STRUCTURE DETERMINATION OF ALDOSE REDUCTASE - JOYS AND TRAPS OF LOCALSYMMETRY AVERAGING

The structure of aldose reductase, a monomeric enzyme of 314 amino aci ds which crystallizes in space group P1 with four monomers per asymmet ric unit, has been solved using a combination of single isomorphous re placement (SIR), solvent flattening and local symmetry averaging. The self rotation showed evidence of 222 local symmetry. The map calculate d from die original single isomorphous replacement phases showed a cle ar solvent envelope but was uninterpretable. A first averaging attempt failed because the molecular envelope obtained from the SIR map weigh ted with monomer correlation was too small and the averaging was biase d by low-resolution truncation. A second attempt with an enlarged enve lope and including low-resolution reflections succeeded in refining ph ases at 3.5 angstrom resolution but failed to extend them correctly. R igid-body refinement of a partial model based on the 3.5 angstrom map calculated from refined phases showed significant departures from the 222 symmetry. A third averaging attempt using the improved symmetry su cceeded in producing a clear map with phases extended to 3.07 angstrom resolution. This map revealed a (beta/alpha)8 fold, not previously fo und in NADPH-dependent enzymes. This work shows the importance of mask definition and local symmetry elements accuracy for averaging, and de scribes a method for improving these parameters.