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. 2015 Aug 4;109(3):647-60.
doi: 10.1016/j.bpj.2015.06.059.

Mapping the Dynamics Landscape of Conformational Transitions in Enzyme: The Adenylate Kinase Case

Dechang Li  1 , Ming S Liu  2 , Baohua Ji  3
Affiliations

Mapping the Dynamics Landscape of Conformational Transitions in Enzyme: The Adenylate Kinase Case

Dechang Li et al. Biophys J. .

Abstract

Conformational transition describes the essential dynamics and mechanism of enzymes in pursuing their various functions. The fundamental and practical challenge to researchers is to quantitatively describe the roles of large-scale dynamic transitions for regulating the catalytic processes. In this study, we tackled this challenge by exploring the pathways and free energy landscape of conformational changes in adenylate kinase (AdK), a key ubiquitous enzyme for cellular energy homeostasis. Using explicit long-timescale (up to microseconds) molecular dynamics and bias-exchange metadynamics simulations, we determined at the atomistic level the intermediate conformational states and mapped the transition pathways of AdK in the presence and absence of ligands. There is clearly chronological operation of the functional domains of AdK. Specifically in the ligand-free AdK, there is no significant energy barrier in the free energy landscape separating the open and closed states. Instead there are multiple intermediate conformational states, which facilitate the rapid transitions of AdK. In the ligand-bound AdK, the closed conformation is energetically most favored with a large energy barrier to open it up, and the conformational population prefers to shift to the closed form coupled with transitions. The results suggest a perspective for a hybrid of conformational selection and induced fit operations of ligand binding to AdK. These observations, depicted in the most comprehensive and quantitative way to date, to our knowledge, emphasize the underlying intrinsic dynamics of AdK and reveal the sophisticated conformational transitions of AdK in fulfilling its enzymatic functions. The developed methodology can also apply to other proteins and biomolecular systems.

