Enyzme Complexes Use Hydrocarbons for Energy

Biological Strategy

Pseudomonas putida

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Modify Electron Transport

Electron transport is essential for helping organisms to produce energy and survive. Several species can modify the movement of electrons in different ways, allowing them to survive in a variety of conditions.

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Modify Oxidation State

Oxidation state refers to the degree to which an element has gained or lost electrons–it can have a significant impact on the characteristics of that element. For example, hexavalent chromium with an oxidation number of +6, is a known carcinogen whereas the less oxidized (i.e., “reduced”) trivalent form, with an oxidation number of +3, is less toxic. The bacterium, Acidiphilium cryptum, can enzymatically convert chromium from the hexavalent to the trivalent form.

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Modify Free Radical Reactivity

Free radicals are molecules react very easily with other molecules, which can cause them to breakdown or cease to function in the same way. They are natural by-products of many biochemical reactions in the body, including metabolism and immune responses. Different organisms have adapted to modify or control free radical activity, in order to maintain the integrity of their biochemical processes and improve their fitness.

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Chemically Generate Flow of Electrons (Redox)

Electron flows, or electron transport, is a critical step in numerous biochemical pathways. For example, the flow of electrons along the electron transport chain between membrane-bound proteins of mitochondria during cellular respiration or the chloroplast during photosynthesis triggers a flow of protons across the same membrane. This flow provides the energy for cellular processes similar to the flow of water across a hydroelectric dam.

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Chemically Break Down Inorganic Compounds

The vast majority of biochemical assembly and breakdown processes–even by the most complex organisms–occur within cells. In fact, cells are able to perform hundreds, even thousands of chemical transformations at the same time under life-friendly conditions (ambient temperature and pressure in an aqueous environment). For example, inorganic pyrophosphate is hydrolyzed to form two phosphate groups in the cellular degradation pathway of fatty acids.

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Bacteria

Domain Bacteria (“small staff”): L. acidophilus, Staphylococcus

From inside of our own bodies to the deepest parts of the ocean, bacteria have adapted to almost every possible environment. All bacteria are unicellular, microscopic, and lack a membrane-bound nucleus and organelles, and come in three basic shapes: spherical (cocci), rod (bacilli), and spiral (spirilla). Some strains can be deadly, like Staphylococcus aureus (MRSA) while others, like Lactobacillus acidophilus, found in our intestinal microbiome, keep us healthy. Cyanobacteria, also known as blue green algae, caused the Great Oxidation Event 2.4 billion years ago, producing Earth’s first oxygenated atmosphere.

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Membrane-bound enzymes produced by the bacteria Pseudomonas putida produce energy from trapped hydrocarbon-based substrates by producing oxygen free radicals that break down the substrates.

Hydrocarbon-based substances like gasoline and parrafin wax do not spontaneously combust with air under normal circumstances. An intense, focused energy input (e.g., a spark) is required to overcome the activation energy for the overall energy-releasing reaction. In some organisms, the oxygen-mediated break down of hydrocarbons and the accompanying release of energy is not activated by intense, focused heat; instead it is activated by enzymes and oxygen free-radicals.

The bacterium Pseudomonas putida oxidizes various alkanes primarily with a membrane-bound enzyme called AlkB that forms a hydrophobic pocket attractive to hydrocarbon-based substrates. The enzyme converts oxygen to free radicals to break down the trapped substrates. This process is capable of providing net energy input for the bacteria. Potential substrates include propane, n-butane, and other alkanes with carbon lengths between 5 and 13. That includes many hydrocarbon components of crude oil.

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