Mode of action of glycine on the biosynthesis of peptidoglycan

The mechanism of glycine action in growth inhibition was studied on 8 different species of bacteria of various genera representing the 4 most common peptidoglycan types. To inhibit the growth of the different organisms to 80%, glycine concentrations from 0.05 to 1.33 M were applied. The inhibited cells showed morphological aberrations. It was demonstrated that glycine is incorporated into the nucleotide activated peptidoglycan precursors. The amount of incorporated glycine was equivalent to the decrease in the amount of alanine. With 1 exception glycine is also incorporated into the peptidoglycan. Studies on the primary structure of both the peptidoglycan precursors and the corresponding peptidoglycan revealed that glycine can replace L alanine in position 1 and D alanine residues in positions 4 and 5 of the peptide subunit. Replacement of L alanine in position 1 of the peptide subunit together with an accumulation of uridine diphosphate muramic acid (UDP MurNAc), indicating an inhibition of the UDP MurNAc:L Ala ligase, was found in 3 bacteria (Staphylococcus aureus, Lactobacillus cellobiosus and L. plantarum). However, discrimination against precursors with glycine in position 1 in peptidoglycan synthesis was observed only in S. aureus. Replacement of D alanine residues was most common. It occurred in the peptidoglycan with 1 exception in all strains studied. In Corynebacterium sp., C. callunae, L. plantarum, and L. cellobiosus most of the D alanine replacing glycine occurs C terminal in position 4, and in C. insidiosum and S. aureus glycine is found C terminal in position 5. It is suggested that the modified peptidoglycan precursors are accumulated by being poor substrates for some of the enzymes involved in peptidoglycan synthesis. 2 mechanisms leading to a more loosely cross linked peptidoglycan and to morphological changes of the cells are considered. First, the accumulation of glycine containing precursors may lead to a disrupture of the normal balance between peptidoglycan synthesis and controlled enzymatic hydrolysis during growth. Second, the modified glycine containing precursors may be incorporated. Since these are poor substrates in the transpeptidation reaction, a high percentage of muropeptides remains uncrosslinked. The second mechanism may be the more significant in most cases.