Bacterial cell walls, amino acids

Although all the chiial amino acids obtained from proteins have the l configuration at then a carbon, that should not be taken to mean that D-amino acids are unknown. In fact, quite a number of D-anino acids occur naturally. D-Alanine, for example, is a constituent of bacterial cell walls and D-seiine occurs in brain tissue. The point is that D-fflnino acids are not constituents of proteins. 

The first known 1-carboxyethyl ether of a sugar was 2-amino-3-0-[(/ )-l-carboxyethyl]-2-deoxy-D-glucose or muramic acid (37). It is a component of the polysaccharide moiety of the peptidoglycan in the bacterial cell-wall. It is partially replaced by the mamo isomer, 2-amino-3-6>-[(/ )-l-carboxy-ethyl]-2-deoxy-D-mannose, in the peptidoglycan from Micrococcus lyso-deikticus. 

All amino acids except glycine exist in these two different isomeric forms but only the L isomers of the a-amino acids are found in proteins, although many D amino acids do occur naturally, for example in certain bacterial cell walls and polypeptide antibiotics. It is difficult to differentiate between the D and the L isomers by chemical methods and when it is necessary to resolve a racemic mixture, an isomer-specific enzyme provides a convenient way to degrade the unwanted isomer, leaving the other isomer intact. Similarly in a particular sample, one isomer may be determined in the presence of the other using an enzyme with a specificity for the isomer under investigation. The other isomer present will not act as a substrate for the enzyme and no enzymic activity will be demonstrated. The enzyme L-amino acid oxidase (EC 1.4.3.2), for example, is an enzyme that shows activity only with L amino acids and will not react with the D amino acids. 

For all other amino acids, with the exception of glycine (Gly), the a-carbon is bonded to four different groups, and the two stereoisomers are mirror images that cannot be superimposed. Eukaryotic proteins are always composed of L-amino acids although D-amino acids are found in certain peptide antibiotics and some peptides of bacterial cell walls. ° ° The physical properties of amino acids are influenced by the degree of ionization at different pH values. 

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 ... 

Griggs and coworkers studied the hydrolysis of canine submaxillary mucin (CSM) by 0.5, 3, and 6 M hydrochloric acid for 1.5, 3, 4.5, 6, and 24 h at 100°. They found that use of 3 M hydrochloric acid for 3 h gives the maximal release of neutral and amino monosaccharides, with the minimal degradation of the monosaccharides liberated, although recoveries of neutral monosaccharides were only 76-88%. Guerrant and Moss also used hydrolysis with 3 M hydrochloric acid, although for 16 h at 75°, for the hydrolysis of bacterial cell-walls. 

The pathway from simple molecules to the peptidoglycan of the bacterial cell wall is lengthy and complex. Many of the details are well known but need not concern us here. Suffice it to say that long carbohydrate chains are synthesized, subsequently decorated with shorter amino acid chains, and these are finally cross-linked to provide a strong strnctnre. It is this final cross-linking step that is inhibited by the p-lactam antibiotics. The consequence is that cell wall biosynthesis cannot be completed and cell death ensnes. Again, the mammalian host carries out no similar reactions so that similar consequences do not ensne for the host orgaiusm. 

Although free amino acids and those in proteins in eukaryotes are entirely of the L-form (except glycine, which is not optically active), D-amino acids do occur in nature, for example in bacterial cell walls (D-alanine and D-glutamate). Consequently, they enter the body from bacteria in food and from the digestion of bacteria in the... 

Cycloserine (Fig- 4) is produced by several species of Streptomyces. One of the basic glycosyl components of the bacterial cell wall, n-acetyl-muramic acid (the product of Mur A and MurB), is modified by the addition of the first three amino acids sequentially by MurC, MurD and MurE enzymes. A dipeptide, D-alanyl-D-alanine is then added to make the pentapeptide. In bacteria, L-alanine is the native form and it is converted to D-alanine form by alanine racemase (Air). Two D-alanines are joined by D-ala-D-ala ligase (DdlA) to synthesize the dipeptide. Cycloserine resembles the substrate for Air and Ddl and inhibits their respective reactions in stage I of the peptidoglycan biosynthesis (Fig. 2). 

This enzyme catalyzes the transamination of a wide spectrum of a-amino acids and a-keto (or 2-oxo) acids, demonstrating absolute specificity for their D-isomers. The most likely physiologic role is to provide D-amino acids for peptidoglycan synthesis in bacterial cell wall formation. 

An important molecular target of the B-lactam antibiotics is an enzyme that acts as a transpeptidase in the stepwise polymerization leading to a thickened, strong bacterial cell wall. Several amino acids are present in addition to the terminal -alanyl- -alanyl unit which the Strominger hypothesis suggests has the same overall shape and reactivity as... 

Mechanism of Action Anantitubercularthat inhibits cell wall synthesis by competing with the amino acid, o-alanine, for incorporation into the bacterial cell wall. Therapeutic Effect Causes disruption of bacf erial cell wall. Bactericidal or bacteriostatic. Pharmacokinetics Readily absorbed from the gastrointestinal (GI) tract. No protein binding. Widely distributed (including cerebrospinal fluid [CSF ). Metabolized in liver. Primarily excreted in urine. Removed by hemodialysis. Half-life 10 hr. 

A good example of this interaction in catalysis is the hydrolysis of the bacterial cell wall polysaccharide by lysozyme. This enzyme contains two carboxylic gronps at its active site and, in active enzyme one must be in dissociated—COO, the other in the undissociated—COOH form. Therefore, the pK s of the two carboxylic groups ate different. This difference in dissociation constant is a consequence of the neighbouring amino acid residues and of the interactions between the functional groups in the microenvironment. 

Nearly all biological compounds with a chiral center occur naturally in only one stereoisomeric form, either d or L. The amino acid residues in protein molecules are exclusively L stereoisomers. D-Amino acid residues have been found only in a few, generally small peptides, including some peptides of bacterial cell walls and certain peptide antibiotics. 

TThe primary function of D-amino acid oxidase, present at high levels in the kidney, is thought to be the detoxification of ingested D-amino acids derived from bacterial cell walls and from cooked foodstuffs (heat causes some spontaneous racemization of the l-amino acids in proteins). Oxalate, whether obtained in foods or produced enzymatically in the kidneys, has medical significance. Crystals of calcium oxalate account for up to 75% of all kidney stones. ... 

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... 

The a-carbon of each amino acid is attached to four different chemi cal groups and is, therefore, a chiral or optically active carbon atom. Glycine is the exception because its a-carbon has two hydro gen substituents and, therefore, is optically inactive. [Note Amino acids that have an asymmetric center at the a-carbon can exist in two forms, designated D and L, that are mirror images of each other (Figure 1.8). The two forms in each pair are termed stereoisomers, optical isomers, or enantiomers.] All amino acids found in proteins are of the L-configuration. However, D-amino acids are found in some antibiotics and in bacterial cell walls. (See p. 250 for a discus sion of D-amino acid metabolism.)... 

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