Oxyanion hole

File:Serine protease oxyanion hole.png

Oxyanion hole of a serine protease (black) stabilises negative charge build-up on the transition state of the substrate (red) using hydrogen bonds from enzyme's backbone amides (blue).

An oxyanion hole is a pocket in the active site of an enzyme which stabilizes transition state negative charge on a deprotonated oxygen or alkoxide. The pocket typically consists of backbone amides or positively charged residues. Stabilising the transition state lowers the activation energy necessary for the reaction, and so promotes catalysis. For example, proteases such as chymotrypsin contain an oxianion hole to stabilise the tetrahedral intermediate anion formed during proteolysis. Additionally, 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). Enzymes that catalyse multi-step reactions can have multiple oxyanion holes which stabilise different transition states in the reaction.

See also

• Enzyme catalysis
• Active site
• Transition state
• Serine proteases#Catalytic mechanism