The “Alternative Oxidase” Pathway
The wide variety of organisms living in the planet today arises from centuries of evolution. The only way they were able to exist such conditions is if they learn to evolve new traits, new skills, and new mechanisms. They would need to rely greatly on their environment, and the current conditions of their habitat would ultimately dictate them to adjust their lifestyle in order to cope up with the changing world.
This eventually led to a number of interesting things. First is the emergence of a diverse number of creatures. The world is populated by billions of organisms, each having their own mini-earth to live on. Secondly is the evolution of several traits that are new to old species. These would, as mentioned above, help them survive the ever changing world and passing on these traits would lead to stronger organisms. Finally, there is the presence of several alternative mechanisms which some organisms learned to adapt to, just in case something goes wrong with, or the original system becomes faulty.
One of these mechanisms is the Alternative Oxidase respiration by plants. By normal accounts, organisms undergo respiration using the basic Cytochrome pathway. It is something that has evolved through, and was tested by time to be effective in certain organisms. But for plants and some algae, the preferred system is the use of the Alternative Oxidase, primarily as an effect of difference in environmental niches and their structural organizations.
This paper would tackle about this alternative pathway, how it differs from the regular respiration and what advantages it brings to the organisms that uses it.
In order to fully understand the topic at hand, one must first be well versed with the basics of respiration. Respiration is the process of obtaining energy by breaking down molecules. Glucose, a 6-carbon unit is normally broken down into smaller pieces in a two step pathway. The first is glycolysis, on which the glucose is converted into pyruvate, a 3-carbon unit. Energy here is produced and used to generate ATP, or adenotriphosphate, which is used in many processes of an organism’s cell (UIC, 2009). Normally, glycolysis occurs in the cell’s cytosol. The amount of energy produced here is not enough for normal cell processes, and so a second step is required.
Following glycolysis is the Kreb’s cycle. This procedure takes the pyruvate that was created during glycolysis, and turns it into acetyl COA. At the same time, several molecules of ATP, NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide) are produced (UIC, 2009). All of these molecules would play essential roles in the next steps to happen which would eventually lead to energy production. This happens in the mitochondria, and much more molecules of ATP are produced.
The amount of energy produced in the glycolysis-Kreb’s cycle steps are, however, not even sufficient to support a cell’s respiratory needs. Bulk of the energy is produced in a third and last step, the electron transport chain. This also happens in the mitochondria, and produces a massive 32 molecules of ATP. What happens is that the NADH and FADH2 are broken down, causing H ions to be pumped out of the mitochondrial membrane. A gradient of positive charges is then generated, in this is used by the electron transport system to systematically pass on electrons, producing ATPs in the process (Raven et al, 2005). It is also at this point where the difference of a regular and an alternative respiration can be observed.
The electron transport system is composed of several units, and the pathway of the electrons through which they passed through is the defining factor on which respiration is done. Below is a figure of the mitochondrial membrane depicting the units involved in the cytochrome pathway.
Figure 1. The cytochrome pathway (Yaniv, 2002).
This system begins with Complex I, on which NADH is oxidized to NAD+. One H+ is transported out into the intermembrane space while the two electrons are taken up by a protein called the ubiquinone (Q). Its function is to deliver these electrons to the next unit of the chain, Complex III. Again, Complex III transfers a H+ to the intermembrane space, while the electrons are transferred to another protein, the cytochrome C. Finally, the cytochrome C brings the electrons to Complex IV, which works just like the other complexes in transferring protons to the outside. The only difference here is that oxygen is consumed, and the return of the electrons facilitates the formation of water (Buchanan et al, 2002).
The overall effect of this pathway is the production of several protons on the intermembrane space. Its heavy density causes it to be forced back in the mitochondria through the ATP synthase. This is a machine-complex that produces ATPs by phosphorylating ADPs (adenosine diphosphate) (UIC, 2009). The influx of the H+ from the intermembrane space runs the enzyme, and produces enough ATPs for the organs to use.
Gaining an understanding on the normal, or the cytochrome respiration pathway is essential before learning the alternative pathway. The beginning steps are similar, but the end producta are very different from each other. To begin, below is a picture of the alternative oxidase pathway.
Figure 1. The alternative oxidase pathway (Yaniv, 2002).
Just like the other pathway, this begins with the oxidation of NADH into NAD+ at Complex I and the uptake of electrons by ubiquinone. But instead of passing through Complex III, the electrons are taken by another protein, the Alternative Oxidase (AOX).
The AOX is a dimer protein, each unit containing an iron binding site to contain the needed electrons. Upon activation, the AOX splits into two, consumes oxygen and produces water as its final product. Comparing to the cytochrome pathway, the alternative oxidase pathway produces very little energy. It is capable of releasing a very small number of protons on the intermembrane space, and the transport of the electrons stop at a very early point. As a matter of fact, this pathway practically produces no energy when compared to that of the cytochrome’s capability.
If the alternative oxidase produces so little energy, why did the system evolved in the first place?
The AOX system is prevalent in plants and some algae, organisms which are very much plastic – they are able to quickly adapt to their surroundings and adjust their mechanisms to suit different needs. There is, therefore, a strong evidence of the advantages of the AOX system to compensate for its low energy production. And of course, the fact that this system evolved means that it serves a purpose.
First off, one can think of the AOX system as a bypass system, something that the organism does in order to avoid the other pathway. Taking into account the effects of the cytochrome pathway, the numerous protons on the intermembrane space can lead to a drastic change of acidity. The plant can then use another pathway, one that doesn’t involve the generation of too much protons to balance the cell’s pH. The AOX pathway was also believed to be a buffer system for the generation of oxygen. The system can limit the amount of oxygen produced, which if in huge amounts, can be harmful in certain plant organs (Parsons et al, 1999).
Another possible reason for the pathway lies in its resistance to inhibition by certain chemicals. The cytochrome pathway in plants, for instance, becomes cut off in the presence of cyanide. This results to a complete shut down of the plant’s ability to respire, and thus produce energy. Having an alternative system, such as the AOX, one that does not get affected by the cyanide can prove to be useful in the survival of the plant (Parsons et al, 1999).
The alternative oxidase system, although feeble in energy production, still has a number of advantages in its belt. It was evolved into by plants in order to cope up with the changing environment; certain events that could threaten their existence such as acidity, harmful chemicals, and inhibitors. Its resistance to certain molecules that could restrict the normal respiration of a cell makes it useful and effective as a survival strategy. This pathway should be seen as a bypass system, on which the plant’s respiration goes into in cases of fault and errors.
Buchanan BB., Gruissem W. and Jones, RL. 2002. Biochemistry & molecular biology of plants. New Jersey: Wiley
Parsons, HL., Yip, JY. and Vanlerberghe, GC. Increased respiratory restriction during phosphate-limited growth in transgenic tobacco cells lacking alternative oxidase.
Plant Physiology 121: 1309-1320, December 1999
Raven, PH., Evert, RF. and Eichhorn, SE. 2005. Biology of plants. New York: W.H. Freeman and Company
UIC lectures. 2009. Glycolysis, Krebs cycle, and other energy-releasing pathways. University of Illinois at Chicago website. Retrieved March 22, 2009 from http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect12.htm
Yaniv, E. 2002. Alternative respiration. Tel Aviv University website. Retrieved March 21, 2009 from http://www.tau.ac.il/~ecology/virtau/2-elitsur/ey.htm