Conformational Transitions of the Three Recombinant Domains of Human Serum Albumin Depending on pH

Human serum albumin (HSA) is a protein of 66.5 kDa that is composed of three homologous domains, each of which displays specific structural and functional characteristics. HSA is known to undergo different pH-dependent structural transitions, the N-F and F-E transitions in the acid pH region and the N-B transition at slightly alkaline pH. In order to elucidate the structural behavior of the recombinant HSA domains as stand-alone proteins and to investigate the molecular and structural origins of the pH-induced conformational changes of the intact molecule, we have employed fluorescence and circular dichroic methods. Here we provide evidence that the loosening of the HSA structure in the N-F transition takes place primarily in HSA-DOM III and that HSA-DOM I undergoes a structural rearrangement with only minor changes in secondary structure, whereas HSA-DOM II transforms to a molten globule-like state as the pH is reduced. In the pH region of the N-B transition of HSA, HSA-DOM I and HSA-DOM II experience a tertiary structural isomerization, whereas with HSA-DOM III no alterations in tertiary structure are observed, as judged from near-UV CD and fluorescence measurements.