Cooperative Role of Arg45 and His64 in the Spectroscopic A3 State of Carbonmonoxy Myoglobin: Molecular Dynamics Simulations, Multivariate Analysis, and Quantum Mechanical Computations

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• 作者：Brita G. Schulze and Jeffrey D. Evanseck
• 刊名：Journal of the American Chemical Society
• 出版年：1999
• 出版时间：July 14, 1999
• 年：1999
• 卷：121
• 期：27
• 页码：6444 - 6454
• 全文大小：248K
• 年卷期：v.121,no.27(July 14, 1999)
• ISSN：1520-5126

文摘

The structural, dynamic, and electronic origin of the spectroscopically observed carbonmonoxymyoglobin (MbCO) A states has been investigated by using molecular dynamics to sample conformationalspace, multivariate analysis to aid in structural interpretations, and quantum mechanics to compute ligandstretch frequencies. Ten short (400 ps) and two longer time (1.2 ns) molecular dynamics simulations, startingfrom five different crystallographic and solution phase structures centered in a 37 Å radius sphere of water,were used to sample the native-fold of MbCO. Three discrete conformational substates resulted where theprimary structural differences corresponded to a variable strength nonbond interaction between His64, Arg45,and the bound ligand. To correlate the structures from the computed substates with the experimentally observedligand stretch frequencies, Hartree-Fock theory with the 6-31G(d) basis set was used to carry out constrainedminimizations and vibrational analysis on representative model geometries from each conformational substate.The A₀ state (out conformation) was determined to have both Arg45 and His64 removed from the heme pocketwith negligible electrostatic effect on the ligand. Alternatively, His64 was determined to induce the red-shifted frequencies characteristic of the A states (A_1-3) by forming a weak hydrogen bond between its protonatedN_{ges/gifchars/delta.gif" BORDER=0 >} and the ligand (in/N_{ges/gifchars/delta.gif" BORDER=0 >} conformation). The A_1,2 state was specifically assigned to the in/N_{ges/gifchars/delta.gif" BORDER=0 >} conformationwith Arg45 removed from His64 (ges/gifchars/Delta.gif" BORDER=0 >ges/gifchars/nu.gif" BORDER=0 >_comp = -10.0 ± 1.8 cm^-1). The second and faster translational motionengaged Arg45 in an additional and cooperative electrostatic interaction with His64 that distinguished betweenthe A_1,2 and A₃ states. The strongest red-shifted ligand stretch frequency (A₃ state) was computed when Arg45interacted with His64 in the in/N_{ges/gifchars/delta.gif" BORDER=0 >} conformation. The polarizing effect of the distal histidine on the CO ligand(ges/gifchars/Delta.gif" BORDER=0 >ges/gifchars/nu.gif" BORDER=0 >_comp = -19.0 ± 6.8 cm^-1) was increased by the positive charge on Arg45. Consequently, a new A-statemodel, which rationalizes the A₃ state based upon the fluctuating electrostatic field generated by the gate-likedynamics of His64 and Arg45, is presented, which is consistent with previously reported time scales for substateinterconversion.