Fatty acyl-AMP

Further examples of acylphosphates are found in fatty acyl-AMPs (see Section 15.4.1) and aminacyl-AMPs (see Section 13.5), activated intermediates in the metabolism of fatty acids and formation of peptides respectively. Each of these is attacked on the C=0 by an appropriate S or O nucleophile, displacing the phosphate derivative AMP. 

This means that the energy demands (ATP AMP) are equivalent to two ATP ADP transformations. However, the product fatty acyl-AMP is actually a reactive mixed anhydride and may be attacked... 

Fatty acids must be activated to acyl CoA derivatives before they can participate in 3-oxidation and other metabolic pathways (Fig. 23.2). The process of activation involves an acyl CoA synthetase (also called a thiokinase) that uses ATP energy to form the fatty acyl CoA thioester bond. In this reaction, the p bond of ATP is cleaved to form a fatty acyl AMP intermediate and pyrophosphate (PPi). Subsequent cleavage of PPi helps to drive the reaction. 

Fig. 23.2. Activation of a fatty acid by a fatty acyl CoA synthetase. The fatty acid is activated by reacting with ATP to form a high-energy fatty acyl AMP and pyrophosphate. The AMP is then exchanged for CoA. Pyrophosphate is cleaved by a pyrophosphatase.

R.COOH -b CoASH -e ATP R.CO.SCoA -b AMP-b PPj fatty acyl-CoA -b carnitine —> fatty acyl-camitine-b CoASH... 

The fatty acid is initially converted into an acyl-AMP derivative by attack of the carboxylate as a nucleophile onto the P=0 system of ATP, with loss of diphosphate as a leaving group. This reaction is far from favourable, and the equilibrium is disturbed by subsequent pyrophosphatase-catalysed hydrolysis of diphosphate into two molecules of phosphate. 

Long-chain fatty acyl-CoA synthetase [EC 6.2.1.3] catalyzes the reaction of ATP with a long-chain carboxylic acid and coenzyme A to produce an acyl-CoA, AMP, and pyrophosphate. While utilizing a wide range of long-chain saturated and unsaturated fatty acids as substrates, enzymes from different tissues vary in their specificity. 

The first step in the activation of a fatty acid— either for energy-yielding oxidation or for use in the synthesis of more complex lipids—is the formation of its thiol ester (see Fig. 17-5). The direct condensation of a fatty acid with coenzyme A is endergonic, but the formation of fatty acyl-CoA is made exergonic by stepwise removal of two phosphoiyl groups from ATP. First, adenylate (AMP) is transferred from ATP to the carboxyl group of the fatty acid, forming a mixed anhydride... 

The carboxyl group of a fatty acid provides a point for chemical attack. The first step is a priming reaction in which the fatty acid is converted to a water-soluble acyl-CoA derivative in which the a hydrogens of the fatty acyl radicals are "activated" (step a, Fig. 17-1). This synthetic reaction is catalyzed by acyl-CoA synthetases (fatty acid CoA ligases). It is driven by the hydrolysis of ATP to AMP and two inorganic... 

The activating enzyme occurs in the mitochondria and belongs to a class of enzymes known as the ATP-dependent acid CoA ligases (AMP) but has also been known as acyl CoA synthetase and acid-activating enzyme. It appears to be identical to the intermediate chain length fatty acyl-CoA-synthetase. 

Answer Activation of carboxyl groups by ATP could in theory be accomplished by three types of reactions the formation of acyl-phosphate + ADP of acyl-ADP + P, or of acyl-AMP + PP,. All these reactions are readily reversible. To create an activation reaction with a highly negative AG ° (effectively irreversible), the third type of reaction can be coupled to a pyrophosphatase reaction, as in the synthesis of fatty acyl-CoA molecules. 

Figure 19.12 Regulation of liver acetyl-CoA carboxylase and cholesterol biosynthesis by phosphorylation. ACC indicates acetyl-CoA carboxylase. Bold arrow indicates activation of kinase kinase by fatty acyl-CoA. (Reproduced by permission from Hardie DG, Carling D, Sim ATR. The AMP-activated protein kinase a multi-substrate regulator of lipid metabolism. Trends Biochem Sci 14 20-23, 1989.)...

O. .. which expels adenosine monophosphate (AMP) as leaving group and yields the fatty acyl CoA. 

Long chain fatty acids are are bound to Fatty acid binding protein for transport within the cytosol. They are impermeable to the inner mitochondrial membrane. They are thus esterified in the cytosol by microsomal Fatty acyl CoA synthetase in a reaction identical to the one shown above. Again the reaction is driven by the hydrolysis of pyrophosphate. The enzyme involves an acyl AMP intermediate ... 

C. Fatty acids cross the inner mitochondrial membrane on a carnitine carrier. This process is inhibited during fatty acid synthesis by malonyl CoA. Fatty acids are very insoluble in water and are transported in the blood by serum albumin. They cross the plasma membrane and are converted to fatty acyl CoA by CoASH and ATP. In the process, ATP is converted to AMP, so fatty acid activation utilizes the equivalent of 2 ATP. In mitochondria, fatty acids are oxidized to C02 and H20. They cannot be oxidized in red blood cells, which lack mitochondria. 

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... 

Fatty acid synthesis and degradation are reciprocally regulated so that both are not simultaneously active. Acetyl CoA carboxylase, the essential control site, is phosphorylated and inactivated by AMP-dependent kinase. The phosphorylation is reversed by a protein phosphatase. Citrate, which signals an abundance of building blocks and energy, partly reverses the inhibition by phosphorylation. Carboxylase activity is stimulated by insulin and inhibited by glucagon and epinephrine. In times of plenty, fatty acyl CoAs do not enter the mitochondrial matrix, because malonyl CoA inhibits carnitine acyl transferase I. 

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