Segmentation and the Entropic Elasticity of Modular Proteins

• Published in: The journal of physical chemistry letters Vol. 9; no. 16; pp. 4707 - 4713
• Main Authors: Berkovich, Ronen, Fernandez, Vicente I, Stirnemann, Guillaume, Valle-Orero, Jessica, Fernández, Julio M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.08.2018
• Subjects: Chemical Sciences; or physical chemistry; Theoretical and
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.8b01925

Single-molecule force spectroscopy utilizes polyproteins, which are composed of tandem modular domains, to study their mechanical and structural properties. Under the application of external load, the polyproteins respond by unfolding and refolding domains to acquire the most favored extensibility. However, unlike single-domain proteins, the sequential unfolding of the each domain modifies the free energy landscape (FEL) of the polyprotein nonlinearly. Here we use force-clamp (FC) spectroscopy to measure unfolding and collapse-refolding dynamics of polyubiquitin and poly­(I91). Their reconstructed unfolding FEL involves hundreds of k B T in accumulating work performed against conformational entropy, which dwarfs the ∼30k B T that is typically required to overcome the free energy difference of unfolding. We speculate that the additional entropic energy caused by segmentation of the polyprotein to individual proteins plays a crucial role in defining the “shock absorber” properties of elastic proteins such as the giant muscle protein titin.