Thioredoxins are ubiquitous small-molecular-weight proteins (typically
100–120 amino-acid residues) containing
an extremely reactive disulphide bridge with a highly conserved sequence
-Cys-Gly(Ala/Pro)-Pro-Cys-. In
bacteria and animal cells, thioredoxins participate in multiple reactions
which require reduction of disulphide
bonds on selected target proteins/enzymes. There is now ample biochemical
evidence that thioredoxins exert very
specific functions in plants, the best documented being the redox regulation
of chloroplast enzymes. Another area
in which thioredoxins are believed to play a prominent role is in reserve
protein mobilization during the process of
germination. It has been discovered that thioredoxins constitute a large
multigene family in plants with different
subcellular localizations, a unique feature in living cells so far. Evolutionary
studies based on these molecules will
be discussed, as well as the available biochemical and genetic evidence
related
to their functions in plant cells.
Eukaryotic photosynthetic plant cells are also unique in that they possess
two different reducing systems, one
extrachloroplastic dependent on NADPH as an electron donor, and the other
one chloroplastic, dependent on
photoreduced ferredoxin. This review will examine in detail the latest
progresses
in the area of thioredoxin
structural biology in plants, this protein being an excellent model for
this
purpose. The structural features of the
reducing enzymes ferredoxin thioredoxin reductase and NADPH thioredoxin
reductase will also be described.
The properties of the target enzymes known so far in plants will be detailed
with special emphasis on the structural
features which make them redox regulatory. Based on sequence analysis,
evidence will be presented that redox
regulation of enzymes of the biosynthetic pathways first appeared in
cyanobacteria possibly as a way to cope with
the oxidants produced by oxygenic photosynthesis. It became more elaborate
in the chloroplasts of higher plants
where a co-ordinated functioning of the chloroplastic and extra
chloroplastic metabolisms is required.