engleski

Comparison of two reaction schemes for the the hydrolysis of acetylthiocholine by butyrylcholinesterase

Catalytic parameters calculated from the schemes derived by Webb (Model A) and by Stojan (Model B) were compared for the hydrolysis of acetylthiocholine (ATCh) by purified horse serum butyrylcholinesterase (BChE) (measured by conventional and stopped-flow techniques). In Model A the acetylated enzyme is omitted from the scheme, while in Model B the Michaelis complex is omitted. The enzyme-substrate dissociation constants Ks and Kss in Model A were 0.25 and 2.0 mM (conv. tech.) and 0.17 and 6.3 mM (stopped-flow tech.). These were attributed to binding of ATCh to the catalytic and peripheral site of BChE. The constants K1 and K2 in Model B (stopped-flow tech.) were 0.223 and >1000 mM. These are attributed to binding of ATCh to an unidentified site in the free enzyme and to the peripheral site in the acetylated enzyme. As the Ks and K1 values are almost the same, both constants are likely to refer to the same enzyme-substrate complex. The constants Kss and K2 are very different ; they are both attributed to peripheral binding of ATCh, but they obviously refer to different complexes. Both models postulate that peripheral binding of a substrate affects rate constants of substrate hydrolysis ; this also follows from the evaluated constants for ATCh hydrolysis.

butyrylcholinesterase; catalytic parameters; kinetic models

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