Enoyl-ACP reductase (FabI) of Haemophilus influenzae: steady-state kinetic mechanism and inhibition by triclosan and hexachlorophene

Steady-state kinetics, equilibrium binding, and primary substrate kinetic isotope effect studies revealed that the reduction of crotonyl-CoA by NADH, catalyzed by Haemophilus influenzae enoyl-ACP reductase (FabI), follows a rapid equilibrium random kinetic mechanism with negative interaction among the substrates. Two biphenyl inhibitors, triclosan and hexachlorophene, were studied in the context of the kinetic mechanism. IC(50) values for triclosan in the presence and absence of NAD(+) were 0.1 +/- 0.02 and 2.4 +/- 0.02 microM, respectively, confirming previous observations that the E-NAD(+) complex binds triclosan more tightly than the free enzyme. Preincubation of the enzyme with triclosan and NADH suggested that the E-NADH complex is the active triclosan binding species as well. These results were reinforced by measurement of binding kinetic transients. Intrinsic protein fluorescence changes induced by binding of 20 microM triclosan to E, E-NADH, E-NAD(+), and E-crotonyl-CoA occur at rates of 0.0124 +/- 0.001, 0.0663 +/- 0.002, 0.412 +/- 0.01, and 0.0069 +/- 0.0001 s(-1), respectively. The rate of binding decreased with increasing crotonyl-CoA concentrations in the E-crotonyl-CoA complex, and the extrapolated rate at zero concentration of crotonyl-CoA corresponded to the rate observed for the binding to the free enzyme. This suggests that triclosan and the acyl substrate share a common binding site. Hexachlorophene inhibition, on the other hand, was NAD(+)- and time-independent; and the calculated IC(50) value was 2.5 +/- 0.4 microM. Steady-state inhibition patterns did not allow the mode of inhibition to be unambiguously determined, but binding kinetics suggested that free enzyme, E-NAD(+), and E-crotonyl-CoA have similar affinity for hexachlorophene, since the k(obs)s were in the same range of 20-24 s(-1). When the E-NADH complex was mixed with hexachlorophene ligand, concentration-independent fluorescence quenching at 480 nm was observed, suggesting at least partial competition between NADH and hexachlorophene for the same binding site. Mutual exclusivity studies, together with the above-discussed results, indicate that triclosan and hexachlorophene bind at different sites of H. influenzae FabI.

Characterization of recombinant, soluble beta-secretase from an insect cell expression system

The beta-site amyloid precursor protein-cleaving enzyme (BACE) cleaves the amyloid precursor protein to produce the N terminus of the amyloid beta peptide, a major component of the plaques found in the brains of Alzheimer's disease patients. Sequence analysis of BACE indicates that the protein contains the consensus sequences found in most known aspartyl proteases, but otherwise has only modest homology with aspartyl proteases of known three-dimensional structure (i.e., pepsin, renin, or cathepsin D). Because BACE has been shown to be one of the two proteolytic activities responsible for the production of the Abeta peptide, this enzyme is a prime target for the design of therapeutic agents aimed at reducing Abeta for the treatment of Alzheimer's disease. Toward this ultimate goal, we have expressed a recombinant, truncated human BACE in a Drosophila melanogaster S2 cell expression system to generate high levels of secreted BACE protein. The protein was convenient to purify and was enzymatically active and specific for cleaving the beta-secretase site of human APP, as demonstrated with soluble APP as the substrate in novel sandwich enzyme-linked immunosorbent assay and Western blot assays. Further kinetic analysis revealed no catalytic differences between this recombinant, secreted BACE, and brain BACE. Both showed a strong preference for substrates that contained the Swedish mutation, where NL is substituted for KM immediately upstream of the cleavage site, relative to the wild-type sequence, and both showed the same extent of inhibition by a peptide-based inhibitor. The capability to produce large quantities of BACE enzyme will facilitate protein structure determination and inhibitor development efforts that may lead to the evolution of useful Alzheimer's disease treatments.

Helicobacter pylori-selective antibacterials based on inhibition of pyrimidine biosynthesis

We report the discovery of a class of pyrazole-based compounds that are potent inhibitors of the dihydroorotate dehydrogenase of Helicobacter pylori but that do not inhibit the cognate enzymes from Gram-positive bacteria or humans. In culture these compounds inhibit the growth of H. pylori selectively, showing no effect on other Gram-negative or Gram-positive bacteria or human cell lines. These compounds represent the first examples of H. pylori-specific antibacterial agents. Cellular activity within this structural class appears to be due to dihydroorotate dehydrogenase inhibition. Minor structural changes that abrogate in vitro inhibition of the enzyme likewise eliminate cellular activity. Furthermore, the minimum inhibitory concentrations of these compounds increase upon addition of orotate to the culture medium in a concentration-dependent manner, consistent with dihydroorotate dehydrogenase inhibition as the mechanism of cellular inhibition. The data presented here suggest that targeted inhibition of de novo pyrimidine biosynthesis may be a valuable mechanism for the development of antimicrobial agents selective for H. pylori.

A second dihydroorotate dehydrogenase (Type A) of the human pathogen Enterococcus faecalis: expression, purification, and steady-state kinetic mechanism

We report the identification, expression, and characterization of a second Dihydroorotate dehydrogenase (DHODase A) from the human pathogen Enterococcus faecalis. The enzyme consists of a polypeptide chain of 322 amino acids that shares 68% identity with the cognate type A enzyme from the bacterium Lactococcus lactis. E. faecalis DHODase A catalyzed the oxidation of l-dihydroorotate while reducing a number of substrates, including fumarate, coenzyme Q(0), and menadione. The steady-state kinetic mechanism has been determined with menadione as an oxidizing substrate at pH 7.5. Initial velocity and product inhibition data suggest that the enzyme follows a two-site nonclassical ping-pong kinetic mechanism. The absorbance of the active site FMN cofactor is quenched in a concentration-dependent manner by titration with orotate and barbituric acid, two competitive inhibitors with respect to dihydroorotate. In contrast, titration of the enzyme with menadione had no effect on FMN absorbance, consistent with nonoverlapping binding sites for dihyroorotate and menadione, as suggested from the kinetic mechanism. The reductive half-reaction has been shown to be only partially rate limiting, and an attempt to evaluate the slow step in the overall reaction has been made by simulating orotate production under steady-state conditions. Our data indicate that the oxidative half-reaction is a rate-limiting segment, while orotate, most likely, retains significant affinity for the reduced enzyme, as suggested by the product inhibition pattern.