The Immunochemistry of Peptidoglycan

The precipitin reaction between peptidoglycan solubilized by ultrasonic treatment and the sera of rabbits immunized with Group A variant streptococci was inhibited with various ammo acid derivatives and synthetic peptides.The peptides were synthesized by the solid phase method.The following conclusions were drawn from these experiments:1.The pentapeptide L-Ala-D-Glu-y-L-Lys-D-Ala-o-Ala is the antigenic determinant of the peptide moiety of the peptidoglycan.2. The antibodies are directed against the COOH-terminal portion of the pentapeptide.3. The terminal n-alanyl-n-alanine is the immunodominant site of the antigen.Thus, the antibodies are directed against the "growing" site of the peptidoglycan.The antibodies bind to the same site as the enzyme transpeptidase, which catalyzes the last step in the biosynthesis of the peptidoglycan.Penicillin, a structural analogue of the COOH-terminal D-Ala-D-Ala and thus an inhibitor of the transpeptidase reaction, did not inhibit the precipitin reaction, an indication that the antibodies distinguish between the structure of penicillin and D-Ala-D-Ala.4. The contribution of the other amino acids to the antigenie site of peptidoglycan is of less importance (L-Lys, D- Glu) or is insignificant (L-Ala).5. The upper limit of the combining site of the antibodies is complementary to that of a peptide ranging between the sizes of a tetrapeptide and a pentapeptide.Peptidoglycan, a component of the bacterial cell wall, is a heteropolymer composed of polysaccharide strands which are cross-linked through short peptide chains (l-5).The polysaccharide strands are made up of repeating units of N-acetyl-