Photorespiration - Prelude

We should all be familiar with respiration. We breath in oxygen from the atmosphere (kindly provided by plants!) and use it to "burn" carbohydrates (also ultimately provided by plants!), releasing energy (NADH and ATP), carbon dioxide (CO₂) and water. The carbohydrates we use as fuel for mitochondrial respiration were generated by plants in the process of photosynthesis using energy from light (see my previous posts on The Light Reactions of Photosynthesis and The Calvin Cycle of C3 Photosynthesis). The investment of NADPH and ATP in carbohydrate biosynthesis by plants is recovered by us as NADH and ATP when we eat plant-derived carbohydrates. In essence, mitochondrial respiration is the reverse of photosynthesis:

Photosynthesis (chloroplast):

CO₂ + H₂O + ATP + NADPH ---> carbohydrates + O₂

Respiration (mitochondrion):

O₂ + carbohydrates ---> CO₂ + H₂O + ATP + NADH

Plants do have mitochondria and perform mitochondrial respiration much like animals, and they use this process to provide energy for growth in the dark when no photosynthesis is possible, in organs that do not express photosynthesis machinery (e.g. flowers) and organs that are not exposed to light (e.g. roots). I will discuss mitochondrial respiration by plants in a subsequent post.

Here I would like to introduce an unusual second form of respiration that occurs in C3 plants that is light-dependent and is called "photorespiration" (i.e. light-dependent O₂ consumption accompanied by CO₂ release).

Because the goal of photosynthesis is to fix CO₂ in the light, photorespiration apparently defeats this objective by liberating CO₂ in the light. Thus, photorespiration is generally considered to be a wasteful biochemical process. So why do plants do this? After 2 to 3 billion years of evolution one might suppose that plants should have evolved mechanisms to minimize such "wasteful" biochemistry and maximize their productivity? These are questions which are still the area of active research today, and I will attempt to address them in this and future articles in this series.

To understand photorespiration it is necessary to revisit the first enzyme of photosynthesis in C3 plants, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) discussed in my previous post on The Calvin Cycle of C3 Photosynthesis.

While the carboxylase activity of this enzyme is responsible for CO₂ fixation, the enzyme (as its full name implies) has another activity ... an oxygenase activity. In this reaction, O₂ competes with CO₂ at the active site of the enzyme. Whereas during CO₂ fixation the carboxylase activity of RuBisCO generates 2 molecules of 3-phosphoglycerate from 1 molecule of ribulose-1,5-bisphosphate and 1 molecule of CO₂, the oxygenase activity of RuBisCO generates 1 molecule of 3-phosphoglycerate + 1 molecule of 2-phosphoglycolate from 1 molecule of ribulose-1,5-bisphosphate and 1 molecule of O₂:

Carboxylase activity of RuBisCO:

CO₂ + ribulose-1,5-bisphosphate --> 2 x 3-phosphoglycerate

Oxygenase activity of RuBisCO:

O₂ + ribulose-1,5-bisphosphate --> 3-phosphoglycerate + 2-phosphoglycolate

2-Phosphoglycolate is a 2-carbon compound and its production poses a number of problems for C3 plants. It disrupts the stoichiometry of photosynthesis and the regeneration of ribulose-1,5-bisphosphate. 2-Phosphoglycolate is also an inhibitor of enzymes of the Calvin cycle. It has to be disposed of and recycled back to 3-phosphoglycerate in order not to deplete intermediates of the Calvin cycle.

In today's atmosphere of about 405 ppm (0.045%) CO₂ and 21% O₂ (i.e a ratio of CO₂:O₂ of about 1:519!) it is not difficult to comprehend that the oxygenase activity of RuBisCO represents a severe problem for C3 plants, despite a couple of billion years of evolution to improve RuBisCO's ability to distinguish between these 2 gases. Only minor changes can be made to the active site of RuBisCO to discriminate between these 2 gases without impairing its CO₂ fixation capacity. These minor adjustments have already been implemented to their maximal capacity during plant evolution.

We also have to bear in mind that plants evolved in an atmosphere where CO₂ levels were much higher than today, and where O₂ levels were much lower. Photorespiration in C3 plants may actually be a fairly recent problem resulting from the success of plants in oxygenating our atmosphere and depleting CO₂! As we will see in future posts, this problem of photorespiration has led to the evolution of alternative photosynthesis strategies by plants, C4 photosynthesis and Crassulacean Acid Metabolism (CAM). These are relatively recent innovations by angiosperms occurring during the last 10 to 30 million years when CO₂ levels were perhaps as low as 240 ppm (0.024%) (i.e. when the ratio of CO₂ to O₂ was about 1:875). As we will see, C4 and CAM plants have essentially evolved mechanisms for CO₂ concentration at the active site of RuBisCO, effectively suppressing photorespiration by blocking the oxygenase activity of this enzyme with excess CO₂.

To illustrate the photorespiration problem resulting from RuBisCO's ability to bind either CO₂ or O₂ at its active site, I performed a simulation of the velocities of the carboxylase and oxygenase activities of the enzyme at different CO₂ and O₂ levels, using kinetic constants for the enzyme derived from Susanne von Caemmerer's book (2000) (Biochemical models of leaf photosynthesis. CSIRO Publishing. Collingwood, Australia).

RuBisCO kinetics with respect to CO₂ and O₂ concentrations (D. Rhodes)

The simulation program was written in Visual Basic and calculates Vc and Vo (the velocities of the carboxylase and oxygenase activities, respectively) at different partial pressures of O₂ (millibars = mbar) and CO₂ (microbars = ubar) (see upper graphs). In this particular program air was assumed to be composed of 20.95% O₂ and 0.036% CO₂. In the lower graphs, I have also plotted 1/Vc versus 1/CO₂, and Vc/CO₂ versus Vc (these are standard plots used by enzymologists to calculate kinetic parameters of enzymes). The most important aspects of these plots are that CO₂ and O₂ are competitive with one another, i.e. as O₂ levels rise, the velocity of the carboxylase activity of RuBisCO decreases. As CO₂ levels rise, the oxygenase activity of RuBisCO decreases.

The small single circles in these graphs represent the activity of RuBisCO in air (20.95% O₂ and 0.036% CO₂) at 25^oC, at sea level, and at normal atmospheric pressure. Here the relative velocities of the carboxylase and oxygenase activities are calculated to be 30.4 and 7.13, respectively. In other words a typical C3 plant growing in a meadow on the coast would be wasting about 23% of its carbon as a result of photorespiration!

Since I wrote this program (in the year 2000) the CO₂ concentration of the atmosphere has risen to 405 - 410 ppm (0.0405 - 0.0410%) due primarily to burning of fossil fuels by humans:

Full Mauna Loa CO₂ record Image Source: NOAA

While this trend will certainly benefit C3 plants (by suppressing photorespiration), it is clearly a major concern for the planet with respect to global warming (the greenhouse effect), and ocean acidification:

H₂O + CO₂ --> HCO₃^- + H⁺ --> CO₃^2- + 2H⁺

see: Ocean acidification - Wikipedia

In the next post in this series (Photorespiration - Biochemistry) I will describe in detail how 2-phosphoglycolate is metabolized and recycled to 3-phosphoglycerate. In this process, CO₂ is generated and energy is expended (in the form of reduced ferredoxin and ATP consumption) contributing to the apparent wastefulness of this pathway.