Triose Phosphate Isomerase: Difference between revisions
Gregg Snider (talk | contribs) |
Gregg Snider (talk | contribs) This paragraph was obtained from http://www.proteopedia.org/wiki/index.php?title=Triosephosphate_Isomerase. The authors are Paula Grabowski, Jacqueline Townsend, Kara Pryke and Regina D. Kettering |
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Similar to the loop spanning residues, the Ω loop 6 hinge residues share high sequence homology amongst species. The role of both the N- and C-terminal hinge regions of Ω loop 6 have been extensively studied including the replacement of conserved hinge residues with glycine, which resulted in a 2500-fold drop in ''k''<sub>cat</sub>. The insertion of glycine into the hinge region significantly increases the flexibility of the loop due to glycine's conformational freedom, which in turn allows the loop to sample many more conformations. This has thermodynamic ramifications as these glycine-rich hinge mutants prompted a large entropy gain (+ΔS) compared to WT, effectively altering the entropic activation energy. Specifically, WT TPI is able to overcome the initial entropic gain (order to disorder), caused by dispelling water molecules from the active site, by forming a more ordered enzyme-substrate complex. Conversely, the glycine-rich hinge mutants again promote an initial entropy gain due to water loss but are unable to pay the entropic penalty due to their inability to bind substrate tightly. Since catalysis will only occur when the closed conformation has been sampled it reasons that the likelihood of sampling this conformation is greatly reduced with the glycine-rich hinge mutants, as evidence by a significant drop in ''k''<sub>cat</sub>. As their overall biological role in the enzyme, the loop hinges act to limit the motion of the loop which effectively restricts the number of conformations accessible to the enzyme. In this manner, TPI acts like an entropy trap. | Similar to the loop spanning residues, the Ω loop 6 hinge residues share high sequence homology amongst species. The role of both the N- and C-terminal hinge regions of Ω loop 6 have been extensively studied including the replacement of conserved hinge residues with glycine, which resulted in a 2500-fold drop in ''k''<sub>cat</sub>. The insertion of glycine into the hinge region significantly increases the flexibility of the loop due to glycine's conformational freedom, which in turn allows the loop to sample many more conformations. This has thermodynamic ramifications as these glycine-rich hinge mutants prompted a large entropy gain (+ΔS) compared to WT, effectively altering the entropic activation energy. Specifically, WT TPI is able to overcome the initial entropic gain (order to disorder), caused by dispelling water molecules from the active site, by forming a more ordered enzyme-substrate complex. Conversely, the glycine-rich hinge mutants again promote an initial entropy gain due to water loss but are unable to pay the entropic penalty due to their inability to bind substrate tightly. Since catalysis will only occur when the closed conformation has been sampled it reasons that the likelihood of sampling this conformation is greatly reduced with the glycine-rich hinge mutants, as evidence by a significant drop in ''k''<sub>cat</sub>. As their overall biological role in the enzyme, the loop hinges act to limit the motion of the loop which effectively restricts the number of conformations accessible to the enzyme. In this manner, TPI acts like an entropy trap. | ||
===Why is the enzyme an obligate dimer?=== | |||
An interdigitating loop (residues 71-77) is found on each subunit. The loop extends from one subunit into a pocket near the active site of the other subunit. Notably, there is a hydrogen bond network involving Lys12 and Glu97 of one subunit and Thr75 of the other subunit. This may explain why the dimer is required for catalytic activity. | |||
== Disease == | == Disease == | ||