Robert DiSilvestro wrote:
>I had posted about use of the term isozyme.
>Since I have gotten a few questions about how
>"subtle" the structural differences are between
>forms, let me just give some specifics. I had
>hesitated on this to avoid possibly "blowing
>my cover" as a reviewer. The enzymes are the
>superoxide dismutases (SODs). Two of them have
>active sites which contain copper and zinc, the
>other has an active site with manganese...
After reading your original question, but before seeing the above, I was
going to comment that until the growth of molecular genetics superoxide
dismutase was the only well known example of the sort of thing you are
talking about. Maybe that is still true, as I can't immediately think of
another example. In general I wouldn't talk about isoenzymes when the
enzymes occur in different organisms, but only when two proteins
catalysing the same reaction occur in the same organism. So far as
superoxide dismutase is concerned this largely deals with your problem,
because nearly all organisms either have Cu/Zn enzymes or they have Mn
enzymes; they don't usually have both. However, the light-emitting
bacterium Photobacter leiognathi has a Cu/Zn enzyme, more usually found
in higher organisms, and it probably has a Mn enzyme as well (I don't
remember). If so, I think it has isoenzymes regardless of how different
in structure they are. The Cu/Zn enzyme of Photobacter leiognathi was
reported by Martin and Fridovich in J. Biol. Chem. 256, 6080-6089
(1981), incidentally.
Athel Cornish-Bowden
