Two subunits connected by only one disulfide bridge: quaternary structure? I've always simply assumed quaternary structure to be characterized by non-covalent interactions such as hydrogen bonding, van der Waals interactions and whatnot. However, if two distinct polypeptides were _only_ connected by one covalent disulfide bridge, would this be considered as quaternary structure, assuming that non-covalent interactions between the subunits are either negligible or even repulsive? In other words, **can a disulfide bridge, on its own, convey quaternary structure?** On a side note, are there any notable examples of this type of interaction?

I don't know of any examples of this but I would say no doubt, that's quarternary structure. Quarternary isn't so much defined by the kind of interaction but much more the fact that it's between different polypeptides; all lower-level structures are within one polypeptide. (Wikipedia agrees.)