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I think the opening sentence of this article needs changing - it says that expansins are nonenzymatic but they have a catalytic domain. They catalyse the breakdown of bonds between polysaccharides in the cell wall and so I think they should be termed enzymes. The scientific literature seems a bit confused - there are quite a lot of hits for "expansin enzyme" on google. Any ideas/suggestions? Smartse (talk) 13:15, 7 April 2009 (UTC)
Response: The phrase "catalytic domain" does not appear here anymore. However, a deeper issue concerns the definition of "enzyme". If we accept the definition in wikipedia, that "enzymes are proteins that catalyze (i.e., increase or decrease the rates of) chemical reactions", then expansin is not an enzyme because there is no chemical reaction, no change in the covalent structure of the wall, as far as anyone knows. It does, however, catalyze (promote) cell wall extension, which is a biophysical re-arrangement of wall polymers, but not a chemical modification. An ion channel in a membrane might be an apt analogy. It facilitates the transport of ions across the membrane, so in this sense it is a catalyst of transport, but a biochemist would not consider a channel to be an enzyme. Likewise, expansins can be thought of as catalysts of wall extension, but not enzymes. Aquilegia007 (talk) 20:35, 16 December 2010 (UTC)