ABSTRACT
The loop involving residues 130-141 (bovine SOD numbering) delineates one side of the active site cleft, as shown in Fig. 5. This loop also appears to be fundamental in determining the optimal electrostatic potential for driving superoxide anions to the active site copper, which is the rate-limiting step of the enzymatic reaction under nonsaturating conditions. Thus, in the human Cu/Zn SOD, the electrostatic triad of charged residues comprising E132, E133 and K136 (residues 130, 131, and 134, respectively in the bovine enzyme), together with R143 (bovine residue 141), is thought to play a key role in directing the long range approach of the negatively charged superoxide anion to the active site. Various studies, such as computational electrostatic calculations and pulse radiolysis [40], have been employed to test this hypothesis; as indicated in Table 1, the structures of SOD mutants have been eluci dated. The crystal structure of the K136E human SOD mutant [40], in which the glutamate side chain takes a number of different conformations in the 10 subunits within the asymmetric unit, clearly indicates how the electrostatic potential can be perturbed.
