As we saw earlier in the case of mitochondria, reactive oxy gen species (ROS) generated during electron transport (see Figure 1) can both serve as signals to regulate organelle function and cause damage to a variety of biomolecules. The same is true of chloroplasts. For example, hydrogen peroxide (H2O2) regulates gas-exchange pores (stomata) in plants subjected to drought stress to prevent dehydration and controls cyclic electron flow, a process we will describe shortly.
Fig1. The phosphate and ATP/ADP transport system in the inner mitochondrial membrane. The coordinated action of two antiporters (purple and green), which results in the uptake of one ADP3− and one HPO42− in exchange for one ATP4− and one hydroxyl, is powered by the outward translocation of one proton (mediated by the proteins of the electron-transport chain, blue) during electron transport. The outer membrane is not shown here because it is permeable to molecules smaller than 5000 Da.
Even though the PSI and PSII photosystems, with their as sociated light-harvesting complexes, are remarkably efficient at converting radiant energy to useful chemical energy in the form of ATP and NADPH, they are not perfect. Depending on the intensity of the light and the physiological conditions of the cells, a relatively small—but significant—amount of the energy absorbed by chlorophyll in the light-harvesting antennas and reaction centers results in the chlorophyll being converted to an activated state called triplet chlorophyll. In this state, the chlorophyll can transfer some of its energy to molecular oxygen (O2), converting it from its normal, relatively unreactive ground state, called triplet oxygen (3O2), to a very highly reactive (ROS) singlet state, 1O2. Some of this 1O2 can be used for signaling to the nucleus to communicate the metabolic state of the chloroplast to the rest of the cell. However if the majority of the 1O2 is not quickly quenched by reacting with specialized 1O2 “scavenger molecules,” it will react with, and usually damage, nearby molecules. This damage, called photoinhibition, can suppress the efficiency of thylakoid activity.
Carotenoids (polymers of unsaturated isoprene groups, including β-carotene, which gives carrots their orange color) and α-tocopherol (a form of vitamin E) are hydrophobic small molecules that are known to play important roles as 1O2 quenchers to protect plants. For example, inhibition of tocopherol synthesis in the unicellular green alga Chlamydomonas reinhardtii by the herbicide pyrazolynate can result in greater light-induced photoinhibition. The carotenoids, which very efficiently siphon off energy from dangerous trip let chlorophyll when they are in close proximity to it, are the quantitatively most important molecules for preventing 1O2 formation. The PSII monomer from the cyanobacterium Thermosynechococcus elongatus contains about 11 carotenoid molecules compared with its 35 chlorophylls, indicating the importance of carotenoid-mediated quenching of 1O2.
Under intense illumination, photosystem PSII is especially prone to generating 1O2, whereas PSI will produce other ROS, including superoxide, hydrogen peroxide, and hydroxyl radicals. One of the protein subunits in the PSII reaction center, called D1, is subjected to almost constant 1O2-mediated damage even under low light conditions. A damaged reaction center moves from the grana to the un stacked regions of the thylakoid, where the D1 subunit is degraded by a protease and replaced by newly synthesized D1 protein in what is called the D1 protein damage-repair cycle. This rapid replacement of damaged D1, which re quires a high rate of D1 synthesis, helps PSII recover from photoinhibition and maintain sufficient activity. An important component in the damage-repair cycle is a chaperone of the HSP70 family called HSP70B. This chaperone binds to the damaged PSII and helps prevent loss of the other components of the complex as the D1 subunit is replaced. The extent of photoinhibition can depend on the amount of HSP70B available to the chloroplasts.
