Depending on the zinc-binding site, it may not bind Fe(II) at all.
Zn(II) and Fe(II) have very different preferred ligand binding
environments. For many zinc-metalloenzymes, substitution with Fe(II)
would be difficult to impossible. In general, you will find it very
difficult to make your non-defined expression medium zinc-deficient.
Zinc is a very common component of complex media, and is also a very
common adventitious contaminant. Ideally, you will want to include the
metal ion in the expression medium so that it can be incorporated at the
time of protein synthesis. In many cases, this will enhance the
stability of the synthesized protein. For bacterial overexpression at
very high protein levels, 10-100 uM metal ion is more than plenty. More
than that is actually toxic in bacterial systems, as it may impede
critical iron transport into cells. But we have found that complex media
already contains more than enough zinc to populate overexpressed
proteins. We only supplement when we are trying to make
metallosubstituted protein, in which case we use defined zinc-deficient
media and supplement with a compatible metal ion (e.g., Co(II)) at
10-100 uM maximum in bacterial systems. Even that is tricky, as we need
some trace metals to populate other metalloenzymes without introducing
too much in the way of zinc-containing impurities.
Most zinc-metalloenzymes will be immune to metal chelation by DTT or
BME, as the protein-metal binding constants will be orders of magnitude
higher. (Values > 10^(12) are typical.) Even EDTA is not enough for many
(most?) Cys(2)His(2) or Cys(2)His(OH2) sites. It is very unlikely that
1-5 mM DTT will be able to extract Zn from a metalloenzyme binding site.
(We stored a particularly unstable Cys-rich zinc-metalloprotein in 100
mM DTT(!) and 2 mM EDTA (!!) and 50% glycerol and it is stable
indefinitely at -20 deg C without detectable Zn loss.)
You may have to evaluate the metal-binding strength of your protein
experimentally or by comparison to homologous proteins. If the binding
constan is expected to be >10^(10), I don't think you need to worry too
much about DTT or BME.
Roger S. Rowlett
Gordon & Dorothy Kline Professor
Department of Chemistry
13 Oak Drive
Hamilton, NY 13346
On 8/15/2014 11:03 AM, Harvey Rodriguez wrote:
Sorry for the non-crystallographic question. Currently I am working on
a zinc binding protein which is expressed in insect cells and may
contain 4-6 zinc ions. As we know, so many zinc binding proteins can
absorb the iron ions from the culture medium and the protein looks
from yellow to dark red when concentrated. But when I concentrate the
protein, I didn’t see the red color even in the very high
concentration. I am just wondering if a zinc binding protein is
expressed from insect or mammalian cells, can the zinc binding sites
grab the irons instead of zinc or the zinc binding site can be empty
loaded if there is not enough zinc in the culture medium? If so, do I
need to include some zinc salt into the culture medium when doing
expression or I can add some zinc ions when purifying? Usually, how
much zinc and at which step of purification can we add the zinc into
the solution when doing purification?
Another question is that we know DTT can react with the heavy atoms to
form the insoluble sulfide precipitates and if the zinc binding
protein is purified with DTT at a final concentration of 1-5 mM, can
it strip the zinc ions from the protein?
I am appreciated if someone has this kind of experimental experiences
and thanks in advance!