Thermodynamics of Metal-Protein Interactions

Dartmouth College, Hanover NH

Investigators

Abstract

With this award, the Chemistry of Life Processes Program in the Chemistry Division is funding Dr. Dean Wilcox from Dartmouth College to investigate the binding of charged metal particles (metal ions) to proteins as well as the properties of metal ions when they are bound to proteins. Essential metals (e.g., zinc, copper, iron) have beneficial properties for living organisms. Proteins play key roles in the import, transport, storage and export of these essential metals, as well as in the sequestration and removal of other metals that are toxic to living systems (e.g., mercury, lead, cadmium). Experiments that measure metal ion binding to proteins provide fundamental insight into how proteins select the essential trace metals and disregard or remove toxic metals. Experiments conducted by Dr. Wilcox's research group reveal how proteins tune the properties of metals for essential reactions. This knowledge is important because both beneficial and toxic metals have distinct reactivities when they are bound to a protein (e.g., metalloenzymes). The protocols and analysis developed in this research and the insight gained from these studies are valuable for biology and medicine. Students working at the interface of inorganic, biological and physical chemistry gain important knowledge and valuable skills for their careers in science and education, thereby enhancing the scientific workforce. This research seeks to quantify the thermodynamics of metal ions binding to proteins, to provide new insight into important biological processes, and to reveal how proteins modulate the redox properties of metal ions. First, the metal binding, which is the basis for biological function, is being investigated. The thermodynamics of the binding of mercury(II), methylmercury and related species to the Mer proteins in the bacterial mer pathway for mercury detoxification are being quantified to elucidate the mechanism of individual steps in this pathway. Other studies focus on the quantification of binding of copper(I) to the neuronal isoform of metallothionein, which is a protein involved in metal storage and sequestration in the brain, and to unique copper storage proteins (Csp's) in methane-metabolizing bacteria. In both cases, several metal ions bind to the protein. The mechanism of binding and release of these metal ions, including cooperativity, is being elucidated. Second, a protocol that determines the metalloprotein reduction thermodynamics from calorimetric measurements of metal binding, is being used to investigate selected metalloproteins and metalloenzymes for new insight into the protein modulation of the metal reduction potential. Several variants of azurin with higher and lower reduction potentials are being studied, as are selected copper enzymes. This method is being extended to studies of metalloenzymes involved in oxygen redox reactions, specifically superoxide dismutases that contain copper, iron or manganese. In addition to determining the redox thermodynamics of these metalloproteins, the thermodynamics and accompanying proton transfer of the two steps of the overall reaction, O2- oxidation and O2- reduction, are being quantified. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.