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enzyme definition - help

David Moss moss at irch.kfk.de
Fri Feb 16 14:54:29 EST 1996

In article <4fs2pc$32r at news.uni-c.dk>, hteicher at biobase.dk says...
>David Moss (moss at irch.kfk.de) wrote:
>: 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
>: 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
>: and
>: plastocyanin as their substrate.
>: David Moss
>: moss at ifia.fzk.de
>as far as i see it, a redox mediator like plastocyanin could be both a
>substrate and an enzyme, part of a long chain of photosynthetic oxido-
>reductases, acting in sequence upon each other. why should the size or
>complexity of the cyt b6-f complex determine the status of plastocyanin?

Because if you don't allow the size question to decide which is the
enzyme and which is the substrate, you have to allow plastoquinone to
be an enzyme as well: I think this would be absurd, and your original
posting seems to imply that you agree.

Of course, any enzyme can be the substrate of another reaction
(e.g. of a protease), but I can't see what purpose is served by
considering plastocyanin to both substrate and catalyst of a
single reaction.

>it seems that many of the arguments used to define whether a photosynthetic
>electron carrier is an enzyme or not are based on defintions of enzymes
>which may no longer be valid, and which are no longer used in the newest

Perhaps I should add that in my student days in the 1970s I was taught
that the concept of "enzyme" and "substrate" are not particularly
useful when considering electron transfer chains!

David Moss

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