enzyme definition - help

David Moss moss at irch.kfk.de
Fri Feb 9 15:46:07 EST 1996

In article <4fg0da$kkd at charm.magnus.acs.ohio-state.edu>,
dopearso at magnus.acs.ohio-state.edu says...

>i personally have a very tight means of
>defining what an enzyme is [to wit, a protein that catalyzes a
>condensation, hydrolysis, or other major chemical change in a
molecule; proton
>or electron transfer reactions do not fall in the "major chemical
>category] and everything else falls into an outlier category, such
as "electron
>transport proteins" [which i use to refer to my beloved
plastocyanin and
>cytochrome f].  other people have different definitions of
"enzyme."  the truly
>important question is not what we call the thing, rather what does
the thing

Well... I agree that plastocyanin is not an enzyme. Cyt f is a part
of the
cytochrome bf complex, which is usually regarded as an enzyme
plastocyanin oxidoreductase). If you exclude proteins that catalyze
transfer reactions, you leave out the largest single group of
(oxidoreductases). As you say, the definition of "enzyme" isn't very
but it is important that scientists use terms with the same meaning,
so that
they understand each other. To that end, it's sensible to adhere to
internationally agreed classifications, like the EC system (see

To the original question: plastocyanin could be called "cytochrome
bf complex-
photosystem I complex oxidoreductase", but people don't call it that
because it's
more sensible to consider the larger protein complexes as enzymes
plastocyanin as their substrate.

David Moss
moss at ifia.fzk.de

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