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enzyme definition - help

Harald Teicher hteicher at biobase.dk
Fri Feb 9 10:44:09 EST 1996

Could somebody please enlighten me on the following problem:

1) the _enzyme_ ferredoxin:nadph oxidoreductase (fnr) mediates the transfer of
electrons from the Fe-S centre of ferredoxin to the nicotinamide base of

2) the _metalloprotein_ ferredoxin mediates the transfer of electrons
from the Fe-S centres FA/B to the FAD group of fnr

3) the _metalloprotein_ plastocyanin mediates the transfer of electrons
from the heme group of cytochrome f to the chlorophyll dimer P700

Question: When fnr, plastocyanin and ferredoxin are involved in catalysing
the process of oxidoreduction, and are not consumed in the process, why is
only fnr classified as an enzyme....?

My guess: plastocyanin and ferredoxin are electron transfer proteins, while
fnr is also involved in the transfer of H+. On the other hand, the definition
of an enzyme no longer seems to include the word _protein_ (ie. ribozymes) so
plastoquinone, which is a hydrogen/electron carrier might also fall under the
category enzyme.(?) Taking things to extremes, might the entire photosynthetic
electron transport apparatus not be defined as an H2O:NADP+ oxidoreductase..;-)

I would appreciate any help on this subject.


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