Figure 2. Three-dimensional structure of β-galactosidase. (a) Ribbon representation of the β-galactosidase tetramer showing the largest face of the molecule. Contacts between red/green and blue/yellow dimers form the long interface. Contacts between the red/yellow and blue/green dimers form the activating interface. Formation of the tetrameric particle results in two deep clefts that run across opposite faces of the molecule. Each contain two active sites. (b) Ribbon diagram of the blue/green dimer viewed down the molecular 2-fold axis, showing the composition of the activating interface. Residues 1–50 from each chain, which form the α-complementation region (see text), are shown in red. The interface includes contacts between the respective complementation peptides, between two helices from the respective monomers that pack together to form a four-helix bundle, and between an extended loop (residues 272–288) from each monomer that reaches across the interface and extends into the active site region of the neighbouring monomer stabilizing the active site structure. (c) Stereo ribbon diagram of the β-galactosidase monomer showing the domain organization of the chain. Residues corresponding to successive domains are coloured in successive spectral colours. Reprinted from Jacobson RH, Zhang XJ, DuBose RF and Matthews BW (1994) Three-dimensional structure of β-galactosidase of E. coli. Nature 369;761–766, with permission.