Tyr74 is essential for the formation, stability and function of Plasmodium falciparum triosephosphate isomerase dimer

Plasmodium falciparum triosephosphate isomerase (PfTIM) is known to be functional only as a homodimer. Although many studies have shown that the interface Cys13 plays a major role in the stability of the dimer, a few reports have demonstrated that structurally conserved Tyr74 may be essential for the stability of PfTIM dimer. To understand the role of Tyr74, we have performed molecular dynamics (MD) simulations of monomeric and dimeric PfTIM mutated to glycine and cysteine at position 74. Simulations of the monomer revealed that mutant Tyr74Gly does not produce changes in folding and stability of the monomer. Interestingly, comparison of the flexibility of Tyr74 in the monomer and dimer revealed that this residue possesses an intrinsic restricted mobility, indicating that Tyr74 is an anchor residue required for homodimerization. Tyr74 also appears to play an important role in binding by facilitating the disorder-to-order transitions of loops 1 and 3, which allows Cys13 to form favorable interactions with loop 3 and Lys12 to be locked in a favorable position for catalysis. High-temperature MD simulations of the wild-type and Tyr74Gly PfTIM dimers showed that the aromatic moiety of Tyr74 is necessary to preserve the geometry and native contacts between loops 1 and 3 at the interface of the dimer. Disulfide cross-linking between mutant Tyr74Cys and Cys13 further revealed that Tyr74 stabilizes the geometry of loop 1 (which contains the catalytic residue Lys12) and the interactions between loops 1 and 3 via aromatic-aromatic interactions with residues Phe69, Tyr101, and Phe102. Principal component analysis showed that Tyr74 is also necessary to preserve the collective motions in the dimer that contribute to the catalytic efficiency of PfTIM dimer. We conclude that Tyr74 not only plays a role in the stability of the dimer, but also participates in the dimerization process and collective motions via coupled disorder-to-order transitions of intrinsically disordered regions, necessary for efficiency in the catalytic function of PfTIM.