Stereoelectronic requirements for optimal hydrogen-bond-catalyzed enolization

Area of Science:

• Biochemistry
• Structural Biology
• Computational Chemistry

Background:

• Enolizing enzymes utilize oxyanion holes to stabilize reaction intermediates.
• The precise stereoelectronic arrangement of hydrogen-bond donors (HBDs) in these oxyanion holes is crucial for catalytic efficiency.
• Previous models did not fully explain the observed out-of-plane orientation of HBDs.

Purpose of the Study:

• To propose a novel stereoelectronic model for the oxyanion holes of enolizing enzymes.
• To elucidate the role of HBD orientation in enzyme catalysis.
• To reconcile structural observations with quantum chemical calculations.

Main Methods:

• Protein crystallography of enolizing enzyme active sites.
• Structural analysis of small molecule crystal structures containing hydrogen-bonded carbonyls.
• Quantum chemical calculations on model enolization reactions.

Main Results:

• A new stereoelectronic model is proposed, explaining the out-of-plane HBD orientation.
• Computational results show reduced lone-pair directionality of HBDs upon enolization.
• Enolates exhibit minimal directional preference for hydrogen bonding.
• Perpendicular HBD positioning induces strain, released upon enolization, favoring kinetics and thermodynamics over in-plane arrangements.

Conclusions:

• The out-of-plane orientation of HBDs in enolizing enzyme oxyanion holes is energetically favorable.
• This orientation facilitates catalysis by releasing strain during the enolization step.
• The findings provide a deeper understanding of enzyme catalytic mechanisms.