L-Alanyl-D-alanine

L-alanyl-D-alanine (—), and L-leucyl-L-proline (---) with fluorescamine in 0.05 M phosphate... 

These are only selected examples of the numerous model systems proposed for the study of racemization, yet, even in such a brief treatment an approach based on enantio-selective enzymes should be mentioned. Coupling of benzyl-oxycarbonyl-L-alanyl-D-alanine to L-alanyl-L-alanine benzyl ester yields a blocked intermediate from which on catalytic hydrogenation the free L-Ala-D-Ala-L-Ala-L-Ala is obtained. This compound is completely resistant to hydrolysis catalyzed by aminopeptidases, because the first bond to be cleaved links the N-terminal residue to a D-amino acid. If however, racemization took place during coupling, this changed the activated residue, D-alanine to L-alanine and after deblocking the tetrapeptide L-Ala-L-Ala-L-Ala-L-Ala is obtained. The latter is completely digestible with aminopeptidases. The liberated alanine is determined and it is a rather exact measure of racemization because for each residue inverted four molecules of alanine are found in the analysis. 

Figure 22 Structure of the labeled poly(lsocyano-L-alanyl-D-alanine methyl ester) together with a representation of the helix formation the polymer adopts.

Fig. 8.1 Biosynthesis of peptidoglycan. The large circles represent A -acetylglucosamine orN-acetylmuramic acid to the latter is linked initially a pentapeptide chain comprising L-alanine, D-glutamic acid and meso-diaminopiraelic acid (small circles) terminating in two D-alanine residues (small, darker circles). The lipid molecule is undecaprenyl phosphate. In the initial (cytoplasm) stage where inhibition by the antibiotic D-cycloserine is shown, two molecules of Dalanine (small circles) are converted by an isomerase to the D-forms (small, darker circles), alter which a ligase joins the two D-alanines together to produce a D-alanyl-D-alanine dipeptide.

Nieto, M. and Perkins, H.R., Physicochemical properties of vancomycin and iodovancomycin and their complexes with diacetyl-L-lysyl-D-alanyl-D-alanine, Biochem. /., 123, 773, 1971. 

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). 

A series of derivatives of the ester 37, having amino acid or peptide residues linked through the carboxyl group of the lactic acid residue, has been described. Their exact structures depend on the growth conditions and the bacterial strain used. The chain attached may contain L-alanine 7,151,154,155 the dipeptide L-alanyl-D-glutamic acid 156,157 the tripeptides L-alanyl-y-D-glutamyl-L-lysine,157-160 L-al-... 

RIS theory, in the form appropriate for branched molecules, is used to calculate the mean-square unperturbed radius of gyration, < s2>0, for cross-linked polyglycine, poly(L-alanine),poly(L-proline),poly(i-alanyl-D-alanine),poly(i.-prolyl-L-pro lylglycine),poly(L-prolyl-i.-alanylglycine ,poly(glycyl-L-alanyl-L-pro line), andpoly(L-aianyl-L-alanylgIycine).Thecentral amino acid residue in each polypeptide chain is replaced by the L-cysteinyl residue involved in cross-link formation. Each cross-linked molecule is considered to contain two trifunctional branch points, the a-carbon atoms of the two... 

Mutasynthesis of New Antibiotics K. L. Rinehart, Pure Appl. Chem., 1977, 49,1361-1384. The Significance of D-Alanyl-D-Alanine Termini in the Biosynthesis of Bacterial Cell Walls and the Action of Penicillin, Vancomycin and Ristocetin H. R. Perkins and M. Nieto, Pure Appl. Chem., 1973, 35, 371-381. 

D-Alanine is found in bacterial cell wall peptidogly-can. l-Alanine is converted to D-alanine by a racemase that contains pyridoxal phosphate as a cofactor. The racemiza-tion is followed by the formation of a D-alanyl-D-alanine dipeptide, which is accompanied by the conversion of ATP to ADP. The dipeptide is subsequently incorporated into the glycopeptide (see fig. 16.16). 

EJJ Lugtenberg, A van Schijndel-vanDam. Temperature-sensitive mutants of Escherichia coli K-12 with low activities of the L-alanine adding enzyme and the D-alanyl-D-alanine adding enzyme. J Bacteriol 110 35 40, 1972. 

Figure 10.4 Proteins showing sequence similarities to CphA. Motifs conserved among the proteins are indicated as well as identified binding sites for ATP, primer and substrates [105], D-Ala, D-Alanine MurE, UDP-N-acetylmuramoyl-L-alanyl-D-glutamate meso-diaminopimelate...

Figure 6-1. A peptidoglycan structure. (R is an L-alanyl-D-glutamyl-L-lysyl-D-alanine D-amino acid residues occur here.)...

Staphylococcus aureus - The polysaccharide chain is an alternating polymer of N-acetylglucosamine and N-acetylmuramie acid. The carboxyl groups of all the N-acetylmuramic acid resides are linked to the terminal group of the tetrapeptide-L-alanyl-D-Y-isoglutaminyl-L-lysyl-D-alanine. The biosynthesis can be considered to occur in the following three distinct stages ... 

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