Researchers from Hyderabad have identified an enzyme that plays a crucial role in the enlargement and growth of bacteria, by studying E. coli. The enzyme MepK helps in cutting a particular class of bonds that connect the peptidoglycan, which is a sac-like molecule that envelops the cell. This action allows more material to be added to the cell wall, making a larger compartment for the cell to reside in.
One of the most important features of a bacterium is its cell wall which protects it from external environmental conditions and also internal pressure and keeps it in shape. Harming the cell wall causes irreversible damage to the bacterium and eventually kills it.
For example, E. coli are rod shaped bacteria often experimented with in the lab. The bacteria die when the integrity of the cell wall is destroyed. Its crucial role in maintaining the wellbeing of the bacterium makes the cell wall a target of study, especially by scientists interested in developing new drug strategies to combat the bacteria.
In this context, understanding how the bacterial cell wall develops during growth and division of cells is an important question being addressed in Manjula Reddy’s lab at the Centre for Cellular and Molecular Biology (CSIR-CCMB) in Hyderabad for a decade now.
In an earlier work, done in 2012, Dr Reddy’s group showed that opening the cell wall by hydrolysing enzymes is crucial for the new material to be incorporated into it, leading to the cell’s expansion and elongation.
The cell wall is made up mostly of a single net-like molecule (peptidoglycan). This consists of many sugar polymers interconnected by short peptides. It encloses the bacterial cytoplasmic membrane very much like a jute bag. The peptides connecting the baglike structure are cross-linked in several ways. Of significance to this work are the links between particular amino acid residues located on adjacent peptide chains. This is a rare component present only in bacterial cell walls and is known as mDAP for short..
In a paper published recently in the Proceedings of National Academy of Sciences (PNAS), the group identified an enzyme (MepK) which helps in breaking down the bond between two mDAP residues. This leads to cutting the molecular mesh and thus aiding the growth (or enlargement) of the cell. “By cleaving these cross-links, MepK [along with other known enzymes] contributes to growth and enlargement of sac-like peptidoglycan... This emphasises the fundamental role of cross-link cleavage in bacterial cell wall synthesis,” says Pavan Kumar Chodisetti, from CSIR-CCMB and the first author of the paper.
“The class of enzymes reported in this paper was not known earlier, and identifying this enzyme [MepK] gave us lot of excitement,” says Dr. Reddy. “[The study] has higher significance in organisms like Clostridia and M. tuberculosis because cell walls of these bacteria have very high levels of mDAP-mDAP type of cross-links. Therefore, MepK-like enzymes will be very important for the growth of these bacteria.”
These cross-links constitute approximately 10% of total cross-links in Gram-negative bacteria like E. coli and Pseudomonas. However, they are predominant in many Gram-positive bacteria such as Mycobacteria and Clostridia (occur up to 80% of total cross-links)
The next step according to Dr. Reddy is “identifying small-molecule inhibitors for this class of enzymes and also to understand the molecular mechanisms by which the cell wall growth is initiated”.