Oxidation acetyl-CoA

In stage three, the citric add (Krebs, or tricarboxylic acid [TCA]) cycle oxidizes acetyl CoA to COj- The energy released in this process is primarily conserved by reducing NAD to NADH or FAD to FADHj. 

The citric acid cycle, also known as the TCA (tricarboxylic acid) cycle or Krebs cycle (after its discoverer in 1937), is used to oxidize the pyruvate formed during the glycolytic breakdown of glucose into C02 and H20. It also oxidizes acetyl CoA arising from fatty acid degradation (Topic K2), and amino acid degradation products (Topic M2). In addition, the cycle provides precursors for many biosynthetic pathways. 

Protein synthesis Glycogen synthesis Lactate uptake Amino acid uptake Glutaminase Glycine oxidation Ketoisocaproate oxidation Acetyl-CoA carboxylase Urea synthesis from amino acids Glutathione (GSH) efflux Taurocholate excretion into bile Actin polymerization Microtubule stability Lysosomal pH... 

Cofactors for the Citric Acid Cycle Suppose you have prepared a mitochondrial extract that contains all of the soluble enzymes of the matrix but has lost (by dialysis) all the low molecular weight cofactors. What must you add to the extract so that the preparation will oxidize acetyl-CoA to C02 ... 

The cycle oxidizes acetyl-CoA, and to perform this task, it must convert acetyl-CoA to citrate. For this to be achieved, oxaloacetate must be available. If the removal of intermediates results in a decrease in the amount of oxaloacetate for this purpose, acetyl-CoA cannot be removed and will accumulate. This will inhibit the pyruvate dehydrogenase complex and activate pyruvate carboxylase, leading to the conversion of pyruvate to oxaloacetate. This product is now available to condense with the acetyl-CoA to produce citrate, which will restore the status quo. Reactions like that of pyruvate carboxylase that provide molecules for the replacement of intermediates of the citric acid cycle are known as anaplerotic reactions (Greek, meaning to fill up ana = up + plerotikos from pleroun = to make full ). 

Some plants and bacteria that can use acetate as their sole source of carbon are able to oxidize acetyl-CoA via the citric acid cycle, or the acetate can be converted to carbohydrates via a pathway that is a modification of the citric acid cycle. This pathway is known as the glyoxylate cycle (Fig. 12-10)... 

D. Ketone bodies are synthesized in the liver from fatty acids derived from the blood. During the cytosolic activation of the fatty acid, ATP is converted to AMP. Carnitine is required to carry the fatty acyl group across the mitochondrial membrane. In the mitochondrion, the fatty acid is oxidized. Acetyl CoA and acetoacetyl CoA are produced and react to... 

The tricarboxylic acid (TCA) cycle (also called the citric acid cycle or Krebs cycle) consists of eight enzymes that oxidize acetyl-CoA with the formation of carbon dioxide, reducing equivalents in the form of NADH and FADH2, and guanosine triphiosphate (GTP) ... 

Fatty acid oxidation Acetyl-CoA production Ketone body synthesis Fatty acid elongation... 

See also / -Oxidation, Acetyl-CoA, Citric Acid Cycle Reactions, Vitamin B12... 

PDC stimulates its phosphorylation to the inactive form. The substrates of the enzyme, CoASH and NAD, antagonize this product inhibition. Thus, when an ample supply of acetyl CoA for the TCA cycle is already available from fatty acid oxidation, acetyl CoA and NADH build up and dramatically decrease their own further synthesis by PDC. 

Removal of any of the intermediates from the TCA cycle removes the 4 carbons that are used to regenerate oxaloacetate during each turn of the cycle. With depletion of oxaloacetate, it is impossible to continue oxidizing acetyl CoA. To enable the TCA... 

Fatty acids that are oxidized are converted to acetyl CoA and snbseqnently to ketone bodies (acetoacetate and (3-hydroxybntyrate). Enough NADH is generated from oxidation of ethanol and fatty acids that there is no need to oxidize acetyl CoA in the TCA cycle. The very high NADH/NAD ratio shifts all of the oxaloacetate in the TCA cycle to malate, leaving the oxaloacetate levels too low for citrate synthase to synhesize citrate (see Fig. 25.6, circle 4). The acetyl CoA enters the pathway for ketone body synthesis instead of the TCA cycle. 

The citric acid cycle is a metabolic pathway that oxidizes acetyl CoA, producing. molecules... 

The citric acid cycle (also known as the tricarboxylic acid cycle, TCA cycle, Krebs cycle) oxidizes acetyl CoA in mitochondria. The cycle produces CO2, NADH and FADH2. The NADH and FADH2 enter oxidative phosphorylation, where they are oxidized to NAD+ and FAD, ready to be used in the citric acid cycle again. The citric acid cycle is also important in some biosynthetic processes such as lipid synthesis, amino acid synthesis, porphyrin synthesis and gluconeogenesis. 

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