Superposition of the X-ray structures of EF-Tu in its complexes with GDP and GMPPNP. The protein is drawn in ribbon form with domain 1, its GTP-binding domain, magenta in the GDP complex and yellow in the GMPPNP complex. Domain 2 and domain 3, which have the same orientation in both complexes, are green and cyan. The bound GDP and GMPPNP are shown in space-filling form with C gray, N blue, O red, and P orange. [Based on X-ray structures by Morten Kjeldgaard and Jens Nyborg, University of Aarhus, Århus, Denmark. PDBid 1EFT.]