Mainly due to computational limitations, past protein molecular dynamics simulations have rarely been extended to 300 psec; we are not aware of any published results beyond 350 psec. The present work compares a 3000 psec simulation of the protein ubiquitin with the available x-ray crystallographic and solution NMR structures. Aside from experimental structure availability, ubiquitin was studied because of its relatively small size (76 amino acids) and lack of disulfide bridges. An implicit solvent model was used except for explicit treatment of waters of crystallization. We found that the simulated average structure retains most of the character of the starting x-ray crystal structure. In two highly surface accessible regions, the simulation was not in agreement with the x-ray structure. In addition, there are six backbone-backbone hydrogen bonds that are in conflict between the solution NMR and x-ray crystallographic structures; two are bonds that the NMR does not locate, and four are ones that the two methods disagree upon the donor. Concerning these six backbone-backbone hydrogen bonds, the present simulation agrees with the solution NMR structure in five out-of-the six cases, in that if a hydrogen bond is present in the x-ray structure and not in the NMR structure, the bond breaks within 700 psec. Of the two hydrogen bonds that are found in the NMR structure and not in the x-ray structure, one forms at 1400 psec and the other forms rarely. The present results suggest that relatively long molecular dynamics simulations, that use protein x-ray crystal coordinates for the starting structure and a computationally efficient solvent representation, may be used to gain an understanding of conformational and dynamic differences between the solid-crystal and dilute-solution states.

P450SU1 and P450SU2 are herbicide-inducible bacterial cytochrome P450 enzymes from Streptomyces griseolus. They have two of the highest sequence indentities to camphor hydroxylase (P450cam from Pseudomonas putida), the cytochrome P450 with the first known crystal structure. We have built several models of these two proteins to investigate the variability in the structures that can occur from using different modeling protocols. We looked at variability due to alignment methods, backbone loop conformations and refinement methods. We have constructed two models for each protein using two alignment algorithms, and then an additional model using an identical alignment but different loop conformations for both buried and surface loops. The alignments used to build the models were created using the Needleman-Wunsch method, adapted for multiple sequences, and a manual method that utilized both a dot-matrix search matrix and the Needleman-Wunsch method. After constructing the initial models, several energy minimization methods were used to explore the variability in the final models caused by the choice of minimization techniques. Features of cytochrome P450cam and the cytochrome P450 superfamily, such as the ferredoxin binding site, the heme binding site and the substrate binding site were used to evaluate the validity of the models. Although the final structures were very similar between the models with different alignments, active-site residues were found to be dependent on the conformations of buried loops and early stages of energy minimization. We show which regions of the active site are the most dependent on the particular methods used, and which parts of the structures seem to be independent of the methods.