Subject: Cd-tolerance of plants

From: [email-masked] (Stephan Fenik)
Date: 24 Mar 1996 06:09:26 -0800

Dear Biologists,

Is anybody here who study heavy metall tolerance of plants?
There is a question to you.

It's well known that there is a good corelation between synthesis
of phytochelatins (PCs) and the level of Cd-tolerance of many plants.
In PCs Cd2+ binds with SH-groups of cystein.
IMHO minimum two cystein's SH-groups are needed for binding the
one Cd2+ ion.

Dr. Henry Strasdeit with collaborators were studding the features
of Cd(S Cys)4 coordination in the PCs by using extended X-ray
absorption fine structure data. They proposed the structural
model of [Cd3(PC3)4]

_|__|___|___   _|__|___|_       +------------------------------+
  |   |   |    |  |   |         |                              |
  S   S   S    S  S   S         | The molecule    _|__|___|_   |
   \  /    \  /    \  /         | of PC3          |  |   |     |
    Cd      Cd      Cd          |                 S  S   S     |
   /  \    /  \    /  \         |                              |
  S   S   S    S  S    S        +------------------------------+
  |   |   |    |  |    |
____________   __________
   |   |   |    |  |   |

   Strasdeit H., Duhme A.K., Kneer R., Zenk M.N.,
   Hermes C., Nolting H.F.
   Evidence for discrete Cd(SCys)4 units in cadmium phytochelatin
   complexes from EXAFS spectroscopy.
   1991 J.Chem.Soc.Chem. Commun., vol.16, pp.1129-1130.

If Dr. Strasdeit is right than the cystein to Cd ratio in PCs
must be 4:1.

In plant cell all Cd is accumulated in vacuole where pH=~5.0
(Vogeli-Lange & Wagner, 1990). In review "Metal-binding
polypeptides in Plants" Dr. N.Robinson writes:

'At pH=5, half of the Cd isolated bound to polypeptides from
N.tabacum dissociated.'

So, in vacuole only half of all PCs are associated with Cd. Than
the PCs' cystein to Cd ratio must be 8:1. Since, if Cd is
chelated by PCs, than plant cell must accumulate eight
times more sulphur than cadmium!

It's meen that plants hyperaccumulators of Cd is hyperaccumulators
of sulfur also. Is it really???

Stephan Fenik
Department of Botany
Pedagogical Institute
2 Kryvonosa St.,
Ternopil, 282009
Ukraine

root&ecolab.ternopil.ua