The peptidoglycan found in the cell envelope of Gram-negative bacteria is made of:
a. A polysaccharide comprised of a single carbohydrate and repeating units of diverse peptides
b. Two N-acetylglucosamine moieties, one of which has a unique substituent and is attached to a peptide
c. Two N-acetylglucosamine moieties, both of which are attached to a peptide with a 5-glycine extension
d. Teichoic acids e. None of the above
The correct answer and explanation is:
The correct answer is b. Two N-acetylglucosamine moieties, one of which has a unique substituent and is attached to a peptide.
Peptidoglycan, which is a key component of the bacterial cell envelope, plays a vital role in maintaining the cell’s shape and protecting it from osmotic pressure. In Gram-negative bacteria, peptidoglycan is a part of the thin cell wall located between the inner cytoplasmic membrane and the outer membrane.
Peptidoglycan consists of repeating disaccharide units composed of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). These two sugar molecules form a backbone of the structure. The MurNAc unit is linked to a short peptide chain, which typically contains a sequence of amino acids that may vary depending on the bacterial species. In some cases, this peptide is cross-linked to the peptides of adjacent glycan chains, providing structural integrity.
In Gram-negative bacteria, the peptidoglycan layer is relatively thin compared to Gram-positive bacteria. However, it is still crucial for maintaining the cell’s mechanical strength. The structure of the peptidoglycan layer also plays a role in the bacterial resistance to various environmental stresses and antibiotics.
The key point in option b is that one of the N-acetylglucosamine units in the peptidoglycan has a unique substituent that is attached to the peptide chain. This differs from the structure found in Gram-positive bacteria, where the peptidoglycan layer is thicker and often cross-linked with different peptides or glycan chains. Thus, this specific structure in Gram-negative bacteria contributes to their distinct structural and functional characteristics.
Option a is incorrect because it misrepresents the structure by suggesting a polysaccharide made of a single carbohydrate and diverse peptides. Options c and d mention incorrect elements such as glycine extensions and teichoic acids, which are not involved in the peptidoglycan structure in Gram-negative bacteria.