Special effects of UDP-sugar binding to bovine liver uridine diphosphoglucose dehydrogenase

UDP-glucose dehydrogenase activity and optimal downstream cellular function require dynamic reorganization at the dimer-dimer subunit interfaces

UDP-glucose dehydrogenase (UGDH) provides precursors for steroid elimination, hyaluronan production, and glycosaminoglycan synthesis. The wild-type UGDH enzyme purifies in a hexamer-dimer equilibrium and transiently undergoes dynamic motion that exposes the dimer-dimer interface during catalysis. In the current study we created and characterized point mutations that yielded exclusively dimeric species (obligate dimer, T325D), dimeric species that could be induced to form hexamers in the ternary complex with substrate and cofactor (T325A), and a previously described exclusively hexameric species (UGDHΔ132) to investigate the role of quaternary structure in regulation of the enzyme. Characterization of the purified enzymes revealed a significant decrease in the enzymatic activity of the obligate dimer and hexamer mutants. Kinetic analysis of wild-type UGDH and the inducible hexamer, T325A, showed that upon increasing enzyme concentration, which favors the hexameric species, activity was modestly decreased and exhibited cooperativity. In contrast, cooperative kinetic behavior was not observed in the obligate dimer, T325D. These observations suggest that the regulation of the quaternary assembly of the enzyme is essential for optimal activity and allosteric regulation. Comparison of kinetic and thermal stability parameters revealed structurally dependent properties consistent with a role for controlled assembly and disassembly of the hexamer in the regulation of UGDH. Finally, both T325A and T325D mutants were significantly less efficient in promoting downstream hyaluronan production by HEK293 cells. These data support a model that requires an operational dimer-hexamer equilibrium to function efficiently and preserve regulated activity in the cell.

Cofactor binding triggers a molecular switch to allosterically activate human UDP-α-D-glucose 6-dehydrogenase

Human UDP-α-D-glucose dehydrogenase (hUGDH) catalyzes the NAD(+)-dependent oxidation of UDP-α-D-glucose (UDG) to produce UDP-α-D-glucuronic acid. The oligomeric structure of hUGDH is dynamic and can form two distinct hexameric complexes in solution. The active form of hUGDH consists of dimers that undergo a concentration-dependent association to form a hexamer with 32 symmetry. In the presence of the allosteric feedback inhibitor UDP-α-D-xylose (UDX), hUGDH changes shape to form an inactive, horseshoe-shaped complex. Previous studies have identified the UDX-induced allosteric mechanism that changes the hexameric structure to inhibit the enzyme. Here, we investigate the role of the 32 symmetry hexamer in the catalytic cycle. We engineered a stable hUGDH dimer by introducing a charge-switch substitution (K94E) in the hexamer-building interface (hUGDH(K94E)). The k(cat) of hUGDH(K94E) is ~160-fold lower than that of the wild-type enzyme, suggesting that the hexamer is the catalytically relevant state. We also show that cofactor binding triggers the formation of the 32 symmetry hexamer, but UDG is needed for the stability of the complex. The hUGDH(K94E) crystal structure at 2.08 Å resolution identifies loop(88-110) as the cofactor-responsive allosteric switch that drives hexamer formation; loop(88-110) directly links cofactor binding to the stability of the hexamer-building interface. In the interface, loop(88-110) packs against the Thr131-loop/α6 helix, the allosteric switch that responds to the feedback inhibitor UDX. We also identify a structural element (the S-loop) that explains the indirect stabilization of the hexamer by substrate and supports a sequential, ordered binding of the substrate and cofactor. These observations support a model in which (i) UDG binds to the dimer and stabilizes the S-loop to promote cofactor binding and (ii) cofactor binding orders loop(88-110) to induce formation of the catalytically active hexamer.

