  Acylphosphatase (AcP) is a small protein with 98 amino acid residues that
catalyzes the hydrolysis of carboxyl-phosphate bonds. AcP is a typical
two-state protein with slow folding rate due to its relatively large contact
order in the native structure. The mechanical properties and unfolding behavior
of AcP has been studied by atomic force microscope. But the folding and
unfolding dynamics at low forces has not been reported. Here using stable
magnetic tweezers, we measured the force-dependent folding rates within a force
range from 1 pN to 3 pN, and unfolding rates from 15 pN to 40 pN. The obtained
unfolding rates show different force sensitivities at forces below and above
~27 pN, which determines a free energy landscape with two energy barriers. Our
results indicate that the free energy landscape of small globule proteins have
general Bactrian camel shape, and large contact order of the native state
produces a high barrier dominate at low forces.
