Adil AHIRI, Aziz ABOULMOUHAJIR
The European Chemistry and Biotechnology Journal - 2026;3(6):50-72
Protein kinases regulate cell signaling through phosphorylation of serine, threonine, or tyrosine residues on substrate proteins. Their catalytic activity is governed by a set of conserved structural elements, the activation segment (bounded by the DFG and APE motifs), the DFG motif, the activation loop (A-loop), and the alphaC-helix, whose conformational states determine whether a kinase is active or inactive. Despite substantial efforts to classify kinase conformations, most existing schemes are geometric in nature; few integrate a quantitative energetic dimension, and the finer secondary-structure features of the activation segment remain underexploited. We compiled a dataset of 4,670 human and murine protein kinase structures from the RCSB PDB (1,699 tyrosine kinases and 2,971 serine/threonine kinases). Activation segment configurations (IN, OUT, and SWAPPED) were assigned using DSSP-guided structural inspection and geometric criteria. Association diagrams were built for 104 tyrosine kinases and 110 serine/threonine kinases. For 68 catalytic domains with fully resolved activation segments, K-means clustering was applied using four criteria: activation segment interaction energy (INTAA server, AMBER parm03 force field), and the conformational states of the DFG motif, alphaC-helix, and A-loop. Spatial heat maps and per-residue C alpha displacement analysis (VMD) were used to characterize energy distribution and conformational transitions. The activation segment was classified into OUT (active, 55%), IN (inactive, 38%), and a minor SWAPPED conformation (6% in the broad survey; n = 2 in the fully resolved clustering subset), the latter retained as an observation rather than a validated class. K-means clustering defined seven descriptive energy/conformation clusters. Heat maps revealed that the activation segment interacts primarily with the catalytic loop, the beta1 strand, and the alpha EF/alphaF loop, with varying intensities across clusters. Per-residue C alpha displacement analysis showed that the A-loop undergoes the largest conformational change between inactive and active states (up to 19 Å), with smaller kinase-specific differences involving the G-loop, alphaC-helix, and adjacent beta-strands. This study provides an integrated energetic and structural classification of the activation segment across the human and murine kinome, complementing existing conformational catalogues and offering a quantitative comparative basis for understanding kinase activation mechanisms relevant to drug design.
