Dynamics in Photosystem II Structure and Function
13
al. 1984; Jansen et al. 1993) - those amino acids that close up the QA and
Q B site towards the matrix space, as discussed above.
The cleavage site of the D1 protein as part of the rapid turnover can be
prevented by an inhibitor in the QB site, like DCMU (Mattoo et al. 1984;
Schuster et al. 1988), other classical herbicides, and also certain phenol-type
inhibitors (Jansen et al. 1993). This supports the notion that the QB site
controls the conformation of the cleavage site and in this way allows or
prevents access of a protease in the primary cleavage of the D1 protein in
rapid turnover.
After complete degradation of the cleaved D1 protein by so-called housekeeping proteases, the translation machinery of the chloroplast will resynthesize the D1 protein. This is also light-dependent. The new protein will be
processed, reinserted into the membrane and reassembled into functional
photo system II units closing the cycle in rapid turnover.
Viewing this process as a regulatory phenomenon, it should be noted
that an amino acid sequence in a functional membrane protein contains
regulatory sites. In rapid turnover these are those of the extended loop in
the D1 protein (Figs. 1.3 and 1.4).
1.5 Photo inhibition and Environmental Stress
As discussed in the rapid turnover of the D1 protein, the cleavage and
degradation of the D 1 protein already proceeds at moderate light intensity.
As degradation is balanced by resynthesis and reassembly of new D1 protein,
it does not lead to an impairment of photosynthesis. However, under higher
excess light conditions, imbalance in the repair cycle resynthesis does not
compensate the degradation. This leads to a rapid disappearance of all
photo system II subunits and a permanent stall of photosynthesis. This
impairment of photosynthesis by the light sensitivity of photosystem II is
enhanced by stress conditions. Each of the processes - inactivation, the
degradation of the D1 protein, resynthesis and reassembly of PS II components - is subject to control and will be influenced by environmental
factors. This is the case in the stress-induced increase of photoinhibition.
For a recent review see Demmig-Adams and Adams (1992). For example,
air pollutants result in light-dependent bleaching and a decrease in photosynthetic activity which is most likely a consequence of the primary light
inactivation of PS II and degradation of the D1 protein (Godde and
Buchwald 1992). The noxious gases do not react necessaily directly with the
primary events in photosystem II, for a review see Lange et al. (1989), but
they affect the balance of rapid turnover and the repair cycle.
A stressed spruce needle can no longer compensate for the inactivation
and degradation process by its repair capacity, and the system collapses
(Godde and Buchwald 1992; Liitz et al. 1992). Overstraining the balance
13
al. 1984; Jansen et al. 1993) - those amino acids that close up the QA and
Q B site towards the matrix space, as discussed above.
The cleavage site of the D1 protein as part of the rapid turnover can be
prevented by an inhibitor in the QB site, like DCMU (Mattoo et al. 1984;
Schuster et al. 1988), other classical herbicides, and also certain phenol-type
inhibitors (Jansen et al. 1993). This supports the notion that the QB site
controls the conformation of the cleavage site and in this way allows or
prevents access of a protease in the primary cleavage of the D1 protein in
rapid turnover.
After complete degradation of the cleaved D1 protein by so-called housekeeping proteases, the translation machinery of the chloroplast will resynthesize the D1 protein. This is also light-dependent. The new protein will be
processed, reinserted into the membrane and reassembled into functional
photo system II units closing the cycle in rapid turnover.
Viewing this process as a regulatory phenomenon, it should be noted
that an amino acid sequence in a functional membrane protein contains
regulatory sites. In rapid turnover these are those of the extended loop in
the D1 protein (Figs. 1.3 and 1.4).
1.5 Photo inhibition and Environmental Stress
As discussed in the rapid turnover of the D1 protein, the cleavage and
degradation of the D 1 protein already proceeds at moderate light intensity.
As degradation is balanced by resynthesis and reassembly of new D1 protein,
it does not lead to an impairment of photosynthesis. However, under higher
excess light conditions, imbalance in the repair cycle resynthesis does not
compensate the degradation. This leads to a rapid disappearance of all
photo system II subunits and a permanent stall of photosynthesis. This
impairment of photosynthesis by the light sensitivity of photosystem II is
enhanced by stress conditions. Each of the processes - inactivation, the
degradation of the D1 protein, resynthesis and reassembly of PS II components - is subject to control and will be influenced by environmental
factors. This is the case in the stress-induced increase of photoinhibition.
For a recent review see Demmig-Adams and Adams (1992). For example,
air pollutants result in light-dependent bleaching and a decrease in photosynthetic activity which is most likely a consequence of the primary light
inactivation of PS II and degradation of the D1 protein (Godde and
Buchwald 1992). The noxious gases do not react necessaily directly with the
primary events in photosystem II, for a review see Lange et al. (1989), but
they affect the balance of rapid turnover and the repair cycle.
A stressed spruce needle can no longer compensate for the inactivation
and degradation process by its repair capacity, and the system collapses
(Godde and Buchwald 1992; Liitz et al. 1992). Overstraining the balance