Cycle, biochemical carbon

Martens, D. A. (2000). Plant residue biochemistry regulates soil carbon cycling and carbon sequestration. Soil Biol. Biochem. 32,361-369. 

There are >40 distinct molybdenum enzymes that occur in all classes of living systems and are especially important in the biochemical cycles of carbon, nitrogen, and sulfur (24b). The majority of the molybdenum enzymes, with notable exceptions including the nitrogenases (25-28) and a 2-hydroxyglutaryl-CoA dehydratase (10), catalyze a conversion of the type [Eq. 1], that is, the net effect of the catalysis corresponds to the transfer of an oxygen atom to or from the substrate. 

Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast. 

Kreb s cycle Biochemical cycle in cellular aerobic metabolism where acetyl CoA is combined with oxaloacetate to form citric acid the resulting citric acid is converted into a number of other chemicals, eventually reforming oxaloacetate NADH, some ATP, and FADH2 are produced and carbon dioxide is released. 

We can share the view of some authors (Fenchel et al., 1998) that this theory may serve a plausible explanation of thioester world as precursor for the present RNA world. Furthermore, it is attractive in a number of respects too. One has to mention the development of very early, global scale geochemical cycling of carbon, sulfur, and iron. It would present the early interactions between atmosphere and biochemical processes, like thioester-bounded hydrogen production contribution to the formation of reducing species that were critical for any atmospheric organic synthesis. 

In the study of the origin of life on earth, the element carbon is essential. Carbon is a required component of the fundamental molecules of life amino acids, bases, and sugars. In addition, a large variety of carbon compounds is necessary in the complex biochemical cycles of living organisms. The physical and chemical nature and geometry of the carbon atom make it well suited to form the vast array of molecules involved in the chemistry of life. 

In the urea cycle, two molecules of ammonia combine with a molecule of carbon dioxide to produce a molecule of urea and water. The overall cycle involves a series of biochemical reactions dependent on enzymes and carrier molecules. During the urea cycle the amino acid ornithine (C5H12N202) is produced, so the urea cycle is also called the ornithine cycle. A number of urea cycle disorders exist. These are genetic disorders that result in deficiencies in enzymes needed in one of the steps in the urea cycle. When a urea cycle deficiency occurs, ammonia cannot be eliminated from the body and death ensues. 

One of the simplest biochemical addition reactions is the hydration of carbon dioxide to form carbonic acid, which is released from the zinc-containing carbonic anhydrase (left, Fig. 13-1) as HC03-. Aconitase (center, Fig. 13-4) is shown here removing a water molecule from isocitrate, an intermediate compound in the citric acid cycle. The H20 that is removed will become bonded to an iron atom of the Fe4S4 cluster at the active site as indicated by the black H20. An enolate anion derived from acetyl-CoA adds to the carbonyl group of oxaloacetate to form citrate in the active site of citrate synthase (right, Fig. 13-9) to initiate the citric acid cycle. 

PATHWAY. A sequence of reactions, usually of a biochemical nature, in which more-complex substances are converted to simple end products, as in the degradation of the components of foods to carbon dioxide and water. Its course is determined largely by preferential factors involving cocnzymcs and other catalysts. An example is the TCA cycle, which is the common pa ill way in the degradation of foodstuffs and cell constituents to carbon dioxide and water. 

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