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Rescue of K12G triosephosphate isomerase by ammonium cations: the reaction of an enzyme in pieces.

ABSTRACT: The K12G mutation at yeast triosephosphate isomerase (TIM) results in a 5.5 × 10(5)-fold decrease in k(cat)/K(m) for isomerization of glyceraldehyde 3-phosphate, and the activity of this mutant can be successfully "rescued" by NH(4)(+) and primary alkylammonium cations. The transition state for the K12G mutant TIM-catalyzed reaction is stabilized by 1.5 kcal/mol by interaction with NH(4)(+). The larger 3.9 kcal/mol stabilization by CH(3)CH(2)CH(2)CH(2)NH(3)(+) is due to hydrophobic interactions between the mutant enzyme and the butyl side chain of the cation activator. There is no significant transfer of a proton from alkylammonium cations to GAP at the transition state for the K12G mutant TIM-catalyzed reaction, because activation by a series of RNH(3)(+) shows little or no dependence on the pK(a) of RNH(3)(+). A comparison of k(cat)/K(m) = 6.6 × 10(6) M(-1) s(-1) for the wildtype TIM-catalyzed isomerization of GAP and the third-order rate constant of 150 M(-2) s(-1) for activation by NH(4)(+) of the K12G mutant TIM-catalyzed isomerization shows that stabilization of the bound transition state by the effectively intramolecular interaction of the cationic side chain of Lys-12 at wildtype TIM is 6.3 kcal/mol greater than that for the corresponding intermolecular interaction of NH(4)(+) at K12G mutant TIM.

SUBMITTER: Go MK

PROVIDER: S-EPMC2945285 | biostudies-literature | 2010 Sep

REPOSITORIES: biostudies-literature

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Rescue of K12G triosephosphate isomerase by ammonium cations: the reaction of an enzyme in pieces.

Go Maybelle K MK Amyes Tina L TL Richard John P JP

Journal of the American Chemical Society 20100901 38

The K12G mutation at yeast triosephosphate isomerase (TIM) results in a 5.5 × 10(5)-fold decrease in k(cat)/K(m) for isomerization of glyceraldehyde 3-phosphate, and the activity of this mutant can be successfully "rescued" by NH(4)(+) and primary alkylammonium cations. The transition state for the K12G mutant TIM-catalyzed reaction is stabilized by 1.5 kcal/mol by interaction with NH(4)(+). The larger 3.9 kcal/mol stabilization by CH(3)CH(2)CH(2)CH(2)NH(3)(+) is due to hydrophobic interactions ...[more]

PMID: 20822141

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Project description:Two mutations of the phosphodianion gripper loop in chicken muscle triosephosphate isomerase (cTIM) were examined: (1) the loop deletion mutant (LDM) formed by removal of residues 170-173 [Pompliano, D. L., et al. (1990) Biochemistry 29, 3186-3194] and (2) the loop 6 replacement mutant (L6RM), in which the N-terminal hinge sequence of TIM from eukaryotes, 166-PXW-168 (X = L or V), is replaced by the sequence from archaea, 166-PPE-168. The X-ray crystal structure of the L6RM shows a large displacement of the side chain of E168 from that for W168 in wild-type cTIM. Solution nuclear magnetic resonance data show that the L6RM results in significant chemical shift changes in loop 6 and surrounding regions, and that the binding of glycerol 3-phosphate (G3P) results in chemical shift changes for nuclei at the active site of the L6RM that are smaller than those of wild-type cTIM. Interactions with loop 6 of the L6RM stabilize the enediolate intermediate toward the elimination reaction catalyzed by the LDM. The LDM and L6RM result in 800000- and 23000-fold decreases, respectively, in kcat/Km for isomerization of GAP. Saturation of the LDM, but not the L6RM, by substrate and inhibitor phosphoglycolate is detected by steady-state kinetic analyses. We propose, on the basis of a comparison of X-ray crystal structures for wild-type TIM and the L6RM, that ligands bind weakly to the L6RM because a large fraction of the ligand binding energy is utilized to overcome destabilizing electrostatic interactions between the side chains of E168 and E129 that are predicted to develop in the loop-closed enzyme. Similar normalized yields of DHAP, d-DHAP, and d-GAP are formed in LDM- and L6RM-catalyzed reactions of GAP in D2O. The smaller normalized 12-13% yield of DHAP and d-DHAP observed for the mutant cTIM-catalyzed reactions compared with the 79% yield of these products for wild-type cTIM suggests that these mutations impair the transfer of a proton from O-2 to O-1 at the initial enediolate phosphate intermediate. No products are detected for the LDM-catalyzed isomerization reactions in D2O of [1-(13)C]GA and HPi, but the L6RM-catalyzed reaction in the presence of 0.020 M dianion gives a 2% yield of the isomerization product [2-(13)C,2-(2)H]GA.

Project description:Background. Amyloid secondary structure relies on the intermolecular assembly of polypeptide chains through main-chain interaction. According to this, all proteins have the potential to form amyloid structure, nevertheless, in nature only few proteins aggregate into toxic or functional amyloids. Structural characteristics differ greatly among amyloid proteins reported, so it has been difficult to link the fibrillogenic propensity with structural topology. However, there are ubiquitous topologies not represented in the amyloidome that could be considered as amyloid-resistant attributable to structural features, such is the case of TIM barrel topology. Methods. This work was aimed to study the fibrillogenic propensity of human triosephosphate isomerase (HsTPI) as a model of TIM barrels. In order to do so, aggregation of HsTPI was evaluated under native-like and destabilizing conditions. Fibrillogenic regions were identified by bioinformatics approaches, protein fragmentation and peptide aggregation. Results. We identified four fibrillogenic regions in the HsTPI corresponding to the β3, β6, β7 y α8 of the TIM barrel. From these, the β3-strand region (residues 59-66) was highly fibrillogenic. In aggregation assays, HsTPI under native-like conditions led to amorphous assemblies while under partially denaturing conditions (urea 3.2 M) formed more structured aggregates. This slightly structured aggregates exhibited residual cross-β structure, as demonstrated by the recognition of the WO1 antibody and ATR-FTIR analysis. Discussion. Despite the fibrillogenic regions present in HsTPI, the enzyme maintained under native-favoring conditions displayed low fibrillogenic propensity. This amyloid-resistance can be attributed to the three-dimensional arrangement of the protein, where β-strands, susceptible to aggregation, are protected in the core of the molecule. Destabilization of the protein structure may expose inner regions promoting β-aggregation, as well as the formation of hydrophobic disordered aggregates. Being this last pathway kinetically favored over the thermodynamically more stable fibril aggregation pathway.

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Rescue of K12G triosephosphate isomerase by ammonium cations: the reaction of an enzyme in pieces.

Publications

Rescue of K12G triosephosphate isomerase by ammonium cations: the reaction of an enzyme in pieces.

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