O

Figure 3.2 The removal of a proton from the CHS of an acetate derivative is a difficult process, that is, for this reaction the acetate is a very weak acid. Conversion to a malonate derivative increases the acidity almost 1012 times, use of a thiomalonate increases it still more

3.1.1 Biosynthesis

The degradation and synthesis of fatty acids are very similar, so both synthesis and degradation are summarized together (Figure 3.3), and then the steps of the biosynthesis are examined in detail one by one. The enzymes of fatty acid biosynthesis in animals are held together in a complex known as fatty acid synthetase, and during all the stages of synthesis of the fatty acid, the growing chain remains attached to this complex, passing from one enzyme to the next. When the chain has grown to 16 or 18 carbon atoms, it becomes detached from the complex. It requires 64 individual steps from acetic acid to make a molecule of stearic acid.

To make a malonyl group from an acetate (Figure 3.2), nature uses N-carboxybiotin as a carrier of C02. From the biotin (another coenzyme and vitamin) the carboxyl group is transferred to acetyl coenzyme A, converting it to the much more reactive malonyl coenzyme A (Figure 3.4). In micro-organisms and plants the growing chain and a molecule of malonate are both attached to acyl carrier protein (ACP). In animals, including insects, the sequence is a little more complicated. An acetyl group is transferred from coenzyme A (Figure 3.5 Part A) to ACP and

Synthesis (anabolism)

Chapter 3 Degradation (cataboiism)

0 0

Post a comment