Oxanion hole

An oxanion hole is a pocket in the structure of an enzyme which stabilizes a deprotonated oxygen or alkoxide, often by placing it close to positively charged residues. This can contribute to binding affinity in two ways:

• It may directly stabilize the transition state of a reaction by stabilizing the anion (such as in the tetrahedral intermediate formed in the proteolytic reaction catalyzed chymotrypsin)
• It may allow for insertion or positioning of a substrate which would suffer from steric hindrance if it could not occupy the hole (such as BPG in hemoglobin).

References

• Lehinger, Principles of Biochemistry, 4th ed.