Unusual binding of a potential biomarker with human serum albumin

This study investigates the specific binding of a potential biomarker, [2,2'-bipyridyl]-3,3'-diol (BP(OH)2), with human serum albumin (HSA). The binding of BP(OH)2 at the two primary drug-binding sites on HSA (Sudlow's sites I and II) is explored by a competitive-binding study and monitored by considering the green-light emission from its diketo tautomer. Warfarin is used as a marker for site I and dansyl-L-proline (DP) as a competitor for site II. Steady-state and time-resolved fluorescence measurements affirm that neither of Sudlow's sites is the binding locus of BP(OH)2. To gain an idea regarding the probable binding site of BP(OH)2, we perform molecular-docking studies, which reveal a close proximity of the probe to Trp-214 in subdomain IIA of HSA. Confirmation of this contention is achieved by studying the quenching of the fluorescence of Trp-214 in the presence of BP(OH)2. Moreover, static quenching seems to be responsible for the depletion of the fluorescence of Trp-214, as manifested by the invariance of the intrinsic fluorescence lifetime of Trp-214, as a function of the concentration of BP(OH)2. Based on displacement and quenching studies, supported by molecular docking, we propose that BP(OH)2 binds in a cleft that separates subdomains IIIA and IIB, which is in close proximity to Trp-214.