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enzyme definition - help

Harald Teicher hteicher at biobase.dk
Wed Feb 14 11:10:01 EST 1996

David Moss (moss at irch.kfk.de) wrote:
: 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
: specific
: >condensation, hydrolysis, or other major chemical change in a
: molecule; proton
: >or electron transfer reactions do not fall in the "major chemical
: change"
: >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
: >do?
: >

of course, this is the whole point....these metallo-proteins, non-protein
and pigment
protein complexes catalyse redox reactions in a whole series of connected
events known as photosynthetic electron transport. in addition to what they
do, these compounds are isolated, purified and quantified using enzymological

while these reactions
obviously are important, it is just as important for scientists to be able
to use precise names during the mediation of results, which, after all, is
the justification for our research in the first place. thus, _personal
names_ and _different definitions_ lead to confusion, precisely the state of
affairs it is our duty (and pleasure) to eliminate.

far from claiming categorically that we _should_ call these mediators enzymes,
i feel we should refrain from categorically naming them _non-enzymes_, as
they seem to fulfill the requirements of the latest definitions of the term
_enzyme_. i do, however, feel that the present state of confusion regarding
nomenclature is not conducive to clarity.....

: 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?

as a general comment to this thread:
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 (viz. enzymes are proteins, large complexes etc....)......
if anyone would like me to forward the e-mail replies to this thread which
i have received, please mail me at hteicher at biobase.dk

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