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Figures

Figure 1
Figure 1
Conformational transitions in AdK: (a) the open state (e.g., PDB code: 4AKE); and (b) the closed state with ligands (e.g., PDB code: 1AKE). The enzyme consists of three well-defined domains: the rigid CORE (blue, residues 1–29, 60–121, and 160–214); nucleotide triphosphate binding domain LID (green, residues 122–159); and nucleotide monophosphate binding domain NMP (red, residues 30–59). The alpha helices α6 and α7 correspond to residues 112 to 122 and residues 160 to 189, respectively. The ligands Mg2+•ATP/AMP are represented by yellow van der Waals (VDW) spheres. The angle LID-CORE θ1 is formed by the centers of mass of the backbone of residues of LID (a.a. 123–155 (LID), hinge (a.a. 161–165), and CORE (a.a. 1–8, 79–85, 104–110, and 190–198), whereas the angle NMP-CORE θ2 is formed by the centers of mass of the backbone of residues of NMP (a.a. 50–59), CORE (a.a. 1–8, 79–85, 104–110, and 190–198), and hinge (a.a. 161–165). The variable dLN is used to monitor the distance by the centers of mass between domains LID and NMP, respectively. To see this figure in color, go online.
Figure 2
Figure 2
Conformational transitions of AdK (without ligands) observed in LT-MD simulations. (a) Simulation O1 starting from the open state, corresponding to the transition: LID open ↔ LID closed. (b) Simulation O2 starting from the open state, corresponding to the transition: 1st step, LID open → LID closed; 2nd step, NMP open → NMP semi-open. (c) Simulation O3 starting from the open state, corresponding to the transition: 1st step, NMP open ↔ NMP semi-open; 2nd step, LID open → LID closed. (d) Simulation C1 starting from the closed state, corresponding to the transition: 1st step, NMP closed ↔ NMP semi-open → NMP open; 2nd step, LID closed ↔ LID open. (e) Simulation C2 starting from the closed state, corresponding to the transition: 1st step, LID closed → LID open; 2nd step, NMP closed → NMP open. (f) Simulation C3 starting from the closed state, corresponding to the transition: NMP closed → NMP semi-open → NMP open → NMP semi-open. More details can be found in Text S2, Table S3, and Figs. S1–S6. To see this figure in color, go online.
Figure 3
Figure 3
Conformational transitions of AdK (with ligands) observed in LT-MD simulations. (a) Starting from the open state with both ATP and AMP, simulation O-ATP-AMP. (b) Starting from the open state with only ATP, simulation O-ATP. (c) Starting from the open state with only AMP, simulation O-AMP. (d) Starting from the closed state with both ATP and AMP, simulation C-ATP-AMP. More details can be found in Text S2, Table S3, and Figs. S7–S10. To see this figure in color, go online.
Figure 4
Figure 4
The distribution of angles LID-CORE θ1 and NMP-CORE θ2 from LT-MD simulations. (a) Starting from the open state without ligand, corresponding to simulations O1, O2, and O3. The green line indicates the transition pathway that LID closed first and then NMP to close. (b) Starting from the closed state without ligand, corresponding to simulations C1, C2, and C3. The red and pink lines indicate two distinctive transition pathways, one is that NMP open first and then LID to open (pink line), and the other one (red line) is in the opposite direction. (c) Starting from the open or the closed state with ligands, corresponding to simulations O-ATP-AMP, O-ATP, O-AMP, and C-ATP-AMP. The crystal structures of PDB 4AKE and PDB 1AKE are marked. The arrows illustrate the conformational transition and pathways as observed in the LT-MD simulations. (Simulation names can be found in Table S3.) To see this figure in color, go online.
Figure 5
Figure 5
BE-META simulations of AdK without ligands. (a) A multiwell relative free energy landscape against the CVs of angles LID-CORE θ1 and NMP-CORE θ2. The state with the minimum free energy was set as the reference state, i.e., the γ state. The contour map represents the free energy, with the scale bar in kcal/mol units. The states α and ζ correspond to the crystal structures of the open and closed states, respectively.β, γ, δ, and ε indicate the intermediate states corresponding to the semi-open–semi-closed conformations, whereas the states η and λ are more near the closed conformations. The red line indicates the most favorable path. The dashed black lines are possible alternative pathways. The green squares represent the intermediate states. The circular symbols represent the conformational states of AdK crystal structures without ligands (see Table S1). (b) A three-dimensional plot of free energy landscapes of the conformational transitions of AdK without ligands. (c) Representative conformational transition pathways derived from the free energy landscape. The relative free energy of each state, in kcal/mol (1.0 kcal/mol equal to 1.7 kBT with temperature 300 K), is labeled below the corresponding structures. The number above the arrows are in 10−2 ns−1 units, representing the transition rate constants between states as calculated by the Kramers’ transition state theory (see Text S3 in the Supporting Material) (88), with the diffusion coefficient D≈4.47×10−3rad2/ns determined from the LT-MD simulations (89,90). To see this figure in color, go online.
Figure 6
Figure 6
BE-META simulations AdK with ligands. (a) A multiwell relative free energy landscape with two CVs: angles LID-CORE θ1 and NMP-CORE θ2. The state with the minimum free energy was set as the reference state, i.e., the λL state. The contour map represents the free energy, with the scale bar in kcal/mol units. The states αL and ζL correspond to the crystal open and closed conformation, respectively. The states βL, γL, δL, εL and μL are the intermediate ones corresponding to the semi-open–semi-closed conformations, whereas the state λL is a more compact closed conformation. The red lines indicated the minimum free energy paths. The dashed black lines are the possible alternative pathways. The green squares represent the intermediate states. The circular symbols represent the conformational states of AdK crystal structures with ligands (see Table S2). (b) A three-dimensional plot of free energy landscapes of the conformational transitions of AdK with ligands. (c) Representative conformational transition pathways derived from the free energy landscape. The relative free energy of each state, in kcal/mol (1.0 kcal/mol equal to 1.7 kBT with temperature 300 K), is labeled below the corresponding structure. The number above the arrows are in 10−2 ns−1 units, representing the transition rate constants between the states as calculated by the Kramers’ transition state theory (see Text S3 in the Supporting Material) (88), with the diffusion coefficient D≈5.13×10−4rad2/ns determined from the LT-MD simulations (89,90). To see this figure in color, go online.
Figure 7
Figure 7
The distribution of distance (a) dLN between the LID and NMP domains and (b) dLC between the LID and CORE domains based on LT-MD simulations. The insets illustrate the corresponding conformations. The stars illustrate the labeling positions in single-molecule FRET experiments. The bin size of the distance is 0.5 Å. To see this figure in color, go online.
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