UDP-glucose dehydrogenase: structure and function of a potential drug target

Biosynthesis of the glycosaminoglycan precursor UDP-α-D-glucuronic acid occurs through a 2-fold oxidation of UDP-α-D-glucose that is catalysed by UGDH (UDP-α-D-glucose 6-dehydrogenase). Structure-function relationships for UGDH and proposals for the enzymatic reaction mechanism are reviewed in the present paper, and structure-based sequence comparison is used for subclassification of UGDH family members. The eukaryotic group of enzymes (UGDH-II) utilize an extended C-terminal domain for the formation of complex homohexameric assemblies. The comparably simpler oligomerization behaviour of the prokaryotic group of enzymes (UGDH-I), in which dimeric forms prevail, is traced back to the lack of relevant intersubunit contacts and trimmings within the C-terminal region. The active site of UGDH contains a highly conserved cysteine residue, which plays a key role in covalent catalysis. Elevated glycosaminoglycan formation is implicated in a variety of human diseases, including the progression of tumours. The inhibition of synthesis of UDP-α-D-glucuronic acid using UGDH antagonists might therefore be a useful strategy for therapy.

Alteration of the quaternary structure of human UDP-glucose dehydrogenase by a double mutation

There are conflicting views for the polymerization process of human UDP-glucose dehydrogenase (UGDH) and no clear evidence has been reported yet. Based on crystal coordinates for Streptococcus pyogenes UGDH, we made double mutant A222Q/S233G. The double mutagenesis had no effects on expression, stability, and secondary structure. Interestingly, A222Q/S233G was a dimeric form and showed an UGDH activity, although it showed increased Km values for substrates. These results suggest that Ala222 and Ser233 play an important role in maintaining the hexameric structure and the reduced binding affinities for substrates are attributable to its altered subunit communication although quaternary structure may not be critical for catalysis.

Characterization of human UDP-glucose dehydrogenase reveals critical catalytic roles for lysine 220 and aspartate 280

Human UDP-glucose dehydrogenase (UGDH) is a homohexameric enzyme that catalyzes two successive oxidations of UDP-glucose to yield UDP-glucuronic acid, an essential precursor for matrix polysaccharide and proteoglycan synthesis. We previously used crystal coordinates for Streptococcus pyogenes UGDH to generate a model of the human enzyme active site. In the studies reported here, we have used this model to identify three putative active site residues: lysine 220, aspartate 280, and lysine 339. Each residue was site-specifically mutagenized to evaluate its importance for catalytic activity and maintenance of hexameric quaternary structure. Alteration of lysine 220 to alanine, histidine, or arginine significantly impaired enzyme function. Assaying activity over longer time courses revealed a plateau after reduction of a single equivalent of NAD+ in the alanine and histidine mutants, whereas turnover continued in the arginine mutant. Thus, one role of this lysine may be to stabilize anionic transition states during substrate conversion. Mutation of aspartate 280 to asparagine was also severely detrimental to catalysis. The relative position of this residue within the active site and dependence of function on acidic character point toward a critical role for aspartate 280 in activation of the substrate and the catalytic cysteine. Finally, changing lysine 339 to alanine yielded the wild-type Vmax, but a 165-fold decrease in affinity for UDP-glucose. Interestingly, gel filtration of this substrate-binding mutant also determined it was a dimer, indicating that hexameric quaternary structure is not critical for catalysis. Collectively, this analysis has provided novel insights into the complex catalytic mechanism of UGDH.

