Local rigidity of a protein molecule.

Distribution of soft and rigid substructures within a protein molecule has been implicated in several occasions and most recently from the imaging and indentation experiments using an atomic force microscope. In this paper, previously reported result of mechanical extension experiments on the recombinant bovine carbonic anhydrase II, Q253C, is re-analyzed to estimate the distribution of Young's modulus, Y, in this protein. The force vs. extension curve of the enzymatically active, type I conformer gave an estimate of Y increasing from 40 to 220 MPa as the polypeptide chain was extended from 10 to 75 nm indicating the presence of a rigid core structure. The enzymatically inactive type II, in contrast, gave an almost constant modulus of 55+/-15 MPa in the same extension range in agreement with the previous proposal that it lacked a core structure.