C

inactivation. Other kinases may also play a role in phosphorylating the carboxylase and, hence, inactivating it (see Fig. 2.25). Because protein kinases are affected by various signalling pathways, then indirectly the latter will influence the activity of ACC.

Synthesis and degradation

Long-term regulation of ACC in animals can be due to changes in enzyme amounts. The tissue concentration of the carboxylase protein has been shown to vary with the rate of fatty acid synthesis under a variety of nutritional, hormonal, developmental or genetic conditions. Measurements with specific antibodies have shown that, for example, fasted rats have only one-quarter the normal levels of the liver enzyme, while genetically obese mice contain four times as much compared to control animals. The amount of an enzyme protein accumulating is due to the net rates of synthesis and degradation. Depending on the specific trigger for changes in acetyl-CoA carboxylase levels, one or both of these factors may be altered. For example, the increase in enzyme content in fasted/re-fed rats is due only to changes in the rates of its synthesis (Table 2.10). By contrast, the decrease in enzyme content in fasted animals is due both to diminished synthesis and to accelerated breakdown.

Table 2.10 Relative acetyl-CoA carboxylase (ACC) levels and rate of enzyme synthesis in different conditions

Content Synthesis of ACC protein

Content Synthesis of ACC protein

Normal

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