A Molecular Dynamics Simulation Study of the Effects of βGln114 Mutation on the Dynamic Behavior of the Catalytic Site of the Tryptophan Synthase

L-tryptophan (L-Trp), a vital amino acid for the survival of various organisms, is synthesized by the enzyme tryptophan synthase (TS) in organisms such as eubacteria, archaebacteria, protista, fungi, and plantae. TS, a pyridoxal 5′-phosphate (PLP)- dependent enzyme, comprises α and β subunits that typically form an α2β2 tetramer. The enzyme’s activity is regulated by the conformational switching of its α and β subunits between the open (T state) and closed (R state) conformations. Many microorganisms rely on TS for growth and replication, making the enzyme and the L-Trp biosynthetic pathway potential drug targets. For instance, Mycobacterium tuberculosis, Chlamydiae bacteria, Streptococcus pneumoniae, Francisella tularensis, Salmonella bacteria, and Cryptosporidium parasitic protozoa depend on L-Trp synthesis. Antibiotic- resistant salmonella strains have emerged, underscoring the need for novel drugs targeting the L-Trp biosynthetic pathway, especially for salmonella-related infections. A single amino acid mutation can significantly impact enzyme function, affecting stability, conformational dynamics, and active or allosteric sites. These changes influence interactions, catalytic activity, and protein-ligand/protein-protein interactions. This study focuses on the impact of mutating the βGln114 residue on the catalytic and allosteric sites of TS. Extensive MD simulations were conducted on E(PLP), E(AEX1), E(A-A), and E(C3) forms of TS using the WT, βQ114A, and βQ114N versions. The results show that both the βQ114A and βQ114N mutations increase protein backbone RMSD fluctuations, destabilizing all TS forms. Conformational and hydrogen bond analyses suggest the significance of βGln114 drifting away from cofactor/intermediates and forming hydrogen bonds with water molecules necessary for L-Trp biosynthesis. The βQ114A mutation creates a gap between βAla114 and cofactor/intermediates, hindering hydrogen bond formation due to short sidechains, disrupting β-sites. Conversely, the βQ114N mutation positions βAsn114 closer to cofactor/intermediates, forming hydrogen bonds with O3 of cofactors/intermediates and nearby water molecules, potentially disrupting the L-Trp biosynthetic mechanism.