Almokalant glucuronidation in human liver and kidney microsomes: evidence for the involvement of UGT1A9 and 2B7.

1. Almokalant, a class III antiarrythmic drug, is metabolized to form isomeric glucuronides identified in human urine. Synthesis of the total glucuronide was studied in human liver and kidney microsomes. Recombinant UDP-glucuronosyltransferases (UGTs) were screened for activity and kinetic analysis was performed to identify the isoform(s) responsible for the formation of almokalant glucuronide in man. 2. From a panel of recombinant isoforms used, both UGT1A9 and 2B7 catalysed the glucuronidation of almokalant. The Km values in both instances were similar with 1.06 mM for the 1A9 and 0.97 mM for the 2B7. Vmax for 1A9 was fourfold higher than that measured for UGT2B7, 92 compared with 21 pmol min(-1) mg(-1), respectively, but UGT1A9 was expressed at approximately twofold higher level than the UGT2B7 in the recombinant cell lines. Therefore, the contribution of UGT2B7 to almokalant glucuronidation could be as significant as that of UGT1A9 in man. 3. Liver and kidney microsomes displayed similar Km values to the cloned expressed UGTs, with the liver and kidney microsomes at 1.68 and 1.06 mM almost identical to the 1A9. 4. The results suggest a significant role for UGT1A9 and 2B7 in the catalysis of almokalant glucuronidation.

UDP-galactopyranose mutase has a novel structure and mechanism.

Uridine diphosphogalactofuranose (UDP-Galf ) is the precursor of the d-galactofuranose (Galf ) residues found in bacterial and parasitic cell walls, including those of many pathogens, such as Mycobacterium tuberculosis and Trypanosoma cruzi. UDP-Galf is made from UDP-galactopyranose (UDP-Galp) by the enzyme UDP-galactopyranose mutase (mutase). The mutase enzyme is essential for the viability of mycobacteria and is not found in humans, making it a viable therapeutic target. The mechanism by which mutase achieves the unprecedented ring contraction of a nonreducing sugar is unclear. We have solved the crystal structure of Escherichia coli mutase to 2.4 A resolution. The novel structure shows that the flavin nucleotide is located in a cleft lined with conserved residues. Site-directed mutagenesis studies indicate that this cleft contains the active site, with the sugar ring of the substrate UDP-galactose adjacent to the exposed isoalloxazine ring of FAD. Assay results establish that the enzyme is active only when flavin is reduced. We conclude that mutase most likely functions by transient reduction of substrate.

Overexpression, purification, crystallization and data collection of a single-stranded DNA-binding protein from Sulfolobus solfataricus.

Single-stranded DNA-binding proteins are recruited when single-stranded DNA is exposed by disruption of the duplex. Many important biological processes such as DNA replication can only occur when the two strands of the duplex are separated. A defining trait of these proteins is the presence of the so-called OB fold. The single-stranded DNA-binding protein of the crenarchaeote Sulfolobus solfataricus has a number of interesting differences and similarities to both the eubacterial and eukaryotic homologues. It has an extended C-terminal tail with significant sequence identity to a similar region in the eubacterial protein. However, the sequence of the OB fold is much more like the eukaryotic and euryarchaeal proteins. The S. solfataricus protein remains a monomer in the absence of DNA but rapidly polymerizes upon binding - a behaviour not seen in the Escherichia coli protein. The protein has been overexpressed, purified and crystallized. The protein crystallizes in two related forms, both having space group P6(1) (or P6(5)) with approximate unit-cell parameters a = b = 75, c = 69 A, but the crystals are distinguished by their size and morphology. The larger crystals are hexagonal bipyramids and are merohedrally twinned, diffracting to 1.34 A with diffraction observed to 1.2 A. Smaller needle-like crystals diffract to about 2.0 A but are not twinned. Molecular-replacement attempts have failed owing to low identity with available search models. The structure will be determined by multiple-wavelength methods.

Overexpression, purification, crystallization and data collection on the Bordetella pertussis wlbD gene product, a putative UDP-GlcNAc 2'-epimerase.

The Boredetella pertussis wlbD gene product is a putative uridine-5-diphosphate N-acetylglucosamine (UDP-GlcNAc) 2'-epimerase involved in Band A lipopolysaccharide biosynthesis. The wlbD gene is homologous to Escherichia coli rffE (32% identical), an established UDP-GlcNAc 2'-epimerase that is involved in enterobacterial common antigen (ECA) formation. The structure of the rffE protein reveals an unexpected role for a bound sodium ion in orientating a substrate-binding alpha-helix in the enzyme active site. Whilst key active-site residues in rffE are present in the wlbD sequence, the sodium-binding residues outside the active site are absent. This raises questions about the modulation of enzyme activity in these two enzymes. The wlbD gene from B. pertussis has been cloned and overexpressed in E. coli and the resulting protein has been purified to homogeneity. In the current study, crystals of the mutant Gln339Arg wlbD enzyme have been obtained by sitting-drop vapour diffusion. Uncomplexed Gln339Arg and UDP-GlcNAc complex data sets have been collected in-house on a rotating-anode generator to 2.1 A. Combined, the data sets identify the space group as P2(1)2(1)2(1), with unit-cell parameters a = 78, b = 91, c = 125 A, alpha = beta = gamma = 90 degrees. The asymmetric unit contains two monomers and 53% solvent.

Overexpression, purification, crystallization and data collection of 3-methylaspartase from Clostridium tetanomorphum.

3-Methylaspartase (E.C. 4.3.1.2) catalyses the reversible anti elimination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to give mesaconic acid as well as a slower syn elimination from the (2S,3R)-epimer, L-erythro-3-methylaspartic acid. The anti-elimination reaction occurs in the second step of the catabolic pathway for glutamic acid in Clostridium tetanomorphum. The reverse reaction is of particular interest because the addition of ammonia to substituted fumaric acids is highly stereoselective and gives highly functionalized amino acids. The mechanism of the transformation is unusual and of considerable interest. 3-Methylaspartase from C. tetanomorphum has been overexpressed and purified from Escherichia coli. Crystals of the enzyme have been obtained by sitting-drop vapour diffusion. Two native data sets have been collected, one in-house on a rotating-anode generator to 3.2 A and one at the European Synchrotron Radiation Facility to 2.0 A. A 2.1 A data set has been collected on a crystal of selenomethionine protein. Combining the data sets identify the space group as P2(1)2(1)2, with unit-cell parameters a = 110.3, b = 109.9, c = 67.2 A, alpha = beta = gamma = 90 degrees. The asymmetric unit contains two monomers with 42% solvent. A self-rotation function indicates the presence of a twofold axis, consistent with a biological dimer.