Energetically unfavorable protein angles: Exploration of a conserved dihedral angle in triosephosphate isomerase

Over the past 3.5 billion years of evolution, enzymes have adopted a myriad of conformations to suit life on earth. However, torsional angles of proteins have settled into limited zones of energetically favorable dihedrals observed in Ramachandran plots. Areas outside said zones are believed to be disallowed to all amino acids, except glycine, due to steric hindrance. Triosephosphate isomerase (TIM), a homodimer with a catalytic rate approaching the diffusion limit, contains an active site lysine residue (K13) with dihedrals within the fourth quadrant (Φ = +51/Ψ = -143). Both the amino acid and the dihedral angles are conserved across all species of TIM and known crystal structures regardless of ligand. Only crystal structures of the engineered monomeric version (1MSS) show accepted β-sheet dihedral values of Φ = -135/Ψ = +170 but experiments show a 1000-fold loss in activity. Based on these results, we hypothesized that adopting the unfavorable torsion angle for K13 contributes to catalysis. Using both, computational and experimental approaches, four residues that interact with K13 (N11, M14, E97, and Q64) were mutated to alanine. In silico molecular dynamics (MD) simulations were performed using 2JK2 unliganded human TIM as a starting structure. Ramachandran plots, containing K13 dihedral values reveal full or partial loss of disallowed zone angles. N11A showed no detectable catalytic activity and lost the unfavorable K13 dihedral angles across four separate force fields during simulation while all other mutants plus wild type retained activity and retained the conserved K13 dihedral angles.