Acta Cryst. (2014). D70, 544-552 [ doi:10.1107/S1399004713030666 ]
Abstract: The Bürgi-Dunitz angle (BD) describes the trajectory of approach of a nucleophile to an electrophile. The adoption of a stereoelectronically favorable BD can necessitate significant reactive-group repositioning over the course of bond formation. In the context of enzyme catalysis, interactions with the protein constrain substrate rotation, which could necessitate structural transformations during bond formation. To probe this theoretical framework vis-à-vis biocatalysis, Schiff-base formation was analysed in Francisella tularensis transaldolase (TAL). Crystal structures of wild-type and LysMet mutant TAL in covalent and noncovalent complexes with fructose 6-phosphate and sedoheptulose 7-phosphate clarify the mechanism of catalysis and reveal that substrate keto moieties undergo significant conformational changes during Schiff-base formation. Structural changes compelled by the trajectory considerations discussed here bear relevance to bond formation in a variety of constrained enzymic/engineered systems and can inform the design of covalent therapeutics.
PDB references: 4e0c, 3tkf, 3tno and 3te9
Keywords: Bürgi-Dunitz angle; imines; induced fit; nucleophiles; pentose phosphate pathway; Schiff bases.
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