The enzyme triosephosphate isomerase (TIM) is a model of catalytic efficiency. The 11-residue loop 6 at the TIM active site plays a major role in this enzymatic prowess. The loop moves between open and closed states, which facilitate substrate access and catalysis, respectively. The N- and C-terminal hinges of loop 6 control this motion. Here, we detail flexibility requirements for hinges in a comparative solution NMR study of wild-type (WT) TIM and a quintuple mutant (PGG/GGG). The latter contained glycine substitutions in the N-terminal hinge at Val167 and Trp168, which follow the essential Pro166, and in the C-terminal hinge at Lys174, Thr175, and Ala176. Previous work demonstrated that PGG/GGG has a 10-fold higher K_m value and 10³-fold reduced k_cat relative to WT with either [D]-glyceraldehyde 3-phosphate or dihyrdroxyacetone phosphate as substrate. Our NMR results explain this in terms of altered loop-6 dynamics in PGG/GGG. In the mutant, loop 6 exhibits conformational heterogeneity with corresponding motional rates < 750 s⁻¹ that are an order of magnitude slower than the natural WT loop-6 motion. At the same time, ns-timescale motions of loop 6 are greatly enhanced in the mutant relative to WT. These differences from WT behavior occur in both apo PGG/GGG and in the form bound to the reaction-intermediate analog, 2-phosphoglycolate (2-PGA). In addition, as indicated by ¹H, ¹⁵N and ¹³CO chemical-shifts, the glycine substitutions (a) diminished the enzyme’s response to ligand, and (b) induced structural perturbations in apo and 2-PGA-bound forms of TIM that are atypical of those observed in WT. Altogether, these data show that PGG/GGG exists in multiple conformations that are not fully competent for ligand binding or catalysis. These experiments elucidate an important principle of catalytic hinge design in proteins: structural rigidity is essential for focused motional freedom of active-site loops.