Allosteric modulation of myristate and Mn(III)heme binding to human serum albumin

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Abstract
Human serum albumin (HSA) is best known for its extraordinary ligand binding capacity. HSA has a high affinity for heme and is responsible for the transport of medium and long chain fatty acids. Here, we report myristate binding to the N and B conformational states of Mn(III)heme-HSA (i.e. at pH 7.0 and 10.0, respectively) as investigated by optical absorbance and NMR spectroscopy. At pH 7.0, Mn(III)heme binds to HSA with lower affinity than Fe(III)heme, and displays a water molecule coordinated to the metal. Myristate binding to a secondary site FAx, allosterically coupled to the heme site, not only increases optical absorbance of Mn(III)heme-bound HSA by a factor of approximately three, but also increases the Mn(III)heme affinity for the fatty acid binding site FA1 by 10-500-fold. Cooperative binding appears to occur at FAx and accessory myristate binding sites. The conformational changes of the Mn(III)heme-HSA tertiary structure allosterically induced by myristate are associated with a noticeable change in both optical absorbance and NMR spectroscopic properties of Mn(III)heme-HSA, allowing the Mn(III)-coordinated water molecule to exchange with the solvent bulk. At pH = 10.0 both myristate affinity for FAx and allosteric modulation of FA1 are reduced, whereas cooperation of accessory sites and FAx is almost unaffected. Moreover, Mn(III)heme binds to HSA with higher affinity than at pH 7.0 even in the absence of myristate, and the metal-coordinated water molecule is displaced. As a whole, these results suggest that FA binding promotes conformational changes reminiscent of N to B state HSA transition, and appear of general significance for a deeper understanding of the allosteric modulation of ligand binding properties of HSA.