Dynamic requirements for a functional protein hinge.
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 tenfold higher Km value and 10(3)-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, nanosecond 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 diminished the enzyme's response to ligand, and induced structural perturbations in apo and 2-PGA-bound forms of TIM that are atypical of WT. 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.
Kempf
James G
JG
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520, USA.
Jung
Ju-Yeon
JY
Ragain
Christina
C
Sampson
Nicole S
NS
Loria
J Patrick
JP
D000595:Amino Acid Sequence; D000818:Animals; D020134:Catalytic Domain; D002645:Chickens; D005998:Glycine / Q000235:genetics; D008954:Models, Biological; D008958:Models, Molecular; D050505:Mutant Proteins / Q000737:chemistry; D009154:Mutation; D019906:Nuclear Magnetic Resonance, Biomolecular; D011485:Protein Binding; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D014305:Triose-Phosphate Isomerase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D050505:Mutant Proteins; D014305:Triose-Phosphate Isomerase; D005998:Glycine