Triosephosphate Isomerase: The Crippling Effect of the P168A/I172A Substitution at the Heart of an Enzyme Active Site.

The P168 and I172 side chains sit at the heart of the active site of triosephosphate isomerase (TIM) and play important roles in the catalysis of the isomerization reaction. The phosphodianion of substrate glyceraldehyde 3-phosphate (GAP) drives a conformational change at the TIM that creates a steric interaction with the P168 side chain that is relieved by the movement of P168 that carries the basic E167 side chain into a clamp that consists of the hydrophobic I172 and L232 side chains. The P168A/I172A substitution at TIM from Trypanosoma brucei brucei ( Tbb TIM) causes a large 120,000-fold decrease in k cat for isomerization of GAP that eliminates most of the difference in the reactivity of TIM compared to the small amine base quinuclidinone for deprotonation of catalyst-bound GAP. The I172A substitution causes a > 2-unit decrease in the p K a of the E167 carboxylic acid in a complex to the intermediate analog PGA, but the P168A substitution at the I172A variant has no further effect on this p K a . The P168A/I172A substitutions cause a 5-fold decrease in K m for the isomerization of GAP from a 0.9 kcal/mol stabilization of the substrate Michaelis complexes. The results show that the P168 and I172 side chains play a dual role in destabilizing the ground-state Michaelis complex to GAP and in promoting stabilization of the transition state for substrate isomerization. This is consistent with an important role for these side chains in an induced fit reaction mechanism [Richard, J. P. (2022) Enabling Role of Ligand-Driven Conformational Changes in Enzyme Evolution. Biochemistry 61, 1533-1542].