Term Dietary Changes Lead to Adjustments in the , , Levels of Enzymes

Long-Term Dietary Changes Lead to Adjustments in the Levels of Enzymes 

Before closing we should point out that, over an extended period, dietary conditions can alter the levels of enzymes involved in fatty acid metabolism. For example, the concentrations of fatty acid synthase and acetyl-CoA carboxylase in rat liver are reduced four- to fivefold after fasting. When a rat is fed a fat-free diet, the concentration of fatty acid synthase is 14-fold higher than in a rat maintained on standard rat chow diet. Current evidence indicates that the levels of these enzymes are governed by the rate of enzyme synthesis, not degradation. It appears that synthesis of the enzyme, in turn, is controlled by the rate of transcription of DNA into mRNA. A question of current interest is how this transcription of DNA is regulated. 

In this chapter we focused on the synthesis and degradation of long-chain fatty acids and on how these processes are regulated. Most of our discussion was concerned with how these reactions take place in the mammalian liver, although occasionally we referred to other animal tissues and to E. coli. The following points are the most important. 

Fatty acids originate from three sources diet, adipocytes, and de novo synthesis. 

The degradation of fatty acids occurs by an oxidation process in the mitochondria. The breakdown of the 16-carbon saturated fatty acid, palmitate, occurs in blocks of two carbon atoms by a cyclical process. The active substrate is the acyl-CoA derivative of the fatty acid. Each cycle involves four discrete enzymatic steps. In the process of oxidation the energy is sequestered in the form of reduced coenzymes of FAD and NAD+. These reduced coenzymes lead to ATP production through the respiratory chain. The oxidation of fatty acids yields more energy per carbon than the oxidation of glucose because saturated fatty acids are in the fully reduced state. 

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