Rubisco activation

However, the exact mechanism that causes deactivation of Rubisco in high N leaves is unclear. Rubisco activity is regulated by the counteraction of tight binding inhibitors and Rubisco activase that removes these inhibitors Portis, ; Salvucci and Ogren, Recently, some unidentified sugar phosphates were shown to reduce Rubisco activity in the light Keys et al. It appears that CA1P does not play any significant role in the light regulation of Rubisco activity in apple leaves because no difference in total Rubisco activity was detected between measurements made at noon under full sunlight and at night L Cheng and LH Fuchigami, unpublished data.

In addition, Rubisco activities were measured at noon under full sunlight in this experiment when CA1P concentration would have been minimal even if apple leaves had considerable amounts of CA1P at night. Therefore, the observed decrease in Rubisco activation state in relation to leaf N may have nothing to do with leaf CA1P level at night. However, it is not known whether N supply would alter the levels of other tight binding inhibitors. One possibility is that the amount of Rubisco activase may not keep pace with the increase in the amount of Rubisco as leaf N increases, resulting in decreased Rubisco activation state.

In addition, Rubisco activase may be also subject to redox regulation at least in some species Zhang and Portis, This may provide a mechanism to explain how Rubisco activation state responds to N content in apple leaves.

The finding that Rubisco activation state decreases with increasing N content in apple leaves under saturating light conditions supports the idea that Rubisco can serve as a storage protein when the N supply is in excess. Considering that Rubisco is so expensive in terms of N investment, it has been argued that N resources would be wasted if Rubisco were present in great excess.

This argument is generally valid in most cases. However, under an excess N supply, accumulating surplus Rubisco may benefit plants in terms of N acquisition and reutilization because N is such an important resource for plant growth and development. Especially for plants such as apple, which have very low nitrate concentrations in leaves even under a high nitrate supply Lee and Titus, , Rubisco apparently serves as a storage protein.

This has been seen in comparisons between wild tobacco plants and antisense Rubisco plants Quick et al. In conclusion, Rubisco activation state decreases with increasing N content in apple leaves and Rubisco may serve as a storage protein in leaves with a high N content. The mechanism of deactivation of Rubisco in high N leaves under saturating light conditions deserves further research.

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Rubisco activase. Biochimica et Biophysica Acta , 15 — Impact on photosynthesis in ambient growth conditions. Sage RF. Plant Physiology 94 , — The nitrogen use efficiency of C 3 and C 4 plants. An activating CO 2 molecule note, not a substrate for catalysis is added to a specific amino acid residue in the active site Lysine Following carbamylation, the substrate RuBP can bind to the active site, forming an enediol. C-terminal loops of RuBisCO now fold over the active site to create a channel down which the substrate CO 2 or O 2 , see below molecule can diffuse.

Catalysis then occurs when the CO 2 or O 2 molecule binds to the enediol. RuBisCO is now active. As both CO 2 and O 2 are small gaseous molecules, and moreover as RuBisCO evolved at a time when atmospheric oxygen concentrations were negligible, RuBisCO does not have perfect specificity for CO 2 over O 2 , thus both can serve as substrates for its catalytic activity.

The competing carboxylase and oxygenase activities of RuBisCO. Phosphoglycollate cannot be converted directly into sugars, and so is a wasteful loss of carbon. To retrieve the carbon from it, plants and algae employ an energy-expensive process called photorespiration note that many written resources on this topic, including Wikipedia, state that photorespiration is the reaction of oxygen with RuPB, catalysed by RuBisCO — this can be misleading, as this reaction is simply the oxygenase activity of RuBisCO, while photorespiration is the series of processes that must take place following such a reaction.

Photorespiration not only wastes energy and reducing power, but also results in the production of dangerous reactive oxygen species — namely H 2 O 2 , hydrogen peroxide — in a cellular compartment called the peroxisome.

It is important to note that, despite this seeming failure of RuBisCO to carry out its function at maximum efficiency, we really ought to marvel at how well evolution has done the best of a bad job: atmospheric O 2 concentrations are in the order of times higher than CO 2 concentrations and yet, somehow, RuBisCO fixes on average 4 CO 2 for every O 2.

RuBisCO Overview Ribulose-1,5-bisphosphate carboxylase oxygenase, normally shortened to RuBisCO, is the most abundant enzyme on Earth — and arguably one of the most important for life at least as we know it! Structure RuBisCO molecule, with small subunits shown in ribbon form. Like this: Like Loading Learn More. Rubisco is widely accepted as the ultimate rate-limiting step in photosynthetic carbon fixation. Atmospheric oxygen competes with CO 2 as a substrate for Rubisco, giving rise to photorespiration.

Later, Lorimer et al. Indeed, on addition of RuBP, the measured K m CO 2 approached the concentrations of dissolved CO 2 in water; however, the rate of the reaction was sustained for only 5 minutes and then declined rapidly. The missing ingredient needed to uncouple Rubisco and RuBP was found in the intact plant with a separate protein, called Rubisco activase 3.

Rubisco activase itself requires ATP, and its activity is related to the energy charge of the chloroplast 4. Thus, the proportion of Rubisco that is active in a leaf activation state can vary depending on the effectiveness of Rubisco activase in removing bound RuBP. Regulation of Rubisco fine tunes the rate of CO 2 fixation to the rate of photosynthetic electron transport, ensuring that chloroplast metabolites are always optimal for photosynthesis 5.

Activase blue switches Rubisco yellow from an inactive to an active form by the ATP-dependent release of tight-binding sugar phosphates such RuBP. Only the active form of Rubisco is capable of catalyzing CO 2 fixation, the first step in photosynthesis. The binding of RuBP causes conformational changes that produce a dead-end complex consisting of inactive Rubisco and tightly bound RuBP.

Activase physically interacts with Rubisco step 5 , changing the conformation of Rubisco step 6 to one that binds RuBP less tightly. ATP hydrolysis by activase is required for these conformational changes, perhaps for priming activase step 4. Because of the lower affinity, RuBP dissociates from the active site of Rubisco step 7 , which frees the site for subsequent carbamylation step 1 or rebinding of RuBP step 3 , but probably after dissociation of activase step 8.

Crafts-Brandner and Salvucci 6 show that an accelerated rate of Rubisco deactivation occurs at high temperature steps 2 and 3 , which is not matched by a faster rate of activation by activase. The paper by Crafts-Brandner and Salvucci 6 in this issue of PNAS provides evidence that, with plants under heat stress, the activation state of Rubisco and photosynthesis as measured by CO 2 exchange is reduced.

By duplicating the temperature response in the test tube under controlled conditions, and by using Rubisco and Rubisco activase isolated from tobacco, they were able to ascribe the limitation to a specific biochemical event, the inability of Rubisco activase to keep pace with a faster deactivation of Rubisco.