Comparison of crystal structures of S-adenosylhomocysteine (AdoHcy) hydrolase in the substrate-free, NAD(+) form [Hu, Y., Komoto, J., Huang, Y., Gomi, T., Ogawa, H., Takata, Y., Fujioka, M., and Takusagawa, F. (1999) Biochemistry 38, 8323-8333] and a substrate-bound, NADH form [Turner, M. A., Yuan, C.-S., Borchardt, R. T., Hershfield, M. S., Smith, G. D., and Howell, P. L. (1998) Nat. Struct. Biol. 5, 369-376] indicates large differences in the spatial arrangement of the catalytic and NAD(+) binding domains. The substrate-free, NAD(+) form exists in an "open" form with respect to catalytic and NAD(+) binding domains, whereas the substrate-bound, NADH form exists in a closed form with respect to those domains. To address whether domain closure is induced by substrate binding or its subsequent oxidation, we have measured the rotational dynamics of spectroscopic probes covalently bound to Cys(113) and Cys(421) within the catalytic and carboxyl-terminal domains. An independent domain motion is associated with the catalytic domain prior to substrate binding, suggesting the presence of a flexible hinge element between the catalytic and NAD(+) binding domains. Following binding of substrates (i.e., adenosine or neplanocin A) or a nonsubstrate (i.e., 3'-deoxyadenosine), the independent domain motion associated with the catalytic domain is essentially abolished. Likewise, there is a substantial decrease in the average hydrodynamic volume of the protein that is consistent with a reduction in the overall dimensions of the homotetrameric enzyme following substrate binding and oxidation observed in earlier crystallographic studies. Thus, the catalytic and NAD(+) binding domains are stabilized to form a closed active site through interactions with the substrate prior to substrate oxidation.

Amount of unexpended funds: $24,000 (subcontract to Oak Ridge)This project has as its focus the design and synthesis of polyammonium macrocyclic receptors for oxoanions of environmental importance. The basic research aspects of this project involve synthesis (and the search for improved synthetic methods), solid state structure determination and thermodynamics studies (to ascertain structural criteria for and strength of anion binding), and molecular dynamics simulations (to assess solution characteristics of the interactions between anions and their receptors). Applications-oriented goals include the fabrication of more efficient anion-selective electrodes and the use of these compounds in liquidliquid separations. The latter goal is the subcontract with Bruce Moyer at Oak Ridge National Laboratory. This first year we have focused on nitrates and phosphates. Considerable progress has been made in the basic areas of synthesis, solid state Structure, and molecular dynamics. Anion selective electrodes have also be made which show promising selectivities for oxoanions of interest. Below are described the major findings and significance in the categories of synthesis, structure and molecular dynamics, and electrode studies.

This project has as its focus the design and synthesis of polyammonium macrocyclic receptors for oxoanions of environmental importance. The basic research aspects of this project involve: (1) synthesis (and the search for improved synthetic methods); (2) solid state structure determination and thermodynamics studies (to ascertain structural criteria for and strength of anion binding); and (3) molecular dynamics simulations (to assess solution characteristics of the interactions between anions and their receptors). Applications-oriented goals include the fabrication of more selective anionselective electrodes and the use of these compounds in liquid-liquid separations. The latter goal comprises the subcontract with Dr. Bruce Moyer at Oak Ridge National Laboratory. Research Progress and Implications This report summarizes work after 1 year and 7 months of a 3-year project. To date, we have focussed on the design and synthesis of selective receptors for nitrate and phosphate. Synthesis. The synthesis of polyaza macrocycles, which are the focus of these studies, is in many cases tedious and time-consuming. For example, the synthesis of 1 can take as long as one month because of purifications required during the synthetic route. A major breakthrough in year one of the project was to identify other polyaza macrocycles which bind the desired anions, but which are simpler to synthesize via two step Schiff base/reduction processes with high yields. This is truly significant since now we can obtain large quantities of the macrocycles and can do a variety of studies at the same time. During year one most of our studies focused on monocyclic systems containing two diethylenetriamine units separated by spacers, 2-6. In year two (as a result of findings from molecular dynamics simulations), we began to examine bicyclic macrocycles, 7-11, which can be synthesized by the same method starting with the tetraamine known as tren.