Axial arrangement of crossbridges in thick filaments of vertebrate skeletal muscle.

X-ray intensity data to 1.8 Å resolution were collected from native trigonal crystals of bovine trypsinogen. The orientation and position of the trypsinogen molecules within their crystal cells were determined by Patterson search techniques using the refined model of bovine trypsin (Bode &; Schwager, 1975), and by subsequent R factor refinement. The translation functions allowed discrimination between the enantiomorphic space groups P3₂21 and P3₁21. After one constrained crystallographic refinement cycle, which reduced the crystallographic reliability factor (R) from 35% to 31%, a preliminary difference Fourier map showed several interesting details. Several refinement cycles reduced the value of R to 23%. The overall chain folding is very similar to trypsin. The chain segments, including residues 184 to 193² and 217 to 223, which form the specificity pocket in trypsin, are flexible in trypsinogen. The autolysis loop is partially mobile between residues 142 and 152. There is no continuing electron density for the N terminal residues preceding Tyr20. This indicates that the N terminus may be only weakly fixed to the rest of the molecule or may even float freely in solution.