O

attached to this protein, which was therefore called acyl carrier protein (ACP). It is a small molecular mass protein (about 8.8 kDa), has one SH group per mole of protein and is very stable to heat and acid pH. Vagelos and his colleagues worked out the structure of the prosthetic group to which the acyl moieties were attached in the following way. They made acyl-ACP derivatives in which the acyl group was labelled with radioactive carbon atoms (such as 2-[14C]-malonyl-ACP) and then hydrolysed the acyl protein with proteolytic enzymes. This yielded small radioactive peptides with structures that were fairly easy to determine. The prosthetic group turned out to be remarkably similar to coenzyme A:

the acyl groups were bound covalently to the thiol group of 4'-phosphopantetheine, which in turn was bound through its phosphate group to a serine hydroxyl of the protein (Fig. 2.3). To study the biosynthesis of ACP, Vagelos made use of a strain of E. coli that had to be supplied with pantothenate in order to grow. When the bacteria were fed with radioactive pantothenate, the labelled substance was incorporated into the cell's ACP and CoA. However, whereas the ACP level remained constant under all conditions the level of CoA was very much dependent on the concentration of pan-tothenate. When the cells were grown in a medium of high pantothenate concentration, the level of

Table 2.7 Reactions of fatty acid synthesis in E. coli

(1) Malonyl transacylase

(2) Acetyl transacylase

(3) 3-ketoacyl-ACP synthase

(4) 3-ketoacyl-ACP reductase

(5) 3-hydroxyacyl-ACP dehydrase

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