(580h) Chemistry on a Single Protein, Vcam-1, during Forced Unfolding

Proteins of many types experience tensile forces in their normal function, and VCAM-1 (Vascular Cell Adhesion Molecule-1) is typical in this. VCAM has seven immunoglobulin (Ig) domains, and each has a disulfide bond (-S-S-) buried in its core that covalently stabilizes about half of each domain against unfolding. VCAM is extended here by single molecule atomic force microscopy(AFM) in the presence or absence of reducing agents. In the absence of reducing agent, a sawtooth pattern of forced unfolding reveals an average period and total length consistent with disulfide locations in VCAM's primary sequence. With increasing reducing agent, accessible disulfides are specifically reduced (to SH) because the average period for unfolding increases up to saturation together with additional metrics of unfolding. Steered molecular dynamics simulations of unfolding indeed show that the core disulfide bond is solvent-exposed long before full length unfolding. Michaelis-Menten kinetics emerge with reduction catalyzed by force ( τ_reduction ~ 10^-4 sec). The results establish single molecule reduction, one bond at a time, and also point to the important contributions that disulfide bridges can make to CAMs stressed in cell adhesion.