The structural response of Human Serum Albumin to oxidation.

Human Serum Albumin (HSA), the most abundant protein of body fluids, has a “modular” three-domain structure potentially responding to stress by means of changes of conformation. Besides acting as the main carrier protein of the circulation, HSA is also an obvious target of extracellular reactive oxidant species due to its high abundance in plasma. It is for this reason considered the main anti-oxidant defense in blood. This study was aimed at correlating oxidant-induced chemical and structural effects on HSA. The experiments benefited from the use of a multi-technique instrumental platform which combined the simultaneous collection of SAXS, UV-vis absorbance spectra and fluorescence emission on the same sample volume. Despite the chemical modification induced by the oxidant hypochlorite, the native shape is preserved up to oxidant/HSA molar ratio <80, above which a structural transition occurs in the critical range 80−120. This conformational variation involves the drifting of one of the end-domains from the rest of the protein and corresponds to the loss of one-third of the α-helix and a net increase of the protein negative charge. The transition is highly reproducible suggesting that it represents a well-defined structural response typical of this multidomain protein. The ability to tolerate elevated levels of chemical modification in a folded or only partially unfolded state, as well as the stability to aggregation, provides albumin with optimal features as a biological buffer for the local formation of oxidants.