Fatty acid breakdown

Intermediates in fatty acid synthesis are linked covalently to the suifhydryl groups of special proteins, the acyl carrier proteins. In contrast, fatty acid breakdown intermediates are bound to the —SH group of coenzyme A. Fatty acid synthesis occurs in the cytosol, whereas fatty acid degradation takes place in mitochondria. 

Unsaturated fatty acids also seem to undergo oxidative breakdown during cooking. The volatile compounds found in cooked products are generally the same as in the raw product. Frequently there are, however, quantitative differences between the cooked and the raw product. Flowever, not much is known about the thermal fatty acid breakdown, but possibly it involves decomposition of already formed hydroperoxides in the raw product and/or oxidation of already formed volatile compounds. For example, l-octen-3-ol occurs in raw cut mushroom, whereas l-octen-3-one cannot be detected. On the other hand, l-octen-3-one is found in relatively large amounts in cooked mushroom [26]. 

One difference between the peroxisomal and mitochondrial pathways is in the chemistry of the first step. In peroxisomes, the flavoprotein acyl-CoA oxidase that introduces the double bond passes electrons directly to 02, producing H202 (Fig. 17-13). This strong and potentially damaging oxidant is immediately cleaved to H20 and 02 by catalase. Recall that in mitochondria, the electrons removed in the first oxidation step pass through the respiratory chain to 02 to produce H20, and this process is accompanied by ATP synthesis. In peroxisomes, the energy released in the first oxidative step of fatty acid breakdown is not conserved as ATP, but is dissipated as heat. 

Malonyl-CoA, an early intermediate of fatty acid synthesis, inhibits carnitine acyltransferase I, preventing fatty acid entry into mitochondria This blocks fatty acid breakdown while synthesis is occurring. 

Plants can synthesize sugars from acetyl-CoA, the product of fatty acid breakdown, by the combined actions of the glyoxylate cycle and gluconeogenesis. 

After the discovery that fatty acid oxidation takes place by the oxidative removal of successive two-carbon (acetyl-CoA) units (see Fig. 17-8), biochemists thought the biosynthesis of fatty acids might proceed by a simple reversal of the same enzymatic steps. However, as they were to find out, fatty acid biosynthesis and breakdown occur by different pathways, are catalyzed by different sets of enzymes, and take place in different parts of the cell. Moreover, biosynthesis requires the participation of a three-carbon intermediate, malonyl-CoA, that is not involved in fatty acid breakdown. 

A similar situation exists in the case of fatty acid synthesis, which proceeds from acetyl-CoA and reverses fatty acid breakdown. However, both carbon dioxide and ATP, a source of energy, are needed in the synthetic pathway. Furthermore, while oxidation of fatty acids requires NAD+ as one of the oxidants, and generates NADH, the biosynthetic process often requires the related NADPH. These patterns seen in biosynthesis of sugars and fatty acids are typical. Synthetic reactions resemble the catabolic sequences in reverse, but distinct differences are evident. These can usually be related to the requirement for energy and often also to control mechanisms. 

Fatty Acids Originate from Three Sources Diet, Adipocytes, and de novo Synthesis Fatty Acid Breakdown Occurs in Blocks of Two Carbon Atoms... 

In the first section we deal with reactions associated with fatty acid breakdown. The second section covers the pathway for fatty acid biosynthesis. Finally, we consider the regulatory mechanisms that determine the conditions under which each of these processes occurs. 

Fatty Acid Breakdown Occurs in Blocks of Two Carbon Atoms... 

Similarly, factors that stimulate acetyl-CoA carboxylase, the first enzyme in the pathway for fatty acid synthesis, also discourage fatty acid catabolism. This dual effect occurs because the first enzyme in the pathway leads to the formation of malonyl-CoA, which is a potent inhibitor of carnitine acyltransferase I. This inhibition prevents the transport of fatty acids into the mitochondrion, thereby, preventing fatty acid breakdown. 

In chapter 18, Metabolism of Fatty Acids, we discuss the synthesis and breakdown of fatty acids. The chapter starts with a discussion of fatty acid breakdown. A second section covers the pathway for fatty acid biosynthesis. Finally, we consider the regulatory mechanisms that determine the conditions under which each of these processes occurs. As in the case of glucose metabolism, it is convenient to discuss the synthesis and breakdown in the same chapter so that the closely related topic of regulation can be considered alongside. 

Related topics Membrane lipids (El) Triacylglycerols (K4) Fatty acid breakdown (K2) Cholesterol (K5) Fatty acid synthesis (K3)... 

Fatty acid breakdown (also called (3-oxidation) brings about the oxidation of long-chain fatty acids with the production of energy in the form of ATP. The fatty acids are converted into their acyl CoA derivatives and then metabolized by the removal of two-carbon acetyl CoA units from the end of the acyl chain. 

Fatty acid breakdown occurs in the cytosol of prokaryotes and in the mitochondrial matrix of eukaryotes. The fatty acid is activated by forming a thioester link with CoA before entering the mitochondria. 

Fatty acid breakdown involves a repeating sequence of four reactions ... 

Overview Fatty acid breakdown brings about the oxidation of long-chain fatty acids. The... 

Activation Fatty acid breakdown occurs in the cytosol of prokaryotes and in the mito-... 

The major source of free fatty acids in the blood is from the breakdown of triacylglycerol stores in adipose tissue which is regulated by the action of hormone-sensitive triacylglycerol lipase (see Topic K4). Fatty acid breakdown and fatty acid synthesis are coordinately controlled so as to prevent a futile cycle (see Topic K3). 

The cycle oxidizes pyruvate (formed during the glycolytic breakdown of glucose) to C02 and H20, with the concomitant production of energy. Acetyl CoA from fatty acid breakdown and amino acid degradation products are also oxidized. In addition, the cycle has a role in producing precursors for biosynthetic pathways. 

Fatty acid breakdown (K2) Amino acid metabolism (M2)... 

Answer Malonyl-CoA would no longer inhibit fatty acid entry into the mitochondrion and jS oxidation, so there might be a futile cycle of simultaneous fatty acid synthesis in the cytosol and fatty acid breakdown in mitochondria. (See Fig. 17-12.)... 

Intermediates in fatty acid synthesis are covalently linked to the sulphhydryl group of an acyl carrier protein (ACP) whereas intermediates in fatty acid breakdown are bonded to coenzyme A. 

Fatty acid oxidation requires adequate amounts of NAD and FAD. Oxidation of NADH and FADH2 occurs in mitochondria via electron transport and the use of molecular oxygen. However, the electron transport chain can only function if ADP is present, so that fatty acid breakdown occurs at the rate necessary to maintain cellular levels of ATP which decrease if metabolic or other work is performed. Tight coupling of electron transport to oxidative phosphorylation (Chapter 14) is of considerable importance in fuel economy. Without it, fatty acids would be utilized in an unproductive manner. In the liver, fatty acids in excess of requirements are converted to triacylglycerols and secreted as VLDLs. 

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