Model assembly study of the ligand binding by p-hydroxybenzoate hydroxylase: correlation between the calculated binding energies and the experimental dissociation constants

Investigation on an Orientation and Interaction Energy of the Water Molecule in the HIV-1 Reverse Transcriptase Active Site by Quantum Chemical Calculations

To obtain basic information such as interaction between the water molecule and amino acids in the active site of HIV-1 Reverse Transcriptase (HIV-1 RT), ab initio molecular orbital calculations and the two-layer ONIOM method were performed. The energetic results from different methods show that the ONIOM2 (MP2/6-311G:HF/6-31G//HF/6-31G:HF/3-21G) can provide reliable results on the orientation of the water molecule in the HIV-1 RT active site. The interaction between the water molecule and Asp186 was found to be the most preferable. The obtained results from ONIOM2 calculations indicated that the active site model system included six amino acid residues (Asp186, Asp185, Met184, Tyr183, Leu187, and Tyr188) leading a preferable representation of the environment surrounding the water molecule in the more realistic model. The water molecule presented in the active site tends to form H-bonding with Asp186, Tyr183, and Tyr188 as indicated by the distances of O4-H2 = 1.91 A, O3-H7 = 2.36 A, and O3-H17 = 1.73 A, respectively. The stability of this complex system brings to the foundation of the estimated binding energy approximately -15.8 kcal/mol or -8.1 kcal/mol which is more stabilized relative to the smallest model complex. These observations revealed that the water molecule forms both a hydrogen bond donor and a hydrogen bond acceptor in the cavity and plays an important role in the specific conformation of the active site of HIV-1 RT. The H-bonding is a rather strong interaction; thus, the water might induce the conformation of the active site to fit the catalysis process and helpfully attract dNTP to elongate the viral DNA in the replication process of this enzyme.

Ab initio molecular dynamics studies on HIV-1 reverse transcriptase triphosphate binding site: implications for nucleoside-analog drug resistance

Quantum-chemical methods are used to shed light on the functional role of residues involved in the resistance of HIV-1 reverse transcriptase against nucleoside-analog drugs. Ab initio molecular dynamics simulations are carried out for models representing the adduct between the triphosphate substrate and the nucleoside binding site. The triphosphate is considered either deprotonated or protonated at the gamma-position. Although the protonated form already experiences large rearrangements in the ps time scale, the fully deprotonated state exhibits a previously unrecognized low-barrier hydrogen bond between Lys65 and gamma-phosphate. Absence of this interaction in Lys65-->Arg HIV-1 RT might play a prominent role in the resistance of this mutant for nucleoside analogs (Gu Z et al., 1994b, Antimicrob Agents Chemother 38:275-281; Zhang D et al., 1994, Antimicrob Agents Chemother 38:282-287). Water molecules present in the active site, not detected in the X-ray structure, form a complex H-bond network. Among these waters, one may be crucial for substrate recognition as it bridges Gln151 and Arg72 with the beta-phosphate. Absence of this stabilizing interaction in Gln151-->Met HIV-1 RT mutant may be a key factor for the known drug resistance of this mutant toward dideoxy-type drugs and AZT (Shirasaka T et al., 1995, Proc Natl Acad Sci USA 92:2398-2402: Iversen AK et al., 1996, J Virol 70:1086-1090).

Aspects of the mechanism of catalysis of glucose oxidase: a docking, molecular mechanics and quantum chemical study

The complex structure of glucose oxidase (GOX) with the substrate glucose was determined using a docking algorithm and subsequent molecular dynamics simulations. Semiempirical quantum chemical calculations were used to investigate the role of the enzyme and FAD co-enzyme in the catalytic oxidation of glucose. On the basis of a small active site model, substrate binding residues were determined and heats of formation were computed for the enzyme substrate complex and different potential products of the reductive half reaction. The influence of the protein environment on the active site model was estimated with a point charge model using a mixed QM/MM method. Solvent effects were estimated with a continuum model. Possible modes of action are presented in relation to experimental data and discussed with respect to related enzymes. The calculations indicate that the redox reaction of GOX differs from the corresponding reaction of free flavins as a consequence of the protein environment. One of the active site histidines is involved in substrate binding and stabilization of potential intermediates, whereas the second histidine is a proton acceptor. The former one, being conserved in a series of oxidoreductases, is also involved in the stabilization of a C4a-hydroperoxy dihydroflavin in the course of the oxidative half reaction.

Ab initio models for receptor-ligand interactions in proteins. 4. Model assembly study of the catalytic mechanism of triosephosphate isomerase

The catalytic mechanism of triosephosphate isomerase (TIM) was investigated with ab initio quantum mechanical calculations. Electrostatic interactions between the quantum mechanical active site and the protein and solvent environment were modeled using the finite difference Poission-Boltzman method. The complexes of TIM with the substrate dihydroxyacetone phosphate (DHAP), five possible intermediates and the product glyceraldehyde-3-phosphate (GAP) were optimized in the active-site model at the 3-21G(*) level and energy profile for the proton abstraction from DHAP by the active-site Glu 167 was calculated at the MP2/3-21G(*)13-21G(*) level. Calculated energetics of the enzyme reaction were found to be in reasonable agreement with the experimental findings. Calculations revealed that an enediol of the substrate is a probable intermediate in the enzyme reaction. It was suggested that the proton abstracted from the substrate by the active-site glutamate goes to the carbonyl oxygen of the substrate producing enediol intermediate either directly or after it is exchanged with solvent.