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Chemical Biology 3OA3, September 2009 Page 57 7. I NTERPRETING YOUR DATA The following pieces of information will help you interpret your data. (1) Active site serine Esterases use a Ser residue as an active site nucleophile to form a covalent acyl-enzyme intermediate, and release the first product, HO-R 2 (Scheme 2). In the second step, water acts as a nucleophile to complete the reaction, releasing R 1 -COOH. Thus, we know that if a diester substrate is cleaved at a particular site, then that ester group was located near the active site Ser during catalysis. R 1 O R 2 O Scheme 2. HO R 2 R 1 O O esterase HO esterase R 1 COOH H 2 O esterase HO (2) Hydrophobic substrates Esterases tend to favour hydrophobic (non-polar) substrates. The hydrophobic interactions between non-polar groups are relatively non-specific - the tightest binding (and highest rate) is achieved with substrates that fill the active site to the greatest extent without causing steric clashes. Substituents that are too large or too small will be worse substrates. Substituents with polar functional groups may require you to consider electronic effects (electron
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