Variants of human thymidylate synthase with loop 181-197 stabilized in the inactive conformation.
Loop 181-197 of human thymidylate synthase (hTS) populates two major conformations, essentially corresponding to the loop flipped by 180{sup o}. In one of the conformations, the catalytic Cys195 residue lies distant from the active site making the enzyme inactive. Ligands stabilizing this inactive conformation may function as allosteric inhibitors. To facilitate the search for such inhibitors, we have expressed and characterized several mutants designed to shift the equilibrium toward the inactive conformer. In most cases, the catalytic efficiency of the mutants was only somewhat impaired with values of k{sub cat}/K{sub m} reduced by factors in a 2-12 range. One of the mutants, M190K, is however unique in having the value of k{sub cat}/K{sub m} smaller by a factor of {approx}7500 than the wild type. The crystal structure of this mutant is similar to that of the wt hTS with loop 181-197 in the inactive conformation. However, the direct vicinity of the mutation, residues 188-194 of this loop, assumes a different conformation with the positions of C{sub {alpha}} shifted up to 7.2 {angstrom}. This affects region 116-128, which became ordered in M190K while it is disordered in wt. The conformation of 116-128 is however different than that observed in hTS in the active conformation. The side chain of Lys190 does not form contacts and is in solvent region. The very low activity of M190K as compared to another mutant with a charged residue in this position, M190E, suggests that the protein is trapped in an inactive state that does not equilibrate easily with the active conformer.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1005828
- Journal Information:
- Protein Sci., Vol. 18, Issue (8) ; 2009
- Country of Publication:
- United States
- Language:
- ENGLISH
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