Crossover Connection Research Articles

Crystal and molecular structure of chymotrypsin inhibitor 2 from barley seeds in complex with subtilisin Novo

The serine proteinase inhibitor from barley seeds, chymotrypsin inhibitor 2(CI-2), has been crystallized in a molecular complex with subtilisin Novo (EC 3.4.21.14). The crystal structure of this complex has been determined at 2.1-A resolution by the molecular replacement method and partially refined by restrained-parameter least-squares methods. The present crystallographic R factor (SigmaFo[unk] - [unk]Fc/Sigma[unk]Fo[unk]) is 0.193. CI-2 is a member of the potato inhibitor 1 family; it is a serine proteinase inhibitor lacking disulfide bonds. Comparison of the subtilisin molecule in this complex with the native subtilisin shows that the two molecules are very similar in structure. The inhibitor binds in a mode presumably resembling that of a true substrate, but it is not cleaved. This is in accord with the reported structures of other serine proteinase-inhibitor complexes. CI-2 consists of a four-stranded mixed parallel and antiparallel beta-sheet against which an alpha-helix packs to form a hydrophobic core. A wide loop crossover connection between parallel strands 2 and 3 of the beta-sheet contains the reactive-site bond. The conformation of the four residues to either side of the reactive-site bond is similar to that of the analogous residues in the third domain of the turkey ovomucoid inhibitor (Kazal family); the overall polypeptide chain fold of these inhibitors and the location of the reactive site in the respective chains are different.

Representation of short and long-range handedness in protein structures by signed distance maps

A new representation is proposed of short and long-range handedness in protein structures, by signed distance maps. This representation, based on the co-ordinates of the C α atoms of proteins, does not require the assignment of specific regular structures. The short-range handedness along the chain in α-helical, β-strand and turn segments is shown, as well as the handedness between two strands of β-sheet structures and for crossover connections. Results are given for a βαβαβ folding unit of flavodoxin, a βαβ unit of subtilisin, which contains a left-handed crossover connection, and the domain 1 of bovine β-trypsin.

Handedness of crossover connections in beta sheets.

In a crossover connection, the polypeptide chain leaves one end of a beta sheet, forms a loop of any length and any conformation, and reenters the same beta sheet from the opposite end. Of the 85 examples of crossover connections which occur in the known protein structures, 83 are righthanded and only two are lefthanded. It is proposed that consistent handedness, even in long irregular loops, could be produced by the preferred twist direction of extended chain and the righthandedness of alpha-helices, provided certain conditions hold during the protein folding process.

Experience with part-winding resonance in EHV auto-transformers: Diagnosis and corrective measures

Four failures of large EHV auto-transformers on the AEP system are described. The failures were initiated by flashovers in the no-load tap changer during system faults. Investigations and tests attribute the flashovers to part-winding resonance. Test data is presented for various terminal conditions and wave shapes corresponding to system transient conditions. The relationship of these tests to ANSI standard dielectric tests is discussed. Corrective measures for existing transformers include arresters tied to an internal crossover connection and capacitor banks connected to the delta tertiary windings.