Energy metabolism Krebs cycle

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

This is undoubtedly why low-carbohydrate diets have proved so effective for rapid weight loss. As soon as the reserves of carbohydrate are used up, the body resorts to metabolizing fat for its energy needs. This continues whenever carbohydrate intake is limited. It should also be appreciated that although carbohydrate can readily be converted into fat (via acetyl-CoA see Section 15.5), fat is not readily converted into carbohydrate in animals. Fat metabolism produces acetyl-CoA, which is then usually metabolized completely via the Krebs cycle. 

Acyl coenzyme As are introduced into mitochondria following coenzyme A esterification in the cytoplasm. Mitochondrial entry depends upon a double membrane transport involving carnitine acyltransferases II and I. Excess acetyl CoA is used for KB synthesis. KBs are transported in the blood and ultimately metabolized via the Krebs cycle. KBs are necessary to provide energy to the brain during fasting, a true alternative substrate to glucose. 

The citric acid cycle (Krebs cycle, TCA cycle) is a nearly universal central catabolic pathway in which compounds derived from the breakdown of carbohydrates, fats, and proteins are oxidized to C02, with most of the energy of oxidation temporarily held in the electron carriers FADH2 and NADH. During aerobic metabolism, these electrons are transferred to 02 and the energy of electron flow is trapped as ATP. 

Emeretli (1990) demonstrated a convincing link between the activity of succinic dehydrogenase in the red muscle mitochondria of Black Sea species and their motor activity. This enzyme is one of the most important in the Krebs cycle, which controls the intensity of aerobic energy metabolism. 

Krebs cycle The second metabolic pathway in aerobic cell respiration converts carbohydrates and lipids (sugars and fats) into carbon dioxide and water and produces energy-rich compounds, including some ATP. 

Orally administered L-carnitine and propionyl-L-carnitine may have metabolic benefits by providing an additional source of carnitine to buffer the cellular acyl CoA pool. In this way, carnitine may enhance glucose oxidation under ischemic conditions and improve energy metabolism in the ischemic skeletal muscle. Propionyl-CoA generated from propionyl-L-carnitine may also improve oxidative metabolism through its anaphoretic actions in priming the Kreb s cycle, secondary to succinyl-CoA production. 

THE MAIN POWERHOUSE is the key energy source of Biochemistryland and, in fact, is key-shaped. Its Main Hallway (fig. 1.2) leads to a ferris wheel (Krebs cycle) run by a powerful Generator (oxidative phosphorylation). There is an Energy Hall of Fame (HOF). The Main Powerhouse is situated near a Saloon (alcohol metabolism). 

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