Structure of Escherichia coli fragment TR2C from calmodulin to 1.7 A resolution.

Fragment TR2C is the C-terminal part of the calcium-binding protein calmodulin, including residues 78-148. The crystal structure of TR2C was solved by molecular replacement and refined to a conventional R value of 21.8% (R(free) = 22.0%), using all data in the resolution range 20.0-1.7 A. This study shows that the secondary structure of TR2C, a pair of EF-hand motifs with two calcium-binding sites, is similar to the corresponding motifs in intact calmodulin. However, it also indicates that the N-terminus of helix E is closer to the C-terminus of helix H in TR2C than in the intact protein and that the loop connecting the EF-hands shows different conformations in the two structures. The crystal structure of TR2C was further found to be similar to the set of NMR structures of this fragment, although some pronounced differences exist.

Exploring the chiral space within the active site of alpha-thrombin with a constrained mimic of D-Phe-Pro-Arg--design, synthesis, inhibitory activity, and X-ray structure of an enzyme-inhibitor complex.

An indolizidinone motif with strategically placed substitutents was designed and synthesized as a constrained mimic of D-Phe-Pro-Arg. Low nanomolar inhibition of alpha-thrombin validates the design elements in this inhibitor which also exhibits a 20-fold selectivity for thrombin versus trypsin. An X-ray crystal structure of the inhibitor with alpha-thrombin shows the expected interactions with key amino acids within the active site and some notable changes in positions.