Uridine diphosphate glucose pyrophosphorylase: differential heat inactivation and further characterization of human liver enzyme

CryoEM analysis of the essential native UDP-glucose pyrophosphorylase from Aspergillus nidulans reveals key conformations for activity regulation and function

Invasive aspergillosis is one of the most serious clinical invasive fungal infections, resulting in a high case fatality rate among immunocompromised patients. The disease is caused by saprophytic molds in the genus Aspergillus, including Aspergillus fumigatus, the most significant pathogenic species. The fungal cell wall, an essential structure mainly composed of glucan, chitin, galactomannan, and galactosaminogalactan, represents an important target for the development of antifungal drugs. UDP (uridine diphosphate)-glucose pyrophosphorylase (UGP) is a central enzyme in the metabolism of carbohydrates that catalyzes the biosynthesis of UDP-glucose, a key precursor of fungal cell wall polysaccharides. Here, we demonstrate that the function of UGP is vital for Aspergillus nidulans (AnUGP). To understand the molecular basis of AnUGP function, we describe a cryoEM structure (global resolution of 3.5 Å for the locally refined subunit and 4 Å for the octameric complex) of a native AnUGP. The structure reveals an octameric architecture with each subunit comprising an N-terminal α-helical domain, a central catalytic glycosyltransferase A-like (GT-A-like) domain, and a C-terminal (CT) left-handed β-helix oligomerization domain. AnUGP displays unprecedented conformational variability between the CT oligomerization domain and the central GT-A-like catalytic domain. In combination with activity measurements and bioinformatics analysis, we unveil the molecular mechanism of substrate recognition and specificity for AnUGP. Altogether, our study not only contributes to understanding the molecular mechanism of catalysis/regulation of an important class of enzymes but also provides the genetic, biochemical, and structural groundwork for the future exploitation of UGP as a potential antifungal target. IMPORTANCE Fungi cause diverse diseases in humans, ranging from allergic syndromes to life-threatening invasive diseases, together affecting more than a billion people worldwide. Increasing drug resistance in Aspergillus species represents an emerging global health threat, making the design of antifungals with novel mechanisms of action a worldwide priority. The cryoEM structure of UDP (uridine diphosphate)-glucose pyrophosphorylase (UGP) from the filamentous fungus Aspergillus nidulans reveals an octameric architecture displaying unprecedented conformational variability between the C-terminal oligomerization domain and the central glycosyltransferase A-like catalytic domain in the individual protomers. While the active site and oligomerization interfaces are more highly conserved, these dynamic interfaces include motifs restricted to specific clades of filamentous fungi. Functional study of these motifs could lead to the definition of new targets for antifungals inhibiting UGP activity and, thus, the architecture of the cell wall of filamentous fungal pathogens.

Agarose gel isoelectrofocusing of UDP-galactose pyrophosphorylase and galactose-1-phosphate uridyltransferase. Developmental aspect of UDP-galactose pyrophosphorylase

The uridine diphosphogalactose pyrophosphorylase activity has been determined in human adult and fetal tissues as well as blood of various ages by measurement of UDP-galactose production from gal-1-p and UTP. The highest activity was found from adult liver in which the specific activity was about 5% of the gal-1-p uridyltransferase activity. In general adult tissues had a somewhat higher activity than the corresponding fetal tissues except erythrocytes in which fetuses had a 5-10 times higher activity than adults. From normal blood the pyrophosphorylase activity in erythrocytes decreased with age, but in the case of galactosemia the decrease with age was not distinct. According to agarose gel isoelectrofocusing studies, at least two isozyme forms for UDP-galactose pyrophosphorylase exist with the activity bands between pH 6.0-6.15. The patterns of AGIF bands of pyrophosphorylase varied according to the age of the samples, suggesting the development of the isozyme forms of pyrophosphorylase to be age-dependent. Uridyltransferase, on the other hand, resolved into multiple bands between pH 5.1-5.6 on agarose gels and the patterns varied according to the variants but not to the age. Significance of the decrease in the pyrophosphorylase activity in erythrocytes with age as well as of the difference in AGIF bands between normal and the galactosemic were discussed with regard to the pathology of classical galactosemia.

