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.