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The catalytic activity of the pyridoxal 5-phosphatedependent tryptophan synthase ␣ 2 ␤ 2 complex is allosterically regulated. The hydrogen bond between the helix ␤H6 residue ␤Ser 178 and the loop ␣L6 residue Gly 181 was shown to be critical in ligand-induced intersubunit signaling, with the ␣-␤ communication being completely lost in the mutant ␤Ser 178 3 Pro (Marabotti, A., De Biase, D., Tramonti, A., Bettati, S., and Mozzarelli, A. (2001) J. Biol. Chem. 276, 17747-17753). The structural basis of the impaired allosteric regulation was investigated by determining the crystal structures of the mutant ␤Ser 178 3 Pro in the absence and presence of the ␣-subunit ligands indole-3-acetylglycine and glycerol 3-phosphate. The mutation causes local and distant conformational changes especially in the ␤-subunit. The ligand-free structure exhibits larger differences at the N-terminal part of helix ␤H6, whereas the enzyme ligand complexes show differences at the C-terminal side. In contrast to the wild-type enzyme loop ␣L6 remains in an open conformation even in the presence of ␣-ligands. This effects the equilibrium between active and inactive conformations of the ␣-active site, altering k cat and K m , and forms the structural basis for the missing allosteric communication between the ␣-and ␤-subunits.The ␣ 2 ␤ 2 complex of tryptophan synthase (TRPS) 1 (EC 4.2.1.20) is a bifunctional enzyme that is considered a paradigm for the analysis of intersubunit regulatory signals. The enzyme is formed by two ␣-and ␤-subunits, arranged in a linear ␣␤␤␣ geometry (1), and catalyzes the last two steps in the biosynthesis of L-tryptophan in a highly concerted mode. The isolated ␣-and ␤-subunits exhibit an activity about 2 orders of magnitude lower that in the tetramer, indicating that the formation of the complex is accompanied by subunit conformational changes (2, 3). Further levels of regulation are achieved by ligand-induced intersubunit signals that are pH-, temperature-, and cation-dependent and keep the catalytic activities of ␣ and ␤-active sites in phase (4 -11). The mechanism of the ␣-␤ activation and allosteric regulation is based on an open-close transition of both subunits (7, 12, 13) involving the movement of several parts of the ␣-and ␤-subunits. In particular, signal transduction from the ␣-to the ␤-active site involves residues belonging to loop ␣L2, loop ␣L6, and the helix ␤H6 (14, 15), the latter structural element being part of the ␤Ϫsubunit COMM domain, as defined by Schneider et al. (15). The question whether there is a unique or multiple pathways of communication and whether these pathways are specialized in controlling defined functional properties of the enzyme was addressed by investigating the activity and regulatory properties of several mutants of the ␣-and ␤-subunits. It was found that mutants in loop ␣L2 and mutants in helix ␤H6 that interact with loop ␣L2 exhibit altered ␣ and ␤ activities (16 -18), whereas mutants in loop ␣L6 and mutants in helix ␤H6 interacting with loop ␣L6 also exhibit altered alloster...

MAWDBP is the only representative of the phenazine biosynthesis-like protein family in the human genome. Its expression is elevated in several disease processes, including insulin resistance, folate deficiency and hypotension, and it may also be involved in carcinogenesis. The exact molecular function of MAWDBP is unknown. Native and seleno-l-methionine-labelled MAWDBP were expressed in Escherichia coli and crystallized at room temperature from precipitants containing 10 mM KF, 14%(w/v) PEG 3350 and 0.1 M sodium citrate pH 5.4. Crystals belong to space group H32, with unit-cell parameters a = b = 187, c = 241 Å , indicative of three to five monomers per asymmetric unit. Crystals were cryoprotected with 15%(v/v) glycerol and data have been collected to 2.7 Å resolution.