|MadSci Network: Microbiology|
Thanks for your question. I think there are several questions to be answered here, as follows:
Why do aerobic organisms need oxygen?
In aerobic organisms, oxygen is required as a terminal electron receptor. What does this mean? Aerobic metabolism (glycolysis and the TCA cycle) generates reduced molecules (NADH and succinate). These are reoxidised by membrane-bound complexes. In eukaryotes, these complexes are found in the membranes of mitochondria. In prokaryotes, functionally-equivalent complexes are located in the plasma membrane. During the reoxidation reactions, electrons pass through the respiratory chain and protons are pumped across the membrane. This generates a proton motive force which is used to synthesise ATP. So what happens to those electrons that are removed from NADH/succinate? They need a sink - and that sink is oxygen, which is reduced to water by the cytochrome oxidase complex.
Take a look at this diagram from the KEGG database of metabolic pathways. It summarises the process that I described - oxidative phosphorylation. You can see oxygen being reduced to water by Complex IV, cytochrome oxidase, the second from right complex.
The beauty of the chemiosmotic process is that different organisms use many variations of it, yet the fundamentals are the same: (1) small reduced molecules are reoxidised by membrane bound complexes, (2) protons (or sometimes other ions) are used to generated an electrochemical gradient for ATP synthesis and (3) a terminal electron acceptor acts as an electron sink and is reduced.
Let's look at an aerobic microorganism. On the KEGG webpage, go to the select box at the top left and scroll down until you find an organism called Rickettsia prowazekii, then click the "Go" button. A new diagram will be drawn. The genes shaded light green are those that encode the respiratory complexes of this organism. You'll see that many of them are present, including those of cytochrome oxidase. In fact, an alpha-proteobacterium very like R. prowazekii is thought to be the ancestor of the eukaryotic mitochondrion (Andersson et al., Nature, 1998).
How do anaerobic organisms live without oxygen?
In the absence of oxygen as a terminal electron acceptor, organisms have 2 choices. First, they can use glycolysis to ferment glucose to an oxidised compound such as ethanol or lactate. This generates a little ATP which may be enough for survival. However, many microorganisms are capable of using molecules other than oxygen as a terminal electron acceptor.
If you go back to the KEGG webpage and select the organism Rhodobacter sphaeroides, you'll see that it can synthesise an aerobic respiratory chain with oxygen as terminal electron receptor, like the one we saw earlier. However, if you grow R. sphaeroides without oxygen and give it some nitrate, it will grow quite happily. It and many other bacteria are capable of denitrification - the use of nitrate as a terminal electron receptor instead of oxygen. This type of metabolism is explained in detail at the University of Wisconsin Bacteriology website. Scroll down the page and you'll find a diagram of the respiratory chain used for denitrification. You'll see that it's similar to that used for aerobic growth, except that the enzymes are used to reduce nitrate to nitrite, then nitric oxide, then nitrous oxide and ultimately nitrogen gas.
That also answers the question...
...why are some anaerobes facultative and others obligate?
The answer is - because facultative anaerobes can synthesise an aerobic respiratory chain or, in the absence of oxygen, rely on either (1) fermentation or (2) an alternative respiratory chain that uses a different terminal electron acceptor, such as nitrate.
Why is oxygen toxic for obligate anaerobes?
Oxygen is quite a reactive molecule which likes to be reduced (i.e. oxidise something else). It may be toxic to obligate anaerobes for several reasons. A common reason is that oxygen is easily partially reduced to highly reactive species such as hydrogen peroxide or superoxide radicals. These can react with proteins and cell membrane lipids and kill the cell. Aerobic organisms possess enzymes to remove reactive oxygen species, such as catalases, peroxidases and superoxide dismutases, but these are often absent or present in low concentration in obligate anaerobes. See Rocha and Smith, 1999, for a good discussion of this in the organism Bacteroides fragilis.
Another reason for oxygen toxicity in anaerobes is that many of their enzymes are oxygen-sensitive. This is especially so for the metalloenzymes that make up the alternative respiratory chains. These proteins often have metals such as iron, molybdenum or tungsten at the active site which are reactive towards oxygen and make the protein unstable if oxygen is present.
Well, that was a long answer to a short question! I hope it has helped you to think about metabolism, anaerobes versus aerobes and the fundamental unity of chemiosmotic respiration. Bacteria can use all sorts of weird and wonderful molecules as electron acceptors, but it's all for the same end - ATP generation via a proton motive force.Neil
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