The predicted tridimensional structure of TRR1 and KRE2 proteins obtained by homology modeling. The structures of TRR1 (a) and KRE2 (b) proteins. The α-helix is represented by the color red, the β-sheet is represented by yellow and loops are represented by green. (a - left panel) The KRE2 active site presents the conserved residues His292, His357, Asp330 and Glu298. (Mutation of these residues abolished the protein activity in C. albicans.) Additional residues found in the KRE2 active site include Glu216, which interacts with the metal ion Mn2+ and creates the reactive nucleophylic center for the glycosyltransferase reaction, and Tyr189, which coordinates the donor and acceptor binding that allows the transfer of the mannose to the growing oligosaccharide. (b - right panel) The TRR1 protein is composed of two domains that comprise the binding sites of NADPH and FAD. The NADPH binding domain contains the active Cys145 and Cys148 residues. Other important residues of the TRR1 active site are Ala151, Val152, Pro153 and Ile154 that form a hydrophobic region in the NADPH binding domain.