Human serum albumin (HSA) is the most abundant protein in human plasma and has a remarkably high ligand binding capacity, making it a good repository for small molecules. One molecule that forms a complex with this enzyme is heme <i>b</i> (Fe(III) protoporphyrin IX), thereby generating HSA-heme. A crystal structure of this complex indicates a phenolate ligand forming a covalent bond to Fe via Tyr161, as opposed to the proximal histidine ligand found in globins. Interestingly, the kinetics and thermodynamic parameters for binding of diatomic gaseous ligands to wild-type and engineered HSA-heme are similar to those reported for myoglobin (Mb), making HSA-heme an artificial hemoprotein and a model complex for monomeric globins. In this study, the electronic structure and electrochemical properties of the different forms of HSA-heme are investigated using UV-vis, IR and fluorescence spectroscopies, and cyclic and square-wave voltammetries. The artificial hemoprotein is also tested for potential nitrite reductase (NiR) functionality. Preliminary studies demonstrate that nitric oxide binds to the Fe(III) and Fe(II) forms of HSA-heme, but oxygen does not (bind to Fe(II)), unlike Mb. Further, the redox potential of the HSA-heme is much lower than Mb, mirroring the redox potential of catalase which also has a tyrosine as the proximal heme ligand. UV-vis data demonstrate that the reaction between HSA-heme and nitrite generate a product similar to the pure form of HSA-heme Fe(II)-NO, alluding to the possibility that this artificial hemoprotein has NiR activity under optimum conditions. Implications on functionality of this complex based on our findings are presented in this work.

Electronic structure and electrochemical elucidation of human serum albumin–heme complex

249th National Meeting (2015)

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