The Structure of Bacterial Cell Walls

T Although D-amino acids do not generally occur in proteins, they do serve some special functions in the structure of bacterial cell walls and peptide antibiotics. Bacterial peptidoglycans (see Fig. 20-23) contain both D-alanine and D-glutamate. D-Amino acids arise directly from the l isomers by the action of amino acid racemases, which have pyridoxal phosphate as cofactor (see Fig. 18-6). Amino acid racemization is uniquely important to bacterial metabolism, and enzymes such as... 

Tests allowing precise determination of the conditions to protect the soluble protein, and of the temperature at which the reaction was slow enough for X-ray data collection, were sought. To ascertain the best conditions for the determination of the structure of a productive lysozyme-substrate complex, the hydrolysis of bacterial cell walls and oligosaccharides was investigated both in high-salt solutions and in mixed solvents. 

The peptidoglycan structure of bacterial cell walls. The shaded areas represent points of attachment of this macromolecule to the rest of the cell wall. The amino sugar units are joined end to end to form long, straight chains. The peptides form cross-links when the amino group of a meso-diaminopimelic acid in one chain replaces the terminal alanine in another chain. Source ... 

The complex structure of bacterial cell walls is discussed in Chapter 8. However, it is appropriate to mention a few bacterial polysaccharides here. The innermost layer of bacterial cell walls is a porous network of a highly crosslinked material known as pepti-doglycan or murein (see Fig. 8-29). The backbone of the peptidoglycan is a P-l,4-linked... 

TE Benson, CT Walsh, JM Hogle. The structure of the substrate-free form of MurA, an essential enzyme for the synthesis of bacterial cell wall. Structure 4 47-54, 1996. 

Many chemicals can bind to enzymes and either eliminate or drastically reduce their catalytic ability. These chemicals, called enzyme inhibitors, have been used for hundreds of years. When she poisoned her victims with arsenic, Lucretia Borgia was unaware that it was binding to the thiol groups of cysteine amino acids in the proteins of her victims and thus interfering with the formation of disulfide bonds needed to stabilize the tertiary structure of enzymes. However, she was well aware of the deadly toxicity of heavy metal salts like arsenic and mercury. When you take penicillin for a bacterial infection, you are taking another enzyme inhibitor. Penicillin inhibits several enzymes that are involved in the synthesis of bacterial cell walls. 

Figure 3.22 The penicillin molecule interferes with the action of bacteria (and therefore of infections) by mimicking the structure of D-ala-D-ala involved in the production of bacterial cell walls in cell division. This is a rationalisation after the event, namely the discovery of the action of penicillin by chance by Alexander Fleming. By contrast modern rational drug design strategies involve using the knowledge of the structure of receptor sites. From Helliwell (1977b) reproduced with permission of New Scientist.

J Lactam antibiotics. A general name for antibio-tics with a four-membered /3-lactam ring in their skeleton, their bactericidal activities are based on the inhibition of bacterial cell wall synthesis. The natural jS-L. a. can be classified according to structures into 5 classes (see figure). 

Nowadays, antibiotics are primarily classified according to the mechanism of their action, with similarity of chemical structure as a secondary factor. Penicillin and its derivatives inhibit the formation of bacterial cell walls (Fig. 3.38). Cephalosporins have the same active mechanism. Other compounds are taken up into bacterial DNA to form unstable molecules (quinolones, metronidazole) or inhibit peptide synthesis (tetracychnes, aminoglycosides, macrolides). Some antibiotics (e.g. glycopeptides) exert a complex effect. 

Peptidoglycans.—The biosynthesis of the peptidoglycans of bacterial cell walls and the action of penicillin on the biosynthesis of peptidoglycans have been re-viewed. Other reviews have discussed the effects of endogenous and exogenous factors on the primary structures of bacterial peptidoglycans and the com-... 

In general, penicillins exert thek biological effect, as do the other -lactams, by inhibiting the synthesis of essential structural components of the bacterial cell wall. These components are absent in mammalian cells so that inhibition of the synthesis of the bacterial cell wall stmcture occurs with Htde or no effect on mammalian cell metaboHsm. Additionally, penicillins tend to be kreversible inhibitors of bacterial cell-wall synthesis and are generally bactericidal at concentrations close to thek bacteriostatic levels. Consequently penicillins have become widely used for the treatment of bacterial infections and are regarded as one of the safest and most efficacious classes of antibiotics. 

This insertion is accomplished by an enzyme called transpeptidase. -Lactam antibiotics function as substrates for the transpeptidase, thereby establishing selective inhibition of bacterial cell wall synthesis. The structural similarity between -lactam antibiotics and the alanylalanine unit is remarkable as can be seen in Figure 6.8. 

Hen egg-white lysozyme catalyzes the hydrolysis of various oligosaccharides, especially those of bacterial cell walls. The elucidation of the X-ray structure of this enzyme by David Phillips and co-workers (Ref. 1) provided the first glimpse of the structure of an enzyme-active site. The determination of the structure of this enzyme with trisaccharide competitive inhibitors and biochemical studies led to a detailed model for lysozyme and its hexa N-acetyl glucoseamine (hexa-NAG) substrate (Fig. 6.1). These studies identified the C-O bond between the D and E residues of the substrate as the bond which is being specifically cleaved by the enzyme and located the residues Glu 37 and Asp 52 as the major catalytic residues. The initial structural studies led to various proposals of how catalysis might take place. Here we consider these proposals and show how to examine their validity by computer modeling approaches. 

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