Conformational dynamics of active site loops 5, 6 and 7 of enzyme Triosephosphate Isomerase: A molecular dynamics study

Triosephosphate Isomerase is a glycolytic enzyme catalyzing the interconversion of Dihydroxyacetone phosphate to Glyceraldehyde-3-phosphate. The active site is comprised of three distinct loops loop-6, loop-7 and loop-8. Based on loop-6 and loop-7 conformation we describe the enzyme as Open TIM and Closed TIM. Various NMR, X-ray crystallography and QM/MM simulation techniques have provided glimpses of individual events of what is essentially a dynamic process. We studied the conformational changes of two distinct loops (loop-6 and loop-7) enveloping the active site, in the presence of natural substrate, reaction intermediates and inhibitor molecules, by means of microsecond atomistic MD simulations in solution and crystal environment. Our studies have revealed that loop-6 samples open and closed conformations in both apo and holo TIM structures. As seen in solution state NMR experiments, we also observe that loop-6 N-terminus and C-terminus move independently. In our simulations we have also observed that backbone dihedrals of loop-7 residues G210 (G210-phi, G210-psi) and G211 (G211-phi) sample open and closed states in both apo and holo TIM structures. Whereas backbone dihedral angles of G211 (G211-psi) and S212 (S212-phi) adopt closed conformation only when the ligand is bound to the active site. As observed in chain-B of 1R2R crystal structures, we also observe that water molecules can also initiate flip of G211-psi and S212-phi dihedral angles into closed conformation. Except, loop-5, which has a dominant effect on the conformational behaviour of loop-6 N-terminus, we do not observe any influence of either loop-6 or loop-7 on the conformational dynamics of the other.