Biosynthesis of UDP-GlcA, a key metabolite for capsular polysaccharide synthesis in the pathogenic fungus Cryptococcus neoformans

UDP-glucose dehydrogenase catalyses the conversion of UDP-glucose into UDP-GlcA, a critical precursor for glycan synthesis across evolution. We have cloned the gene encoding this important enzyme from the opportunistic pathogen Cryptococcus neoformans. In this fungus, UDP-GlcA is required for the synthesis of capsule polysaccharides, which in turn are essential for virulence. The gene was expressed in Escherichia coli and the 51.3-kDa recombinant protein from wild-type and five mutants was purified for analysis. The cryptococcal enzyme is strongly inhibited by UDP-xylose and NADH, has highest activity at pH 7.5 and demonstrates Km (app) values of 0.1 and 1.5 mM for NAD+ and UDP-glucose respectively. Its activity was significantly decreased by mutations in the putative sites of NAD+ and UDP-glucose binding. Unlike previously reported eukaryotic UDP-glucose dehydrogenases, which are hexamers, the cryptococcal enzyme is a dimer.

UDP-glucose dehydrogenase plays multiple roles in the biology of the pathogenic fungus Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus surrounded by an elaborate polysaccharide capsule that is strictly required for its virulence in humans and other mammals. Nearly half of the sugar residues in the capsule are derived from UDP-glucuronic acid or its metabolites. To examine the role of these nucleotide sugars in C. neoformans, the gene encoding UDP-glucose dehydrogenase was disrupted. Mass spectrometry analysis of nucleotide sugar pools showed that the resulting mutant lacked both UDP-glucuronic acid and its downstream product, UDP-xylose, thus confirming the effect of the knockout and indicating that an alternate pathway for UDP-glucuronic acid production was not used. The mutant was dramatically affected by the lack of specific sugar donors, demonstrating altered cell integrity, temperature sensitivity, lack of growth in an animal model of cryptococcosis, and morphological defects. Additionally, the polysaccharide capsule could not be detected on the mutant cells, although the possibility remains that abbreviated forms of capsule components are made, possibly without proper surface display. The capsule defect is largely independent of the other observed changes, as cells that are acapsular because of mutations in other genes show lack of virulence but do not exhibit alterations in cell integrity, temperature sensitivity, or cellular morphology. All of the observed alterations were reversed by correction of the gene disruption.

cDNA cloning and expression of UDP-glucose dehydrogenase from bovine kidney

We have isolated a cDNA encoding UDP-glucose dehydrogenase from a bovine kidney cDNA-library, the first mammalian cDNA clone published. [After submission of the manuscript, a study appeared describing the molecular cloning and characterization of the human and mouse UDP-glucose dehydrogenase genes (Spicer et al., 1998).] The enzyme catalyzes the conversion of UDP-glucose to UDP-glucuronic acid, an essential precursor in glycosaminoglycan biosynthesis. The cDNA has an open reading frame of 1482 nucleotides coding for a 55 kDa protein. Expression of the enzyme in COS-7 cells showed a 3-fold increase in UDP-glucose dehydrogenase activity; also, the C-terminal 23 amino acids was shown not to be necessary for enzyme activity. Northern blots from human and mouse tissues reveal high expression in liver and low in skeletal muscle. Human tissues have a major transcript size of 3.2 kilobases and a minor of 2.6 whereas mouse tissues have a single 2.6 kilobase transcript. We have also developed a sensitive and direct assay using UDP-[14C]Glc as a substrate for detection of small amounts of UDPGDH activity.

Studies on the unusual behaviour of bovine liver UDP-glucose dehydrogenase in assays at acid and neutral pH and on the presence of tightly bound nucleotide material in purified preparations of this enzyme

Assays of UDP-glucose dehydrogenase at pH 6.0 show long (10-15 min) lag periods before the steady-state rate is established, but at pH 9.0 no lag is observed. At intermediate pH values the lag is progressively shorter as the pH becomes more alkaline. The behaviour of the enzyme in assays at neutral and acid pH depends on the pH and concentration of the enzyme used to initiate the assay. The steady-state rate at pH 6.0 is strongly concentration-dependent. It is suggested that these phenomena arise because of the slow dissociation of an inactive enzyme species to an active one. Purified preparations of the enzyme release approx. 1 mol of a UDP-sugar/mol of enzyme subunit on denaturation. The identity of the UDP-sugar is unknown.