Unfolding pathways of human serum albumin: Evidence for sequential unfolding and folding of its three domains

Human serum albumin (HSA) contains three α-helical domains (I-III). The unfolding process of these domains was monitored using covalently bound fluorescence probes; domain I was monitored by N-(1-pyrene)maleimide (PM) conjugated with cys-34, domain II was monitored by the lone tryptophan residue and domain III was followed by p-nitrophenyl anthranilate (NPA) conjugated with Tyrosine-411 (Tyr-411). Using domain-specific probes, we found that guanidium hydrochloride-induced unfolding of HSA occurred sequentially. The unfolding of domain II preceded that of domain I and the unfolding of domain III followed that of domain I. In addition, the domains I and III refolded within the dead time of the fluorescence recovery experiment while the refolding of domain II occurred slowly. The results suggest that individual domain of a multi-domain protein can fold and unfold sequentially.