4-Uloses as Intermediates in Enzyme-Nicotinamide Adenine Dinucleotide-mediated Oxidoreductase Mechanisms

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

An arginine residue is essential for stretching and binding of the substrate on UDP-glucose-4-epimerase from Escherichia coli. Use of a stacked and quenched uridine nucleotide fluorophore as probe

In the previous paper we demonstrated that uridine-5'-beta-1-(5-sulfonic acid) naphthylamidate (UDPAmNS) is a stacked and quenched fluorophore that shows severalfold enhancement of fluorescence in a stretched conformation. UDPAmNS was found to be a powerful competitive inhibitor (Ki = 0.2 mM) for UDP-glucose-4-epimerase from Escherichia coli. This active site-directed fluorophore assumed a stretched conformation on the enzyme surface, as was evidenced by full enhancement of fluorescence in saturating enzyme concentration. Complete displacement of the fluorophore by UDP suggested it to bind to the substrate binding site of the active site. Analysis of inactivation kinetics in presence of alpha,beta-diones such as phenylglyoxal, cyclohaxanedione, and 2,3-butadione suggested involvement of the essential arginine residue in the overall catalytic process. From spectral analysis, loss of activity could also be directly correlated with modification of only one arginine residue. Protection experiments with UDP showed the arginine residue to be located in the uridyl phosphate binding subsite. Unlike the native enzyme, the modified enzyme failed to show any enhancement of fluorescence with UDPAmNS clearly demonstrating the role of the essential arginine residue in stretching and binding of the substrate. The potential usefulness of such stacked and quenched nucleotide fluorophores has been discussed.

The isolation, purification, and preliminary crystallographic characterization of UDP-galactose-4-epimerase from Escherichia coli

Uridine diphosphogalactose-4-epimerase from E. coli has been crystallized in a form suitable for a high-resolution X-ray crystallographic structural analysis. The enzyme complexed with a substrate analogue, uridine diphosphobenzene (UDP-benzene), crystallizes readily using polyethylene glycol 8000 as the precipitant. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) with unit cell dimensions, a = 76.3 A, b = 83.1 A, and c = 132.1 A. Based on still setting photographs, the crystals diffract to a nominal resolution of 2.3 A and are stable in the X-ray beam. The enzyme used in these experiments was produced by a new expression system and a modified purification scheme.

Reaction of uridine diphosphate galactose 4-epimerase with a suicide inactivator

UDPgalactose 4-epimerase from Escherichia coli is rapidly inactivated by the compounds uridine 5'-diphosphate chloroacetol (UDC) and uridine 5'-diphosphate bromoacetol (UDB). Both UDC and UDB inactivate the enzyme in neutral solution concomitant with the appearance of chromophores absorbing maximally at 325 and 328 nm, respectively. The reaction of UDC with the enzyme follows saturation kinetics characterized by a KD of 0.110 mM and kinact of 0.84 min-1 at pH 8.5 and ionic strength 0.2 M. The inactivation by UDC is competitively inhibited by competitive inhibitors of UDPgalactose 4-epimerase, and it is accompanied by the tight but noncovalent binding of UDC to the enzyme in a stoichiometry of 1 mol of UDC/mol of enzyme dimer, corresponding to 1 mol of UDC/mol of enzyme-bound NAD+. The inactivation of epimerase by uridine 5'-diphosphate [2H2]chloroacetol proceeds with a primary kinetic isotope effect (kH/kD) of 1.4. The inactivation mechanism is proposed to involve a minimum of three steps: (a) reversible binding of UDC to the active site of UDPgalactose 4-epimerase; (b) enolization of the chloroacetol moiety of enzyme-bound UDC, catalyzed by an enzymic general base at the active site; (c) alkylation of the nicotinamide ring of NAD+ at the active site by the chloroacetol enolate. The resulting adduct between UDC and NAD+ is proposed to be the chromophore with lambda max at 325 nm. The enzymic general base required to facilitate proton transfer in redox catalysis by this enzyme may be the general base that facilitates enolization of the chloroacetol moiety of UDC in the inactivation reaction.

Uridine-diphosphate-glucose 4-epimerase from Saccharomyces fragilis. Inactivation by heat and reconstitution of the inactive enzyme

UDP-glucose 4-epimerase from Saccharomyces fragilis is rapidly inactivated by heating at 42 degrees C for 7 min and at 45 degrees C for 4 min. The effector site, specific for sugar phosphates, is destroyed still earlier. The enzyme is inactivated by the dissocation of NAD from it leaving the dimeric structure unaffected. It can be reactivated by mercaptoethanol and NAD, both of which are essential for reactivation, and NAD becomes associated with the dimeric protein moiety.