Figure 21-35


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A ribbon diagram of the bifunctional enzyme tryptophan synthase from S. typhimurium. Only one αβ dimer of the αββα heterotetramer is shown. The α subunit is blue, the β subunit's N-terminal domain is orange, its C-terminal domain is red, and all β sheets are tan. The active site of the α subunit is identified by its bound competitive inhibitor, indolepropanol phosphate (IPP; red ball-and-stick model), whereas that of the β subunit is marked by its PLP coenzyme (yellow ball-and-stick model). The solvent-accessible surface of the ~25-Å-long "tunnel" connecting the α and β active sites is outlined by yellow dots. Several indole molecules (green ball-and-stick models) have been modeled into the tunnel to show that it is wide enough for indole to pass from one active site to the other. [X-ray structure by Craig Hyde, National Institutes of Health.]