Putative active site of EF-EG2. (a) Solvent accessible surface showing the binding cleft. The surface is colored according to the local electrostatic potential as calculated by APBS (Baker et al., 2001) from −3kT (red) to +3kT (blue). (a) is drawn from the identical direction of Fig. 1 and is a close-up view of the left-hand side of Fig. 4(a). (b) Electron densities of putative active residues and crystal reagents at the binding cleft. The dark green contour shows the Fo − Fc (3.5σ) map calculated without TRS, GOL1, 2, 3 molecules and the side-chain atoms Asp74, Asp77 and Glu431. (c) Superposition of residues Asp74, Asp77 and Glu431 from EF-EG2 (magenta) on Asp383, Asp386 and Gln795 from CbhA (cyan). Residues within 4 Å from TRS, GOL1, GOL2 (in EF-EG2 structure, purple) and CTT (in CbhA structure, yellow) are shown as white and gray stick models, respectively. Hydrogen-bonding interactions (within 3.4 Å) in the EF-EG2 structure are represented by black dashed lines. Bond lengths that may be useful for elucidating the catalytic mechanism of EF-EG2 are indicated by red dashed lines.