Separation and characterization of two UTP-utilizing hexose phosphate uridylyltransferases from Entamoeba histolytica

Two UTP-utilizing uridylyltransferases which react with both glucose 1-phosphate and galactose 1-phosphate were isolated from cell-free extracts of Entamoeba histolytica. The more specific of these enzymes, glucose-1-phosphate uridylyltransferase, acts preferentially on glucose 1-phosphate, having a maximum velocity 20-fold greater with this substrate than with galactose 1-phosphate. It was purified 200 fold with a 25% yield and has a molecular weight of 45 000. This enzyme requires a reducing agent for stability. The less specific transferase reacts with both hexose phosphates, having a maximum velocity of 1.35 times greater with galactose 1-phosphate. It was purified 1000 fold with a 20% yield, and has a molecular weight of 40 000. The common Leloir enzyme, UDP glucose-hexose-1-phosphate uridylytransferase (EC 2.7.7.12), was not found in this organism. To avoid confusion with the Leloir enzyme our experience suggests that the less specific enzyme, which is presently referred to in the literature as galactose-1-phosphate uridylyltransferase (EC 2.7.7.10), should be named UTP:hexose-1-phosphate uridylyltransferase (EC 2.7.7.?). The more specific enzyme (EC 2.7.7.9) should be more clearly named UTP:glucose-1-phosphate uridylyltransferase.

Separation and characteristics of galactose-1-phosphate and glucose-1-phosphate uridyltransferase from fruit peduncles of cucumber

Galactose-1-phosphate uridyltransferase (EC 2.7.7.10), responsible for the conversion of galactose-1-phosphate (Gal-1-P) to uridine diphosphate galactose (UDPgal) was examined in fruit peduncles of Cucumis sativus L. Two uridyltransferases (pyrophosphorylases), from I and II, were partially purified and resolved on a diethylamino-ethyl-cellulose column. Form I can utilize glucose-1-phosphate (Glc-1-P), while form II can utilize either Gal-1-P or Glc-1-P, with a preference for Gal-1-P. Form I was more heat stable than form II. Both Glc-1-P and Gal-1-P activities of form II were inactivated at the same rate by heating. The finding of a uridyltransferase with preference for Gal-1-P indicates that cucumber may have a Gal-1-P uridyltransferase (pyrophosphorylase) pathway for the catabolism of stachyose in the peduncles. The absence of the enzyme UDP-glucose-hexose-1-phosphate uridyltransferase (EC 2.7.7.12) in this tissue rules out catabolism by the classical Leloir pathway. The incorporation of carbon from UDPglc into Glc-1-P as opposed to sucrose may be regulated by the activities of the uridyltransferases. Pyrophosphate, in the same concentration range, inhibits UDP-gal formation (Ki=0.58±0.10 mM) and stimulates Glc-1-P formation. The ratio of units of pyrophosphatase to units of Gal-1-P uridyltransferase was higher in peduncles from growing fruit than from unpollinated fruit. Modulation of carbon partitioning through a uridyltransferase pathway may be a factor controlling growth of the cucumber fruit.

Presence of a single enzyme catalyzing the pyrophosphorolysis of UDP-glucose and UDP-galactose in Bifidobacterium bifidum

The enzyme preparation catalyzing the pyrophosphorolyses of UDP-glucose and UDPgalactose almost at the same rate was purified about 900-fold from Bifidobacterium bifidum grown on glucose medium. The two activities were always associated with each other, and their activity ratio was always constant throughout the purification steps. The final preparations was revealed homogeneous by polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate. There was no significant difference in thermal stabilities of the two activities. From these results it was concluded that the UDPglucose and UDPgalactose pyrophosphorylase activities in B. bifidum are catalyzed by a single enzyme protein.