Ester-linked acyl groups

Carboxylic acid derivatives are compounds that possess an acyl group (R—C=0) linked to an electronegative atom, e.g. —Cl, —CO2 R, —OR or —NH2. They can be converted to carboxylic acids via simple acidic or basic hydrolysis. The important acid derivatives are acid chlorides, acid anhydrides, esters and amides. Usually nitriles are also considered as carboxylic acid derivatives. Although nitriles are not directly carboxylic acid derivatives, they are conveniently hydrolysed to carboxylic acids by acid or base catalysts. Moreover, nitriles can be easily prepared through dehydration of amides, which are carboxylic acid derivatives. 

A similar distribution of fatty acids has also been detected in lipid A of other bacteria (Fig. 5). Thus, in Fusobacterium nucleatum, 2 moles of (R)-3-OH-14 0 are ester-bound, one of which is 3-O-acylated by 14 0. In amide linkage, (R)-3-0(14 0)-16 0 is present. In Vibrio cholerae, a dimer of (R)-3-OH-12 0 is bound as an ester while (R)-3-0-(14 0)-14 0 and (R)-3-0-(16 0)-14 0 are amide-linked. The lipid A component of Chromobacterium violaceum possesses 2 moles of (R)-3-OH-10 0 in ester linkage. The amide-bound acyl groups are represented by (R)-3-0H-12 0 residues which are 3-0-acylated by 12 0 and (S)-2-OH-12 0. In P. mirabilis, 3-0H-14 0 is, like in Salmonella, ester-and amide-bound. In this case, however, exclusively 14 0 substitutes the 3-hydroxyl groups of both 0- and N-linked 3-OH-14 0. 

The chemistry and biological activities of lipid A are reviewed in earlier chapters of this volume. As noted there, the components of the lipid A complexes from Salmonella species and j5. coli are derivatives of a disaccharide comprised of two D-glucosamine units linked B,l->-6. The disaccharide is substituted at positions 1 and 4 by phosphate functions and on the amino nitrogens by B-hydroxymyristoyl groups. A variable number (up to 5) of ester-linked fatty acyl residues is also present. In the simplest components the phosphates are present as monoester groups. 

Water is required eventually to hydrolyse the acyl group attached to the serine residue. However, this is a much easier step than the hydrolysis of a peptide link since esters are more reactive than amides. Furthermore, the hydrolysis of the peptide link means that one half of the peptide can drift away from the active site and leave room for a water molecule to enter. 

---