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Human serum albumin （HSA） interacts with a vast array of chemically diverse ligands at specific binding sites. To pinpoint the essential structural elements for the formation of the warfarin binding site on human serum albumin, a defined set of five recombinant proteins comprising combinations of domains and/or subdomains of the N-terminal part were prepared and characterized by biochemical standard procedures, tryptophanyl fluorescence, and circular dichroic measurements, indicating well-preserved secondary and tertiary structures. Affinity constants for binding to warfarin were estimated by fluorescence titration experiments and found to be highest for HSA-DOM I-II and HSA, followed by HSA-DOM IB-II, HSA-DOM II, and HSA-DOM I-IIA. In addition, ultraviolet difference spectroscopy and induced circular dichroism experiments were carried out to get an in depth understanding of the binding mechanism of warfarin to the fragments as stand-alone proteins. This systematic study indicates that the primary warfarin binding site is centered in subdomain IIA with indispensable structural contributions of subdomain IIB and domain I, while domain III is not involved in this binding site, underlining the great potential that lies in the use of combinations of recombinant fragments for the study and accurate localization of ligand binding sites on HSA.
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