non-phenolic lignin models and many other oxidation-proof compounds (chloroorganic pesticides, carcinogenic hydrocarbons, etc.). Data on the ligninase structure and functions not only shed light of the lignin biodegradation but also open new perspectives in peroxidation chemistry and biotechnology. Many aspects of ligninase catalytic mechanism can be understood in comparative studies of congruent chemical reactions, e.g., peroxidisulfate-supported oxidation, as well as of ligninase-like activity of some plant and animal peroxidases which is also manifested at low pH. Ligninases are not only more powerful oxidative agents than other peroxidases, but also, in contrast to latters, appear to be able to control the contributions of C-C and C-O bond splitting in primary radical-cations of substrates. The contribution of the oxidative-hydrolytic dealkylation of radical cations can be considered as one of classification criteria for lignolytic enzymes.