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The crystal structure of the enzyme ribonuclease T 1 complexed with the specific inhibitor 2'-guanylic acid has been refined to an R value of 0.180 by the restrained least-squares method using synchrotron film data in the 6.0-1.9A resolution range. The refinement was based on a complete model of the protein derived from a multiple-isomorphousreplacement electron density map at 2.5 A resolution. Improvement of the protein model involved 15 model-building sessions with FRODO on a vector graphics system and 138 cycles of stereochemically restrained refinement. The refined model consists of 777 protein atoms, 24 inhibitor atoms, and 91 water O atoms. All protein atoms could be located except for the side chain of residue Glul02. The mean temperature factor for protein atoms is 12-3 ~2 and the r.m.s, deviation of bond lengths from ideal values is 0.018 A. The r.m.s, distance of main-chain atoms from their equivalents in the unrefined model is 1.38 .~. Intermolecular lattice contacts in space group P21212~ comprise eight possible hydrogen bonds between protein atoms, mostly involving the polypeptide backbone of the protein, and 62 non-bonded contacts of atoms within 4.0 A of each other. In addition, there are 13 water molecules connecting symmetry-related protein molecules via hydrogen bonds. The bound inhibitor does not participate in intermolecular contacts but several protein active-site groups are involved in lattice contacts.

Abstract.(fl-CD. PEA)2.32H20;

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