A heat-stable nicotinamide-adenine dinucleotide glycohydrolase from Pseudomonas putida KB1. Partial purification and some properties of the enzyme and an inhibitory protein

NADPH-generating systems in bacteria and archaea

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is an essential electron donor in all organisms. It provides the reducing power that drives numerous anabolic reactions, including those responsible for the biosynthesis of all major cell components and many products in biotechnology. The efficient synthesis of many of these products, however, is limited by the rate of NADPH regeneration. Hence, a thorough understanding of the reactions involved in the generation of NADPH is required to increase its turnover through rational strain improvement. Traditionally, the main engineering targets for increasing NADPH availability have included the dehydrogenase reactions of the oxidative pentose phosphate pathway and the isocitrate dehydrogenase step of the tricarboxylic acid (TCA) cycle. However, the importance of alternative NADPH-generating reactions has recently become evident. In the current review, the major canonical and non-canonical reactions involved in the production and regeneration of NADPH in prokaryotes are described, and their key enzymes are discussed. In addition, an overview of how different enzymes have been applied to increase NADPH availability and thereby enhance productivity is provided.

Characterization of Streptococcus pyogenes beta-NAD+ glycohydrolase: re-evaluation of enzymatic properties associated with pathogenesis

The gram-positive pathogen Streptococcus pyogenes injects a beta-NAD(+) glycohydrolase (SPN) into the cytosol of an infected host cell using the cytolysin-mediated translocation pathway. In this compartment, SPN accelerates the death of the host cell by an unknown mechanism that may involve its beta-NAD(+)-dependent enzyme activities. SPN has been reported to possess the unique characteristic of not only catalyzing hydrolysis of beta-NAD(+), but also carrying out ADP-ribosyl cyclase and ADP-ribosyltransferase activities, making SPN the only beta-NAD(+) glycohydrolase that can catalyze all of these reactions. With the long term goal of understanding how these activities may contribute to pathogenesis, we have further characterized the enzymatic activity of SPN using highly purified recombinant protein. Kinetic studies of the multiple activities of SPN revealed that SPN possessed only beta-NAD(+) hydrolytic activity and lacked detectable ADP-ribosyl cyclase and ADP-ribosyltransferase activities. Similarly, SPN was unable to catalyze cyclic ADPR hydrolysis, and could not catalyze methanolysis or transglycosidation. Kinetic analysis of product inhibition by recombinant SPN demonstrated an ordered uni-bi mechanism, with ADP-ribose being released as a second product. SPN was unaffected by product inhibition using nicotinamide, suggesting that this moiety contributes little to the binding energy of the substrate. Upon transformation, SPN was toxic to Saccharomyces cerevisiae, whereas a glycohydrolase-inactive SPN allowed for viability. Taken together, these data suggest that SPN functions exclusively as a strict beta-NAD(+) glycohydrolase during pathogenesis.

Structure and function of an ADP-ribose-dependent transcriptional regulator of NAD metabolism

Besides its function as an essential redox cofactor, nicotinamide adenine dinucleotide (NAD) also serves as a consumable substrate for several reactions with broad impact on many cellular processes. NAD homeostasis appears to be tightly controlled, but the mechanism of its regulation is little understood. Here we demonstrate that a previously predicted bacterial transcriptional regulator, NrtR, represses the transcription of NAD biosynthetic genes in vitro. The NAD metabolite ADP-ribose functions as an activator suppressing NrtR repressor activity. The presence of high ADP-ribose levels in the cell is indicative of active NAD turnover in bacteria, which could signal the activation of NAD biosynthetic gene expression via inhibiting the repressor function of NrtR. By comparing the crystal structures of NrtR in complex with DNA and with ADP-ribose, we identified a "Nudix switch" element that likely plays a critical role in the allosteric regulation of DNA binding and repressor function of NrtR.

NAD+-glycohydrolase acts as an intracellular toxin to enhance the extracellular survival of group A streptococci

Group A streptococci (GAS) produce several secreted products that are thought to enhance pathogenicity by facilitating spread of the organisms through host tissues. Two such products, streptolysin O (SLO) and NAD+-glycohydrolase, appear to be functionally linked, in that SLO is required for transfer of NAD+-glycohydrolase into epithelial cells. However, the effects of NAD+-glycohydrolase on host cells are largely unexplored. We now report that SLO-mediated delivery of NAD+-glycohydrolase to the cytoplasm of human keratinocytes results in major changes in host cell biology that enhance GAS pathogenicity. We derived isogenic mutant strains deficient in the expression of SLO, NAD+-glycohydrolase or both proteins in the background of a virulent, M-type 3 strain of GAS. All three mutant strains were internalized by human keratinocytes more rapidly and in higher numbers than were organisms from the wild-type strain. Association of the mutant strains with keratinocytes also resulted in reduced cytotoxicity and reduced keratinocyte apoptosis compared with wild-type GAS. These results support a model in which NAD+-glycohydrolase contributes to GAS pathogenesis by modulating host cell signalling pathways to inhibit GAS internalization, to augment SLO-mediated cytotoxicity and to induce keratinocyte apoptosis. We conclude that NAD+-glycohydrolase is a novel type of bacterial toxin that acts intracellularly in the infected host to enhance the survival and proliferation of an extracellular pathogen.

NAD(+)-glycohydrolase from Streptococcus pyogenes shows cyclic ADP-ribose forming activity

NAD(+)-glycohydrolase from Streptococcus pyogenes was purified by successive chromatography on CM Sepharose CL-6B, Sephacryl S-200 HR and hydroxyapatite. The purified enzyme possessed synthesis and hydrolysis activities of cyclic ADP-ribose (cADPR), a newly found second messenger for Ca2+ mobilisation, along with cleavage activity of the ribose-nicotinamide bond in NAD+.