PROTEIN STRETCHING II-ASTERISK-1 - RESULTS FOR CARBONIC-ANHYDRASE

The force curve measurement mode of an atomic force microscope was use d to record the force required to stretch a protein molecule that was covalently sandwiched through gold-thiol bonds between a mica substrat e and a silicon nitride tip, both coated with gold. In one experiment, 8 +/- 1 out of 20 lysyl residues of bovine carbonic anhydrase B were randomly derivatized to give free thiols, and grafted on an atomically flat gold (111) surface on mice. Force curves taken on the surface co vered with protein molecules using a gold coated tip occasionally show ed a large downward deflection indicating trapping and subsequent stre tching of protein molecules between the tip and the substrate. In anot her experiment, the same protein was genetically engineered so that cy steine residues were introduced at both the amino and the carboxyl ter minus. Force curves taken in a similar manner as in the first experime nt indicated almost complete extension of a linear polypeptide chain. The result was explained in terms of extension of a pseudo-three-dimen sional gel in both cases, with additional stretching of a linear chain in the second case.