Krebs cycle electron transport

The egg and the embryonic cell are well endowed with bioenergetic pathways. The multiple-enzyme systems involved in glycolysis, the hexose monophosphate shunt, the Krebs cycle, the electron transport chain, and oxidative phosphorylation have all been found in the vertebrate embryo. In the embryonic and in the mature cell, oxidation through the Krebs cycle, electron transport, and coupling of oxidation and phosphorylation occur in mitochondria. The chemical energy provided by these pathways is needed for normal development because if either glycolysis, Krebs cycle, or electron transport chain inhibitors are administered in vivo or added to explanted chick or sea urchin embryos, embryonic development is arrested. 

These are the energy producers within the cell. They generate energy in the form of Adenosine Tri-Phosphate (ATP). Generally, the more energy a cell needs, the more mitochondria it contains. Site for Kreb s Citric Acid Cycle Electron transport system and Oxidative Phosphorylation Fatty acid oxidation Amino acid catabolism Interconversion of carbon skeletons. 

Aerobic respiration occins in prokaryotes in the cytoplasm of the cell but in the eukaryotes it takes place in the cytoplasm and in cell organelles known as mitochondria. These oval- or rod-shaped structures consist of a matrix siuroimded by two membranes between which there is a compartment, as shown in Figine 3.8. The inner membrane is convoluted to increase the surface area and the folds so formed are called cristae which project into the fluid-filled matrix. Respiration takes place in two stages, the first of which occurs in the cytoplasm of the cell and is called glycolysis. The second stage occurs in the mitochondria and involves the citric acid cycle (also known as the Krebs Cycle after its discoverer Hans Krebs) and electron transport. This is discussed below (Figure 3.10). 

Insects poisoned with rotenone exhibit a steady decline ia oxygen consumption and the iasecticide has been shown to have a specific action ia interfering with the electron transport iavolved ia the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD) by cytochrome b. Poisoning, therefore, inhibits the mitochondrial oxidation of Krebs-cycle iatermediates which is catalysed by NAD. 

The citric acid cycle, also called the Krebs cycle or the tricarboxylic add (TCA) cyde, is in the mitochondria. Although oxygen is not directly required in the cyde, the pathway will not occur anaerobically because NADH and FADH will accumulate if oxygen is not available for the electron transport chain. 

The metabolic machinery responsible for the heterotrophic respiration reactions is contained in specialized organelles called mitochondria. These reactions occur in three stages (1) glycolysis, (2) the Krebs or tricarboxylic acid cycle, and (3) the process of oxidative phosphorylation also known as the electron transport chain. As illustrated in... 

Under aerobic conditions, the pyravate is oxidized to CO and H O via the tricarboxylic acid or Krebs cycle and the electron transport system. The net yield for glycolysis followed by complete oxidation is 38 moles ATP per mole glucose, although there is evidence that the yield for bacteria is 16 moles ATP per mole glucose (Aiba et al., 1973). Thus, 673 kcal are liberated per mole glucose, much of which is stored as ATP. 

Fat fuels can only generate ATP via the Krebs cycle and electron transport, so that generation of ATP requires molecular oxygen. Consequently, ATP cannot be generated under anoxic or hypoxic conditions from fat fuels. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

The aerobic pathway involves three series of reactions glycolysis, the Krebs cycle, and the electron transport chain. 

Most anaerobically functioning mitochondria use endogenously produced fumarate as a terminal electron-acceptor (see before) and thus contain a FRD as the final respiratory chain complex (Behm 1991). The reduction of fumarate is the reversal of succinate oxidation, a Krebs cycle reaction catalysed by succinate dehydrogenase (SDH), also known as complex II of the electron-transport chain (Fig. 5.3). The interconversion of succinate and fumarate is readily reversible by FRD and SDH complexes in vitro. However, under standard conditions in the cell, oxidation and reduction reactions preferentially occur when electrons are transferred to an acceptor with a higher standard redox potential therefore, electrons derived from the oxidation of succinate to fumarate (E° = + 30 mV) are transferred by SDH to ubiquinone,... 

The tricarboxylic acid (TCA) cycle (also known as the citric acid cycle and the Krebs cycle) is a collection of biochemical reactions that oxidize certain organic molecules, generating CO2 and reducing the cofactors NAD and FAD to NADH and FADH2 [147], In turn, NADH and FADH2 donate electrons in the electron transport chain, an important component of oxidative ATP synthesis. The TCA cycle also serves to feed precursors to a number of important biosynthetic pathways, making it a critical hub in metabolism [147] for aerobic organisms. Its ubiquity and importance make it a useful example for the development of a kinetic network model. 

Much of the metabolic activity of cells consists largely of central metabolic pathways that transform large amounts of proteins, fats and carbohydrates. Foremost among these pathways are glycolysis, which can occur in either aerobic or anaerobic conditions, and the Krebs cycle, which is coupled to the electron transport chain, which accepts electrons removed from reduced coenzymes of glycolysis and the Krebs cycle. The final electron acceptor of the chain is usually oxygen, but some bacteria use specific, oxidized ions as the final acceptor in anaerobic conditions. 

Actively respiring fungal cells possess a distinct mitochondrion, which has been described as the power-house of the cell (Fig. 4.2). The enzymes of the tricarboxylic acid cycle (Kreb s cycle) are located in the matrix of the mitochondrion, while electron transport and oxidative phosphorylation occur in the mitochondrial inner membrane. The outer membrane contains enzymes involved in lipid biosynthesis. The mitochondrion is a semiindependent organelle as it possesses its own DNA and is capable of producing its own proteins on its own ribosomes, which are referred to as mitoribosomes. 

Krebs cycle Citric acid cycle, TCA cycle, the mitochondrial process by which acetyl groups from acetyl-CoA are oxidized to CO. The reducing equivalents are captured as NADH and FADH, which feed into the electron transport system of the mitochondrion to produce ATP via oxidative phosphorylation. 

---