Medical advances have led to an expanding and diverse immunosuppressed population that is susceptible to opportunistic pathogens, including fungi. Consequently, infection in an immunocompromised host presents a spectrum of clinical, diagnostic and therapeutic challenges, often resulting in a considerable source of morbidity and mortality. Of concern is an epidemiological shift towards invasive fungal infections by Aspergillus
species (Lass-Flörl, 2009
). A. fumigatus
) is responsible for 90% of invasive aspergillosis (IA), in which primarily pulmonary infections can disseminate to any organ (Denning, 1998
). Approximately 24–40% of at-risk patients (for example, those undergoing treatment for haematological malignancies) develop significant disease (Caira et al.
), with mortality rates of up to 90% (Zmeili & Soubani, 2007
), reflecting inherent problems in the diagnosis and treatment of IA. Voriconazole is currently regarded as a first-line therapy (Herbrecht et al.
) for invasive disease, but there are profound drug–drug interactions, toxicity issues and initial reports of resistance (Howard et al.
; Bueid et al.
). Overall, this situation represents considerable medical risk, partly owing to a lack of novel antifungal drug targets in the pipeline of the pharmaceutical industry.
The fungal cell wall is a dynamic, interlaced and only partially defined polysaccharide structure that is essential for survival (Gastebois et al.
). Like glucan and chitin, galactomannan is a major component of the cell wall in A. fumigatus
, forming a linear core of mannan branched with short β(1–5)-linked galactofuranose (Galf
) side chains. Galf
forms the outer edge of the cell wall and is the target of a serological diagnostic test for Aspergillus
(Stynen et al.
). The only source of Galf
is by conversion from galactopyranose (Galp
) by the enzyme UDP-galactopyranose mutase (UGM; Trejo et al.
; Nassau et al.
). UGM (EC 22.214.171.124) is a flavo-containing enzyme that catalyses the isomerization of the six-membered ring (pyranose) form of galactose (Galp
) to the five-membered ring form (Galf
). Deletion of the Af
UGM gene resulted in marked defects on solid media and a reduction in cell-wall thickness and growth rate, and attenuated virulence has been demonstrated in an animal model (Schmalhorst et al.
). These findings were contradicted by a second report of an Af
UGM knockout using a different strain (Lamarre et al.
), leading to uncertainty as to the importance of UGM, and further work is required to resolve this. Crucially, UGM is absent in higher eukaryotes, making it a potential target for structure-based drug design.
biosynthetic pathway has been extensively studied in prokaryotes (Richards & Lowary, 2009
) and structural data have led to insights into the mechanism of prokaryotic UGM (Sanders et al.
; Beis et al.
; Gruber, Borrok et al.
; Gruber, Westler et al.
; Partha et al.
). Although the prokaryotic and eukaryotic UGMs show less than 20% sequence conservation (Bakker et al.
; Beverley et al.
), the active site contains conserved residues (Oppenheimer et al.
Our understanding of the structure and mechanism of AfUGM is very limited as there are no available crystal structures of any eukaryotic UGMs. To further characterize UGM as a potential drug target, detailed structural information on AfUGM is required; this communication describes the cloning, overexpression, purification, crystallization and preliminary X-ray diffraction data of AfUGM, including a selenomethionine (SeMet) derivative.