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
>
>(clip)
>
>: 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
>text-books

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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