Folding and unfolding studies on large multidomain proteins are still rare dsspite their high abundance in genomes of prokaryotes and eukaryotes. Here we investigate the unfolding properties of a 271 residue, two domin ribose binding protein (RBP) from the bacterial periplasm using single-molecule force spectroscopy. We observe that RBP predominantly unfolds via a two-state pathway with an unfolding force of similar to 80 pN and an unfolding contour length of similar to 95 nm. Only a small population (similar to 15%) of RBP follows three state pathways. The ligand binding neither increases the mechanical stability nor influences the unfolding flux of RBP through different pathways. The kinetic partitioning between two-state and three-state pathways which has been reported earlier for other periplasmic proteins is also observed in RBP albeit to a lesser extent. These results provide important insights into the mechanical stability and unfolding processes of large two-domain proteins and highlight the contrasting features upon ligand binding. Protein structural topology diagrams are used to explain the differences in the mechanical unfolding behavior of RBP with other peripalsmic binding proteins.