The great RuBisCO and its amazing carbon fixation

Thanks to its role in photosynthesis, ribulose bisphosphate carboxylase oxidase (RuBisCO) is perhaps the most abundant enzyme in the world. Paul Connell sheds some light on how it works

Dissolved in the stroma of a chloroplast, the plant cell structure responsible for photosynthesis, is the enzyme RuBisCO, which makes up half of the soluble protein of most leaves.

A diagram of the Calvin Cycle

The Calvin cycle, as shown in our photosynthesis poster.

CC BY

Bret Syfert/‘Big Picture: Plants’ (2016)

RuBisCO does a very difficult biochemical job. It kick-starts the first stage of the Calvin cycle, taking carbon dioxide dissolved in the leaf water and placing it into sugar molecules to make them bigger. That chemical reaction is called carboxylation – and the overall process is called carbon fixation.

How the enzyme got its name

An enzyme is named in two parts. The first part is its substrate (the chemical it works on); the second part is its action. RuBisCO’s substrate is the high-energy five-carbon sugar RuBP, or ribulose bisphosphate. Its primary action is carboxylation, but at most temperatures, RuBisCO also oxidises RuBP – which is why the final two words of the name are ‘carboxylase oxidase’.

How fast does it work?

How fast an enzyme works depends on three things: temperature, substrate and concentration.

Enzyme reactions speed up as temperature increases – up to an optimum point.

The optimum point is the temperature at which the enzyme’s active site is best at binding to its substrate. Human enzymes, for instance, have an optimum of 37ºC – our body temperature. Plant enzymes have varying optimums, to match the varying climates the plants grow in. At higher temperatures than its optimum, RuBisCO starts to be better at oxidation than carboxylation. When the rate of carboxylation slows down, so does photosynthesis.

The more substrate there is, the more likely the enzyme’s active site is to be full, so the rate of reaction is faster.

RuBisCO’s substrate is RuBP, which is regenerated as the Calvin cycle turns (see diagram). But it also requires CO2 for the process to start. As the level of CO2 dissolved in the water in the leaf drops, the active site is less likely to be full.

The higher the enzyme concentration, the faster the reaction.

Leaves contain a huge amount of RuBisCO to increase the rate of this vital reaction.

Oxidation vs reduction

Oxidation occurs when chemicals have oxygen added to them or hydrogen taken away. The level of oxidation is indicated numerically (positive numbers for oxidised atoms, negative for reduced ones).

  • Carbon dioxide (CO2) consists of a carbon atom double-bonded to two oxygen atoms. The carbon is highly oxidised, with an oxidation state of +4.
  • Methane (CH4) is a carbon atom with four hydrogen atoms single-bonded to it. The carbon is highly reduced, with an oxidation state of –4.
  • Glucose (C6H12O6) is a carbohydrate. For every individual carbon atom, there is one oxygen atom and two hydrogen atoms. The oxidation state of the carbons in glucose comes out as zero.
  • Glycerate phosphate (GP), which is produced during a turn of the Calvin cycle, is also a carbohydrate. It too has an average carbon atom oxidation of zero.

During photosynthesis, the oxidation state of carbon atoms falls from +4 to zero. This requires energy provided by ATP and reduced NADP from the light-dependent reactions. This reduction takes place in just one step: the carboxylation reaction catalysed by RuBisCO. The energy found in the ATP and reduced NADP is used in subsequent reactions to rearrange GP into glucose and RuBP.

When it’s hot

As temperatures rise over approximately 35ºC, photosynthesis slows down and becomes far less efficient, with a higher rate of oxidation. During the first carboxylation, RuBisCO takes a carbon dioxide molecule but cannot hold on to it.

Many tropical plants such as maize and sugar cane have an extra biochemical pathway bolted on to the Calvin cycle. Their process is called C4 photosynthesis.

Plants that use C4 photosynthesis can concentrate the carbon dioxide locally – with less competition from oxygen – so carbon is fixed faster. If this fixes carbon faster, why haven’t all plants evolved to use C4? The answer is energy: C4 photosynthesis requires more ATP – approximately 30 ATP, compared with 18 ATP for lower-temperature photosynthesis (known as C3) – so at lower temperatures, C4 photosynthesis has no advantage.

References

Questions for discussion

  • Why is sugar cane (Saccharum officinarum) the most important sugar crop in the tropics but sugar beet (Beta vulgaris) the most important sugar crop in Europe?
  • Find an explanation of oxidation and reduction in terms of gain and loss of electrons.
  • How much a chemical is oxidised or reduced is given a number called the ‘oxidation state’. Find the oxidation state of carbon dioxide and glucose and use those figures to explain why photosynthesis is a difficult reaction.
  • Ribulose is not much different from glucose or even ribose (from RNA). The formula for glucose is C6H12O6. Find the formula for ribulose.
  • Where does the energy come from to make RuBP and drive the Calvin cycle?
  • RuBisCO takes carbon dioxide from the leaf water that surrounds it. Explain how this carbon dioxide gets to RuBisCO from the air outside the leaf.
  • Go a little deeper: find out how RuBisCO is switched on by light and switched off in the dark.

About this resource

This resource was first published in ‘Plants’ in May 2016.

Topics:
Cell biology, Statistics and maths, Ecology and environment
Issue:
Plants
Education levels:
16–19, Undergraduate, Continuing professional development