Introduction- Energy Metabolism in Prokaryotes:
Microorganisms can survive whether by adopting chemoorganotropgy, chemolithotrophy or by phototrophy but the final aim of all cells is to conserve some energy released during the energy yielding reactions.
Gibbs Free Energy and Cellular Work:
All the chemical reactions involved in a cell are coupled with the energy changes, energy either being utilized or released during reactions. Although some energy is lost as heat in a cell, some energy is available to do the work called free energy, called Gibb’s free energy. Excess energy in cell can cause cell damage. Rather cell handles energy and store safely in the form of ATP and release it when needed. If the potential energy of product is less than that of reactant, the Gibb’s free energy becomes negative indicating the energy is released and energy is conserved in the form of ATP, energy currency. These reactions are called exergonic reactions and catabolic reaction like oxidation of organic compounds like glucose, fat oxidation is associated with the release of energy and storage in the form of ATP i.e., energy released during redox reactions is conserved in cells by simultaneous synthesis of energy rich compounds like ATP.
Redox Reactions-Oxidation and Reduction:
Variety of reactions occurs in bacterial cells which are essential for proper living and survival of the cell. Redox reactions are one of them. One atom or molecule loses one or more electrons (oxidation) while another atom or molecule gets those electrons (reduction) in a linked chemical reaction known as an oxidation-reduction reaction, or redox reaction. Dehydrogenation is an oxidation process that results in the loss of both an electron and a proton. Hydrogenation is a reduction reaction in which both an electron and a proton are gained. The species with a higher reduction potential receives the electrons from the molecules with a lower reduction potential. Since Gibbs free energy changes according to the reduction potential gradient, the larger the gradient, the more Gibbs free energy is released. Redox reactions involve free energy change. It moves the bacterial system from higher to lower energy state and release energy.
Role of ATP and Phosphoryl Transfer:
Energy donors used in the energy metabolism are called energy source as energy is released when they are oxidized. Phosphorylated molecules in prokaryotes are the main source of chemical energy that is conserved during redox reactions. ATP hydrolysis has a free energy of -30 kJ/mol. ATP is the most significant energy-rich phosphate molecule in cells. Then the phosphate bond in ATP is continuously cleaved to drive anabolic reactions and resynthesized at expense of catabolic reactions i.e., organisms harness energy and channel it into biological work.
When ATP is broken, phosphoryl group transfer takes place and energy is released which is used to do the work like flagellar motility in bacteria. Gibb’s free energy can do chemical work, transport energy and mechanical energy. Example in bacteria, for glucose metabolism, glucose has to be transported into cell by ATP utilization which phosphorylates glucose into cell so that the subsequent oxidation can take place.
Catabolism and High-Energy Intermediates:
High-energy phosphate compounds are created during catabolism, and they are a way to activate a wide range of compounds for additional chemical transformations by transferring phosphoryl groups to them, which gives them free energy and increases the amount of free energy generated during subsequent metabolic transformations.
Oxidative Phosphorylation and Electron Transport Chain (ETC):
Prokaryotic cells harvest energy from glucose by capturing much of its energy as ATP. Oxidative phosphorylation, a major ATP synthesis process, involves electron transfer based of redox reactions via electron carriers finally reaching to the most electronegative species i.e. oxygen. In ETC, electrons are stripped in pairs and transferred to electron carriers and then electrons are passed allowing energy to be captured in the form of electrochemical gradient.
The electron carriers in bacterial electron transport chains differ. Electrons frequently enter many locations and exit through multiple terminal oxidases. Bacterial electron transport chains are often shorter and have lower phosphorus to oxygen (P/O) ratios than mitochondrial transport chains. A tiny quantity of free energy is used three times to move hydrogen ions across a membrane after the electrons have gone through a number of redox processes. This procedure helps create the gradient that chemiosmosis uses. The oxygen molecule at the conclusion of the ETS in aerobic respiration serves as the final electron acceptor, and the final carrier reduces it to water. Cytochrome oxidase, an electron carrier that varies throughout bacterial species, can be utilized to diagnose closely related bacteria.
Cytochrome c oxidase is used, for instance, by the gram-negative cholera-causing Vibrio cholerae and the gram-negative opportunist Pseudomonas aeruginosa. One possible replacement for aerobic respiration is anaerobic respiration, which employs an inorganic molecule other than oxygen as a final electron acceptor.
Bacteria and archaea use a variety of anaerobic respiration processes. As final electron acceptors, nitrate and nitrite are used by important soil bacteria known as denitrifiers to create nitrogen gas. During each journey through the ETS, an electron loses energy; but, in some instances, the energy is preserved as potential energy by the pumping of hydrogen ions across a plasma membrane. In prokaryotic cells, protons are pumped from the periplasmic space, which is the region outside the cytoplasmic membrane in both gram-positive and gram-negative bacteria.
Because protons have a higher concentration (chemical) on one side of the membrane and are positively charged (electrical), there is an unequal distribution of protons across the membrane, creating an electrochemical gradient. The proton motive force (PMF) is the electrochemical gradient created when protons accumulate on one side of the membrane relative to the other. A pH gradient is also created as a result of the protons’ involvement, with the side of the membrane with the higher proton concentration being more acidic.
ATP Synthase-Energy Converter:
ATP synthase is a cytoplasmic membrane-spanning enzyme. Through this protein, protons enter the cytoplasm from outside the cytoplasmic membrane. This proton flux is utilized on the protein’s inner side to convert ADP and Pi into ATP.
In prokaryotes, photophosphorylation—which also involves redox reactions—can also lead to ATP synthesis. Photophosphorylation drives the synthesis of ATP by harnessing the sun’s light energy. The photosynthetic cyanobacteria, the purple and green bacteria, and the Halobacteria are examples of prokaryotes that have the ability to transform light energy into chemical energy. The purple and green bacteria perform bacterial photosynthesis, also known as anoxygenic photosynthesis, while the cyanobacteria do oxygenic photosynthesis, which is the process by which plants undergo photosynthesis.
By directly transferring a high-energy phosphate group from a phosphorylated intermediate metabolic molecule in an exergonic catabolic pathway, prokaryotes can also use substrate-level phosphorylation to produce ATP from ADP.