CAPP

Combining Algal and Plant Photosynthesis

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!). It is the only net carboxylase that we know of – that is, the only enzyme capable of catalysing a net fixation of inorganic carbon into organic molecules in one step. This plays a central role in the process of photosynthesis, as it has since RuBisCO first evolved, about 3.6 billion years ago.

Structure

RuBisCO molecule, with small subunits shown in ribbon form.

RuBisCO comprises a number of identical small subunits (or chains) and identical large subunits/chains. Although there are several variant forms of RuBisCO, with differing numbers of subunits, the most abundant form in nature – present in cyanobacteria, green algae and plants – is called form I, and can be described as L8S8. That is, it comprises 8 large and 8 small subunits. More accurately, it comprises a tetramer of large chain dimers (i.e four pairs of large subunits), and a dimer of small chain tetramers (i.e. two quads of small subunits). In plant and algal cells, the small subunits are encoded by nuclear genes, while the large subunits are encoded by genes located in the chloroplast genome. The RuBisCO protein molecule itself is located in the chloroplast stroma of plants and algae.

The active sites of RuBisCO are located on the large subunits, between the dimer pairs, thus the catalytic properties of RuBisCO are dependent on the large subunits. The small subunits may have many other important roles, as shall be discussed elsewhere on this website (see Pyrenoid). On this latter point, a structural aspect of RuBisCO to note is that, present in the polypeptide chain that makes up one RuBisCO small subunit, are two α-helices which have been heavily implicated in pyrenoid formation in algae.

Catalytic Properties

Despite its huge importance in life, RuBisCO is, by enzyme standards, rather slow, which a catalytic turnover rate (Kcat) of between 3 and 10 molecules per second. This may sound fast, but compare this to, for example, carbonic anhydrases (which you will meet elsewhere on this site, catalysing the interconversion between CO2 and HCO3) which have a  a Kcat of around 500 000 molecules s-1!

– Carboxylase Activity

RuBisCO catalyses the important entry step of CO2 into the Calvin-Benson-Bassham (CBB) cycle, as part of the light-independent reactions of photosynthesis. CO2 is reacted with ribulose-1,5-bisphosphate to produce an unstable 6-carbon intermediate that rapidly dissociates into two molecules of 3-phosphoglycerate (3-PGA). Ultimately this gives rise to triose phophates from which sugars can be derived.

– Activation

The central role of RuBisCO in the fundamental process of photosynthesis means that it must be tightly regulated, to ensure it is active only where and when it should be. One important layer of this regulation is the activation of RuBisCO at the beginning of the day. During the night, the RuBisCO active sites are blocked by inhibitors or misfired reactants. These inhibitors include “daytime” substrates, such as RuBP, as well as specific inhibitors to ensure that RuBisCO is only active when there is a source of free energy from sunlight (otherwise, RuBisCO will use the free energy released by the breakdown of sugars via respiration, which constitutes a futile cycle – RuBisCO uses energy to fix carbon as sugars in the CBB cycle, then breaks these sugars down to re-release the energy so that it can fix more carbon!).

Activation, then, involves firstly the removal of these inhibitors. This is carried out by a light-activated chaperone protein imaginatively called RuBisCO activase. RuBisCO activase uses the free energy of ATP hydrolysis to “clear out” the active sites of RuBisCO.

The second step in activation of RuBisCO is called carbamylation. An activating CO2 molecule (note, not a substrate for catalysis) is added to a specific amino acid residue in the active site (Lysine201) . The carbamate thus formed is stabilised by a magnesium ion (Mg2+) – this is a further checkpoint to ensure RuBisCO is activated during periods of photosythetic activity, as the Mg2+ concentration of the chloroplast stroma increases dramatically during the day when Mg2+ floods out of the photosynthetically active thylakoids.

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 CO2 (or O2, see below) molecule can diffuse. Catalysis then occurs when the CO2 (or O2) molecule binds to the enediol. RuBisCO is now active.

Oxygenase Activity

Competing with CO2 for RuBisCO’s attention are O2 molecules. As both CO2 and O2 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 CO2 over O2, thus both can serve as substrates for its catalytic activity. This is a problem for plants and algae as RuBisCO’s oxygenase activity yields, instead of two 3-PGA per substrate molecule, one 3-PGA and one molecule of phosphoglycollate.

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 H2O2, hydrogen peroxide – in a cellular compartment called the peroxisome. For more detail on photorespiration, see the Undergraduate Teaching Resource entitled “Why do Plants and Algae Need CCMs?”.

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 O2 concentrations are in the order of 500 times higher than CO2 concentrations and yet, somehow, RuBisCO fixes on average 4 CO2 for every O2. It really isn’t as shoddy an enzyme as it’s often painted!

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