Fig. 6

The N- and C-helices of 500 nsec B-YL cross-linked by a symmetry-related tyrosine trimer. Main-chain folding of 500 nsec B-YL is shown in the ribbon-rainbow format, with the N-terminal insertion sequence in blue, the N-terminal helix in blue-green, the turn hairpin in green yellow and the C-terminal helix in yellow-orange. Three of the tyrosine residues (i.e., Y8, Y11, and Y34) are organized as a distorted pinwheel trimer in the hydrophobic interior of the 500 nsec B-YL that links together the N- and C-helices through non-covalent interactions. An approximate threefold axis relating the three tyrosines in Fig. 5a is seen more clearly by rotating the 500 nsec model so that the symmetry axis is perpendicular to the plane of the paper (see “X”). This three-fold axis is designated an “approximate” symmetry axis because it is strictly valid for all three tyrosine ring structures, but not for the hydroxyl of Y8, which faces an opposite direction than those of the hydroxyls for either Y11 or Y34. a Graphical representation of the rotated 500 nsec B-YL model is in ribbon-rainbow format, with tyrosine sidechains as purple sticks. b Graphical representation of the rotated 500 nsec B-YL model is in ribbon-rainbow format, with tyrosine sidechains as space-filling model