AEROBIC RESPIRATION:

AEROBIC RESPIRATION:
When O2 is present and the electron transport chain is operating, pyruvate is next oxidized to acetyl-CoA, the substrate for the TCA cycle.
This reaction yields 2 NADHs because 2 pyruvates arise from a glucose; therefore 6 more ATPs are formed.
Oxidation of each acetyl-CoA in the TCA cycle will yield 1 GTP (or ATP), 3 NADHs, and a single FADH2 for a total of 2 GTPs (ATPs), 6 NADHs, and 2 FADH2s from two acetyl-CoA molecules. As table 9.2 shows, this amounts to 24 ATPs when NADH and FADH2 from the cycle are oxidized in the electron transport chain.
Thus the aerobic oxidation of glucose to 6 CO2 molecules supplies a maximum of 38 ATPs.



The calculations just summarized and presented in table 9.2 are theoretical and based on P/O ratios (the number of ATPs formed per oxygen atom reduced by 2 electrons in electron transport) of 3.0 for NADH oxidation and 2.0 for FADH2.
In fact, the P/O ratios are more likely about 2.5 for NADH and 1.5 for FADH2.
Thus the total ATP aerobic yield from glucose may be closer to 30 ATPs rather than 38.
 Bacterial electron transport systems often have lower P/O ratios than the eucaryotic system being discussed, bacterial aerobic ATP yields can be less.
 E. coli with its truncated electron transport chains has a P/O ratio around 1.3 when using the cytochrome bo path at high oxygen levels and only a ratio of about 0.67 when employing the cytochromes bd branch at low oxygen concentrations.

 aerobic respiration is much more effective than anaerobic processes not involving electron transport and oxidative phosphorylation.
 Many microorganisms, when moved from anaerobic to aerobic conditions, will drastically reduce their rate of sugar catabolism and switch to aerobic respiration, a regulatory phenomenon known as the Pasteur Effect

ANAEROBIC RESPIRATION:
Many bacteria have electron transport chains that can operate with exogenous electron acceptors other than O2. As noted earlier, this energy-yielding process is called anaerobic respiration.
The major electron acceptors are nitrate, sulfate, and CO2, but metals and a few organic molecules can also be reduced.

• Some bacteria can use nitrate as the electron acceptor at the end of their electron transport chain and still produce ATP. Often this process is called dissimilatory nitrate reduction.
• Nitrate may be reduced to nitrite by nitrate reductase, which replaces cytochromes oxidase.

• However, reduction of nitrate to nitrite is not a particularly effective way of making ATP, because a large amount of nitrate is required for growth (a nitrate molecule will accept only two electrons).
• The nitrite formed is also quite toxic. Therefore nitrate often is further reduced all the way to nitrogen gas, a process known as denitrification.
• Each nitrate will then accept five electrons, and the product will be nontoxic.


• There is considerable evidence that denitrification is a multistep process with four enzymes participating: nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase.

• Interestingly, one of the intermediates is nitric oxide (NO). In mammals this molecule acts as a neurotransmitter, helps regulate blood pressure, and is used by macrophages to destroy bacteria and tumor cells.
Two types of bacterial nitrite reductases catalyze the formation of NO in bacteria. One contains cytochromes c and d1 (e.g., Paracoccus and Pseudomonas aeruginosa), and the other is a copper protein (e.g., Alcaligenes).
 A wellstudied example of denitrification is gram-negative soil bacterium Paracoccus denitrificans, which reduces nitrate to N2 Anaerobically.
 Denitrification is carried out by some members of the genera Pseudomonas, Paracoccus, and Bacillus. They use this route as an alternative to normal aerobic respiration and may be considered facultative anaerobes. If O2 is present, these bacteria use aerobic respiration (the synthesis of nitrate reductase is repressed by O2).
 Denitrification in anaerobic soil results in the loss of soil nitrogen and adversely affects soil fertility.
 Two other major groups of bacteria employing anaerobic respiration are obligate anaerobes.
Those using CO2 or carbonate as a terminal electron acceptor are called methanogens because they reduce CO2 to methane .
Sulfate also can act as the final acceptor in bacteria such as Desulfovibrio. It is reduced to sulfide (S2- or H2S), and eight electrons are accepted.

 Anaerobic respiration is not as efficient in ATP synthesis as aerobic respiration—that is, not as much ATP is produced by oxidative phosphorylation with nitrate, sulfate, or CO2 as the terminal acceptors.
 Reduction in ATP yield arises from the fact that these alternate electron acceptors have less positive reduction potentials than O2 (see table 8.1).
 The reduction potential difference between a donor like NADH and nitrate is smaller than the difference between NADH and O2. Because energy yield is directly related to the magnitude of the reduction potential difference, less energy is available to make ATP in anaerobic respiration. Nevertheless, anaerobic respiration is useful because it is more efficient than fermentation and allows ATP synthesis by electron transport and oxidative phosphorylation in the absence of O2.
Anaerobic respiration is very prevalent in oxygen-depleted soils and sediments.