The NAD-dependent glutamate dehydrogenase from the hyperthermophilic archaeon Pyrobaculum islandicum: cloning, sequencing, and expression of the enzyme gene(1)

Construction of a novel bioanode for amino acid powered fuel cells through an artificial enzyme cascade pathway

OBJECTIVE

The construction of a novel bioanode based on L-proline oxidation using a cascade reaction pathway comprised of thermostable dehydrogenases.

RESULTS

A novel multi-enzymatic cascade pathway, containing four kinds of dehydrogenases from thermophiles (dye-linked L-proline dehydrogenase, nicotinamide adenine dinucleotide (NAD)-dependent Δ¹-pyrroline-5-carboxylate dehydrogenase, NAD-dependent L-glutamate dehydrogenase and dye-linked NADH dehydrogenase), was designed for the generation of six-electrons from one molecule of L-proline. The current density of the four-dehydrogenase-immobilized electrode, with a voltage of + 450 mV (relative to that of Ag/AgCl), was 226.8 μA/cm² in the presence of 10 mM L-proline and 0.5 mM ferrocene carboxylate at 50 °C. This value was 4.2-fold higher than that of a similar electrode containing a single dehydrogenase. In addition, about 54% of the initial current in the multi-enzyme cascade bioanode was maintained even after 15 days.

CONCLUSIONS

Efficient deep oxidation of L-proline by multiple-enzyme cascade reactions was achieved in our designed electrode. The multi-enzyme cascade bioanode, which was built using thermophilic dehydrogenases, showed high durability at room temperature. The long-term stability of the bioanode indicates that it shows great potential for applications as a long-lived enzymatic fuel cell.

Enzymatic asymmetric synthesis of chiral amino acids

This review summarizes the progress achieved in the enzymatic asymmetric synthesis of chiral amino acids from prochiral substrates.

Expression and functional analysis of glutamate synthase small subunit-like proteins from archaeon Pyrococcus horikoshii

Glutamate synthase, glutamine α-ketoglutarate amidotransferase (often abbreviated as GOGAT) is a key enzyme in the early stages of ammonia assimilation in bacteria, algae and plants, catalyzing the reductive transamidation of the amido nitrogen from glutamine to α-ketoglutarate to form two molecules of glutamate. Most bacterial glutamate synthases consist of a large and small subunit. The genomes of three Pyrococcus species harbour several open reading frames which show homology with the small subunit of glutamate synthase. There are no open reading frames which may be coding for a large subunit responsible for the glutamate formation in these pyrococcal genomes. In this work, two open reading frames PH0876 and PH1873 from P. horikoshii were cloned and expressed in Escherichia coli as soluble proteins. Both proteins show NADPH-dependent oxidoreductase activity using artificial electron acceptors iodonitrotetrazolium chloride at thermophilic conditions. It is possible that these open reading frames are the products of gene duplication and that they are the early forms of an electron transfer domain in archaea which may have later contributed to many electron transfer enzymes.

Intersubunit interaction induced by subunit rearrangement is essential for the catalytic activity of the hyperthermophilic glutamate dehydrogenase from Pyrobaculum islandicum

The specific activity of recombinant Pyrobaculum islandicum glutamate dehydrogenase (pis-GDH) expressed in Escherichia coli is much lower than that of the native enzyme. However, when the recombinant enzyme is heated at 90 degrees C or exposed to 5 M urea, the activity increases to a level comparable to that of the native enzyme. Small-angle X-ray scattering measurements revealed that the radius of gyration (R(g,z)) of the hexameric recombinant enzyme was reduced to 47 A from 55 A by either heat or urea, and that the final structure of the active enzyme is the same irrespective of the mechanism of activation. Activation was accompanied by a shift in the peaks of the Kratky plot, though the molecular mass of the enzyme was unchanged. The activation-induced decline in R(g,z) followed first-order kinetics, indicating that activation of the enzyme involved a transition between two states, which was confirmed by singular-value decomposition analysis. When the low-resolution structure of the recombinant enzyme was restored using ab initio modeling, we found it to possess no point symmetry, whereas the heat-activated enzyme possessed 32-point symmetry. In addition, a marked increase in the fluorescence emission was observed with addition of ANS to the inactive recombinant enzyme but not the active forms, indicating that upon activation hydrophobic residues on the surface of the recombinant protein moved to the interior. Taken together, these data strongly suggest that subunit rearrangement, i.e., a change in the quaternary structure of the hexameric recombinant pis-GDH, is essential for activation of the enzyme.

L-Threonine dehydrogenase from the hyperthermophilic archaeon Pyrococcus horikoshii OT3: gene cloning and enzymatic characterization

A gene encoding the L-threonine dehydrogenase homologue has been identified in a hyperthermophlic archaeon Pyrococcus horikoshii OT3 via genome sequencing. The gene was cloned and expressed in Escherichia coli. The purified enzyme from the recombinant E. coli was extremely thermostable; the activity was not lost after incubation at 100 degrees C for 20 min. The enzyme (molecular mass: 192 kDa) is composed of a tetrameric structure with a type of subunit (41 kDa). The enzyme is specific for NAD and utilizes L-threonine, L-serine and DL-threo-3-phenylserine as the substrate. The enzyme required divalent cations such as Zn(2+), Mn(2+) and Co(2+) for the activity, and contained one zinc ion/subunit. The K(m) values for L-threonine and NAD at 50 degrees C were 0.20 mM and 0.024 mM, respectively. Kinetic analyses indicated that the L-threonine oxidation reaction proceeds via a random mechanism with regard to the binding of L-threonine and NAD. The enzyme showed pro-R stereospecificity for hydrogen transfer at the C4 position of the nicotinamide moiety of NADH. This is the first description of the characteristics of an L-threonine dehydrogenase from the archaea domain.

Heat effect on the structure and activity of the recombinant glutamate dehydrogenase from a hyperthermophilic archaeon Pyrococcus horikoshii

Glutamate dehydrogenase from Pyrococcus horikoshii (Pho-GDH) was cloned and overexpressed in Escherichia coli. The cloned enzyme with His-tag was purified to homogeneity by affinity chromatography and shown to be a hexamer enzyme of 290+/-8 kDa (subunit mass 48 kDa). Its optimal pH and temperature were 7.6 and 90 degrees C, respectively. The purified enzyme has outstanding thermostability (the half-life for thermal inactivation at 100 degrees C was 4 h). The enzyme shows strict specificity for 2-oxoglutarate and L-glutamate and requires NAD(P)H and NADP as cofactors but it does not reveal activity on NAD as cofactor. K(m) values of the recombinant enzyme are comparable for both substrates: 0.2 mM for L-glutamate and 0.53 mM for 2-oxoglutarate. The enzyme was activated by heating at 80 degrees C for 1 h, which was accompanied by the formation of its active conformation. Circular dichroism and fluorescence spectra show that the active conformation is heat-inducible and time-dependent.