Kinase Active Site and Ligand Binding Pocket Inspector
Explore the 3D architecture of the protein kinase catalytic cleft—including the P-loop, hinge region, gatekeeper residue (T790 in EGFR), catalytic lysine, and DFG activation loop—using experimental PDB coordinates for human EGFR (P00533 / PDB 3IKA). Geometric proximity between a residue and the co-crystallised inhibitor defines the physical binding pocket while remaining distinct from thermodynamic binding affinity. The workspace shows the structure and the selected residue; it computes no distances or contact shells, so the distance and contact definitions below are for reading the structure, not outputs of the tool.
Focuses on the EGFR catalytic domain in PDB 3IKA, a T790M construct covalently bound to the inhibitor WZ4002, with residue 790 selected to examine the steric environment of the ATP/inhibitor cleft.
Map the conserved structural motifs of the kinase catalytic cleft
Eukaryotic protein kinases share a bilobal fold in which the ATP and substrate-binding cleft sits at the interface between a β-sheet-rich N-terminal lobe and a predominantly α-helical C-terminal lobe. Four structural landmarks organize small-molecule binding inside this cleft: (1) the glycine-rich P-loop (phosphate-binding loop) that arches over the nucleotide phosphates, (2) the hinge strand that forms canonical backbone hydrogen bonds with the adenine ring of ATP or the core heterocycle of an inhibitor, (3) the gatekeeper residue (Thr790 in EGFR P00533) guarding the back hydrophobic pocket, and (4) the activation loop beginning with the conserved Asp-Phe-Gly (DFG) motif.
When inspecting PDB 3IKA in the protein workspace with residue 790 selected, examine how the gatekeeper position sits directly behind the hinge region, here already a methionine. Comparing the 3D position of residue 790 with the linear UniProt domain track helps bridge medicinal-chemistry SAR descriptions (Type I, Type II DFG-out, and covalent Cys797 inhibitors) with concrete atomic coordinates.
Keep geometric contact shells distinct from thermodynamic binding claims
Contact analysis rests on Euclidean distances between crystallographic heavy-atom centres; the protein workspace does not compute them, so measure distances in a dedicated structure-analysis program when you need them. By the usual definitions, residues with atoms within 4.0 Å of a bound ligand form the first-shell contact environment, and donor–acceptor pairs within 3.5 Å are candidate hydrogen bonds or salt bridges. However, crystal coordinates represent a time- and lattice-averaged electron density model that typically lacks explicit hydrogens and reflects crystallization pH and cryoprotectant conditions.
Never equate visual proximity in a static crystal structure with quantitative binding affinity or clinical resistance. For EGFR T790M, drug sensitivity depends on the interplay between reversible pocket contacts, covalent bond formation at Cys797, and cellular ATP competition (Km,ATP). Use the structural workbench to verify residue numbering, chain coverage and where the selected residue sits, then pair those observations with biochemical IC50/Ki measurements from the assay workspace.
Equations and variables
Interatomic Euclidean distance (a definition; the workspace computes none)
d(A, B) = sqrt((xA − xB)² + (yA − yB)² + (zA − zB)²)
- d(A, B)
- Center-to-center distance between heavy atom A (protein) and heavy atom B (ligand) (Å)
- (x, y, z)
- Orthogonal Cartesian coordinates from the crystallographic PDB/mmCIF model (Å)
Conventional contact-shell thresholds (definitions; the workspace applies none)
Contact class : { d ≤ 3.5 Å: H-bond candidate (with angle check); d ≤ 4.0–4.5 Å: van der Waals shell }
- 3.5 Å
- Typical upper donor-to-acceptor heavy-atom distance for a hydrogen bond
- 4.0–4.5 Å
- First-shell hydrophobic or steric contact cutoff around a co-crystallized ligand
Scope and limitations
- X-ray crystal structures at typical medicinal-chemistry resolutions (1.8–2.8 Å) do not directly resolve hydrogen atom positions; protonation states and tautomers must be inferred separately.
- Static crystallographic distances do not measure free energy of binding (ΔGbind); a close contact can be either stabilizing or sterically repulsive depending on conformational strain.
- Substituting T790M in a sequence label highlights the wild-type or crystal coordinate position and does not perform molecular dynamics relaxation or alchemical free-energy perturbation.
Sources and provenance
- UniProtKB human EGFR P00533CC BY 4.0
The interactive workspace displays the release of the retrieved record.
- RCSB PDB experimental entry 3IKA: EGFR 696–1022 T790M with WZ4002CC0; wwPDB experimental coordinate file
- Yun et al. 2008 The T790M mutation in EGFR kinase causes drug resistance by increasing the affinity for ATPArticle citation; biochemical mechanism of the EGFR gatekeeper substitution
Technical questions
What role does the gatekeeper residue play in a protein kinase ATP-binding pocket?
Positioned at the back of the adenine-binding cleft adjacent to the hinge region (Thr790 in human EGFR, Thr315 in ABL1), the gatekeeper side chain controls access to a deep hydrophobic back pocket. A small threonine side chain permits entry of quinazoline and pyrimidine warheads, whereas substitution with a bulkier methionine alters pocket sterics and, in the L858R activating-mutant background, increases the affinity for ATP (Yun et al. 2008).
Does a heavy-atom distance under 3.5 Å guarantee a hydrogen bond between protein and ligand?
No. Distance alone is a necessary condition, not a sufficient one. A genuine hydrogen bond requires one heavy atom to carry a donated hydrogen (donor) and the other to present a lone pair (acceptor) with a favorable donor–H···acceptor geometry, rather than two apolar carbons or two unprotonated acceptors sitting in close contact.
Why is T790M resistance not explained solely by steric clash with first-generation inhibitors?
While replacing threonine 790 with methionine modifies the hydrophobic back pocket and displaces conserved water networks, Yun et al. 2008 showed that, in the L858R activating-mutant background, T790M also lowers the Michaelis constant (Km) for ATP back towards wild-type levels, restoring competition from millimolar intracellular ATP against reversible ATP-competitive inhibitors.
Connected workflows
- EGFR sequence and structure inspection
Inspect the full EGFR P00533 sequence-to-structure mapping and annotations.
- 4PL IC50 curve fitting
Fit 4PL dose-response inhibition curves to determine biochemical IC50 values.
- Lipinski rule-of-five property inspection
Evaluate physicochemical druglikeness properties of kinase inhibitor structures.