Characterization of the deoxynucleotide triphosphate triphosphohydrolase (dNTPase) activity of the EF1143 protein from Enterococcus faecalis and crystal structure of the activator-substrate complex

The EF1143 protein from Enterococcus faecalis is a distant homolog of deoxynucleotide triphosphate triphosphohydrolases (dNTPases) from Escherichia coli and Thermus thermophilus. These dNTPases are important components in the regulation of the dNTP pool in bacteria. Biochemical assays of the EF1143 dNTPase activity demonstrated nonspecific hydrolysis of all canonical dNTPs in the presence of Mn(2+). In contrast, with Mg(2+) hydrolysis required the presence of dGTP as an effector, activating the degradation of dATP and dCTP with dGTP also being consumed in the reaction with dATP. The crystal structure of EF1143 and dynamic light scattering measurements in solution revealed a tetrameric oligomer as the most probable biologically active unit. The tetramer contains four dGTP specific allosteric regulatory sites and four active sites. Examination of the active site with the dATP substrate suggests an in-line nucleophilic attack on the α-phosphate center as a possible mechanism of the hydrolysis and two highly conserved residues, His-129 and Glu-122, as an acid-base catalytic dyad. Structural differences between EF1143 apo and holo forms revealed mobility of the α3 helix that can regulate the size of the active site binding pocket and could be stabilized in the open conformation upon formation of the tetramer and dGTP effector binding.     

Crystallization and preliminary x-ray analysis of the flavoenzyme vanillyl-alcohol oxidase from Penicillium Simplicissimum

Vanillyl-alcohol oxidase catalyses the oxidation of several 4-hydroxybenzyl alcohols by using 8-α-(N³-histidyl)-FAD as a covalently bound prosthetic group. Crystals of vanillyl-alcohol oxidase from Penicillium simplicissimum have been grown by using the vapor diffusion technique. The space group was found to be I4, with cell dimensions a = b = 140.5 Å, c = 132.9 Å. Diffraction data have been recorded to 3.2 Å resolution by using a laboratory source and to 2.5 Å resolution on flash freezing the crystal at the ELETTRA Synchrotron X-ray diffraction beam line. Proteins 27:601–603, 1997. © 1997 Wiley-Liss Inc.     

Stabilization of C4a-hydroperoxyflavin in a two-component flavin-dependent monooxygenase is achieved through interactions at flavin N5 and C4a atoms

p-Hydroxyphenylacetate (HPA) 3-hydroxylase is a two-component flavin-dependent monooxygenase. Based on the crystal structure of the oxygenase component (C₂), His-396 is 4.5 Å from the flavin C4a locus, whereas Ser-171 is 2.9 Å from the flavin N5 locus. We investigated the roles of these two residues in the stability of the C4a-hydroperoxy-FMN intermediate. The results indicated that the rate constant for C4a-hydroperoxy-FMN formation decreased ∼30-fold in H396N, 100-fold in H396A, and 300-fold in the H396V mutant, compared with the wild-type enzyme. Lesser effects of the mutations were found for the subsequent step of H₂O₂ elimination. Studies on pH dependence showed that the rate constant of H₂O₂ elimination in H396N and H396V increased when pH increased with pK_a >9.6 and >9.7, respectively, similar to the wild-type enzyme (pK_a >9.4). These data indicated that His-396 is important for the formation of the C4a-hydroperoxy-FMN intermediate but is not involved in H₂O₂ elimination. Transient kinetics of the Ser-171 mutants with oxygen showed that the rate constants for the H₂O₂ elimination in S171A and S171T were ∼1400-fold and 8-fold greater than the wild type, respectively. Studies on the pH dependence of S171A with oxygen showed that the rate constant of H₂O₂ elimination increased with pH rise and exhibited an approximate pK_a of 8.0. These results indicated that the interaction of the hydroxyl group side chain of Ser-171 and flavin N5 is required for the stabilization of C4a-hydroperoxy-FMN. The double mutant S171A/H396V reacted with oxygen to directly form the oxidized flavin without stabilizing the C4a-hydroperoxy-FMN intermediate, which confirmed the findings based on the single mutation that His-396 was important for formation and Ser-171 for stabilization of the C4a-hydroperoxy-FMN intermediate in C₂.     

Site-directed mutagenesis and homology modeling indicate an important role of cysteine 439 in the stability of betaine aldehyde dehydrogenase from Pseudomonas aeruginosa

Betaine aldehyde dehydrogenase (BADH) from the human pathogen Pseudomonas aeruginosa is a tetrameric enzyme that contains a catalytic Cys286 and three additional cysteine residues, Cys353, 377, and 439, per subunit. In the present study, we have investigated the role of the three non-essentials in enzyme activity and stability by homology modeling and site-directed mutagenesis. Cys353 and Cys377 are located at the protein surface with their sulfur atoms buried, while Cys439 is at the subunit interface between the monomers forming a dimeric pair. All three residues were individually mutated to alanine and Cys439 also to serine and valine. The five mutant proteins were expressed in Escherichia coli and purified to homogeneity. Their steady-state kinetics was not significantly affected, neither was their structure as indicated by circular dicroism spectropolarimetry, protein intrinsic fluorescence, and size-exclusion chromatography. However, stability was severely reduced in the Cys439 mutants particularly in C439S and C439V, which were inactive when expressed at 37 degrees C. They also exhibited higher sensitivity to thermal and chemical inactivation, and higher propensity to dissociation by dilution or exposure to low ionic strength than the wild-type enzyme. Size-exclusion chromatography indicates that substitution of Cys439 lead to unstable dimers or to stable dimeric conformations not compatible with a stable tetrameric structure. To the best of our knowledge, this is the first study of an aldehyde dehydrogenase revealing a residue at the dimer interface involved in holding the dimer, and consequently the tetramer, together.