Energy yields in aerobic respiration

Energy yields in aerobic respiration

The energy yield from the aerobic breakdown of one molecule of glucose when the electrons stored in the reduced coenzyme molecules is fed into the electron-transport chain. As shown previously the electrons are transferred stepwise from co-enzyme carriers to molecular oxygen, and this transfer is coupled to the generation of ATP by the oxidative phosphorylation.

Each glucose molecule broken down, there are 12 reduced coenzymes to be oxidized: 2 FADH₂ (1 from each turn of the TCA cycle) and 10 NADH₂ (2from glycolysis; 2 from the gateway step between glycolysis and the TCA cycle, i.e.,pyruvic acid to acetyl-CoA; and 6 from two turns of the TCA cycle). Since 3 ATP are produced from NADH₂ and 2 ATP from FADH₂, there are 34 ATP generated from the reduced coenzymes via oxidative phosphorylation from the respiratory chain. But the total yield of ATP from the aerobic respiration of 1 glucose molecule is 38:34 from the oxidation of reduced coenzymes, 2 glycolysis, and from the side reaction of the TCA cycle, that is, from2 GTP. The total ATP yield per glucose molecule for aerobic respiration is summarized.

The complete oxidation of glucose via glycolysis, TCA cycle, and the respiratory chain is summarized in this overall reaction

C₆H₁₂O₆+6O₂→6CO₂+6H₂O

Catabolism of lipid

Glucose is the single most important source of energy for most cells. However, for many microorganisms, other substances, such as lipid and proteins, may be used as the alternate source of energy. There is a general rule that governs their utilization: they are converted as quickly and efficiently as possible into intermediates of the glycolytic and TCA pathways so that a minimum number of additional enzymes is required to effect complete breakdown. This rule highlights the fact that the glycolytic pathway and TCA cycle serve as a common center around which other catabolic pathways are build.

The breakdown of lipid or fats being with the cleavage of triglycerides by the addition of water to form glycerol and fatty acids by means of the enzyme called lipases. Glycerol as a component of fats can be converted into an intermediate of glycolytic pathway (dihydroxyacetone phosphate) by the following reactions;

Glycerol + ATP →ADP + glycerol 3 phosphate

Glycerol 3 phosphate +NAD⁺ → dihydroxyacetone phosphate + NADH₂

The dihydroxyacetone phosphate form would be broken down by the mechanisms. The fatty acids are oxidized by the successive removal of two carbon fragments in the form of acetyl-CoA, a process known as -oxidation. The acetyl CoA formed can the enter the TCA cycle, and the hydrogen atoms and their electrons enter to the respiratory transport chain, leading to oxidative phosphorylation. There is more energy yield per gram of fat than that of per carbohydrate. However, relatively few microbial species are effective in breaking down the lipids of either simple or complex types, partly because of the limited solubility of lipids.

Catabolism of protein

Many heterotrophic microorganisms can be degraded exogenous proteins, using the products as carbon and nitrogen energy sources. Since protein molecules are too large to pass into the cell, bacteria secret exo-enzymes called proteases that hydrolyze exogenous proteins to peptides, which are then transported into the cell cytoplasm. Bacteria produce peptidases that breakdown of peptides to the individual amino acids which are then broken downs according to the specific amino acid and the species or strain of bacteria breaking it down. This process may be shown as follows:

Proteins → peptides → amino acids

Where amino acid are broken down, the carbon skeleton of the amino acid undergoes oxidation to compounds that may enter the TCA cycle for the further oxidation. Entry into the TCA cycle can be via acetyl-CoA, -ketoglutaric acid, succinic acid, fumaric acid or oxaloacetic acid.

Respiration without oxygen in some bacteria

Some bacteria which are ordinarily aerobic can grow anaerobically if the nitrate is present. For example, Aquaspirillum intersonic, an aquatic bacterium, is dependent on the oxygen unless the potassium nitrate is added to the medium. In such cases nitrate essentially substitutes for oxygen as the final electron acceptor in the respiratory chain. This process is termed anaerobic respiration. The pathways for dissimilation of carbon and energy sources are identical with those in aerobic respiration, and electron transport occurs via a respiratory chainlike to that in aerobic cells. Oxygen is replaced as the terminal electron acceptor by nitrate. However. In some strict anaerobes, other compounds, such as carbon dioxide, or ions, such as sulfate ion, can be the terminal electron acceptors.

Heterotrophic CO₂ fixation

This phenomenon (unrelated to atmospheric CO₂ fixation) is important because it provides a mechanism for the synthesis of compounds of the TCA cycle from the products of carbohydrate metabolism. The two types of CO₂ fixation reactions occurs in Heterotrophic bacteria.

1. This first type of reaction is essentially irreversible and occurs in many bacteria.

Phosphenol pyruvate (PEP )+ CO₂ → oxaloacetate + P (inorganic phosphate)

A variation of this reaction requires a nucleoside by phosphate

PEP + ADP + CO₂↔ oxaloacetate + ATP

2. The second type requires the vitamin biotin for activity:

ATP + pyruvate + CO₂↔ oxaloacetate + ADP + Pi

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.