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only twice among our examples, where the preceding and following residues occur in the same secondary structural elements, while the central cysteine is in coil structure according to the DSSP definition.23

Among the secondary structural elements that are linked by disulfide bonds, the most predominant is the coil-coil linkage followed, in order, by the "regular structure"-coil linkage (sheet-coil and helix-coil) and the "regular structure"-"different regular structure" linkage (e.g., sheet-helix), whereas the sheet-sheet or helix-helix linkage (i.e., the "regular structure"-"similar regular structure") is rare (6.6%) (Table III). This latter linkage occurs only five times, and three of them are in the same molecule: bovine pancreatic prophospholipase (4BP2). It seems that the high frequency of disulfide bridges between linked coils or between a coil and a regular structure is more general and important for the three-dimensional structures of the proteins. This result agrees with the experimental observation of Matsumura and Matthews,7'36 who introduced nonnative disulfide bonds into T4 lysosyme. If the disulfide bond was located in a regular-a-helical-structural region, which was the most rigid part of the protein, there was no observable increase in stability, whereas in case of flexible regions the engineered disulfide bond increased the thermostability of the molecule. The disulfide links in these structural elements appear to be as important as conserved salt bridges with aspartate residues, which occur mainly in coil structures.28

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