|Lysozyme-like phage lysin|
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
Lysins, also known as endolysins or murein hydrolases, are hydrolytic enzymes produced by bacteriophages in order to cleave the host's cell wall during the final stage of the lytic cycle. Lysins are highly evolved enzymes that are able to target one of the five bonds in peptidoglycan (murein), the main component of bacterial cell walls, which allows the release of progeny virions from the lysed cell. These enzymes are being used as antibacterial agents due to their high effectiveness and specificity in comparison with antibiotics, which are susceptible to bacterial resistance.
Double-stranded DNA phage lysins tend to lie within the 25 to 40 kDa range in terms of size. A notable exception is the streptococcal PlyC endolysin, which is 114 kDa. PlyC is not only the biggest and most potent lysin, but also structurally unique since it is composed of two different gene products, PlyCA and PlyCB, with a ratio of eight PlyCB subunits for each PlyCA in its active conformation. All other lysins are monomeric and comprise two domains separated by a short linker region. The N-terminal domain catalyses the hydrolysis of peptidoglycan whereas the C-terminal domain binds to the cell wall substrate.
The catalytic domain is responsible for the cleavage of peptidoglycan bonds. Functionally, five types of lysin catalytic domain can be distinguished:
- N-acetylmuramidase (lysozyme-like)
- N-acetylmuramoyl-L-alanine amidase
- γ-D-glutaminyl-L-lysine endopeptidase
Peptidoglycan consists of cross-linked amino acids and sugars which form alternating amino sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Endo-β-N-acetylglucosaminidase lysins cleave NAGs while N-acetylmuramidase lysins (lysozyme-like lysins) cleave NAMs. Endopeptidase lysins cleave any of the peptide bonds between amino acids, whereas N-acetylmuramoyl-l-alanine amidase lysins (or simply amidase lysins) hydrolyze the amide bond between the sugar and the amino acid moieties. Finally, the recently discovered γ-d-glutaminyl-l-lysine endopeptidase lysins cleave the gamma bond between D-glutamine and L-lysine residues.
Usually, two or more different catalytic domains are linked to a single cell-binding domain. This is typical in many staphylococcal lysins as well as the streptococcal PlyC holoenzyme, which contains two catalytic domains. Catalytic domains are highly conserved in phage lysins of the same class.
The cell-binding domain (CBD) binds to a specific substrate found in the host bacterium's cell wall, usually a carbohydrate. In contrast to the catalytic domain, the cell-binding domain is variable, which allows a great specificity and decreases bacterial resistance. Binding affinity to the cell wall substrate tends to be high, possibly so as to sequester onto cell wall fragments any free enzyme, which could compete with phage progeny from infecting adjacent host bacteria.
It has been proposed that the main mechanism of evolution in phage lysins is the exchange of modular units, a process by which different catalytic and cell-binding domains have been swapped between lysins, which would have resulted in new combinations of both bacterial binding and catalytic specificities.
Mode of action
The lysin catalytic domain digests peptidoglycan locally at a high rate, which causes holes in the cell wall. Since the cross-linked peptidoglycan cell wall is the only mechanism that prevents the spontaneous burst of bacterial cells due to the high internal pressure (3 to 5 atmospheres), enzymatic digestion by lysins irreversibly causes hypotonic lysis. Theoretically, due to the catalytic properties of phage lysins, a single enzyme would be sufficient to kill the host bacterium by cleaving the necessary number of bonds, even though this has yet to be proven. The work by Loessner et al suggests that cleavage is typically achieved by the joint action of multiple lysin molecules at a local region of the host's cell wall. The high binding affinity to the cell wall substrate (close to that of IgG for its substrate) of each lysin appear to be reason why multiple molecules are required, since every lysin binds so tightly to the cell wall that it can't break enough bonds to cause lysis by itself.
In order to reach the cell wall, phage lysins have to cross the cell membrane. However, they lack a signal peptide that would allow them to do so. In order to solve such a problem, phage viruses synthesize another protein called holin which binds to the cell membrane and makes holes in it (hence its name), allowing lysins to reach the peptidoglycan matrix. The prototypical holin is the lambda phage S protein, which assists the lambda phage R protein (lysin). All holins embed themselves in the cell membrane and contain at least two transmembrane helical domains. The hole making process is thought to be achieved by holin oligomerization at a specific moment when progeny virions are set to be released.
Phage lysins are generally species or subspecies specific, which means that the are only effective against bacteria from which they were produced. While some lysins only act upon the cell walls of several bacterial phylotypes, some broad-spectrum lysins have been found. Similarly, some thermostable lysins are known, which makes them easier to use in biotechnology. Regarding their use as antibacterial agents, lysins have been found effective exclusively against Gram-positive bacteria, since Gram-negative bacteria possess an outer membrame that prevents extracellular lysin molecules from digesting peptidoglycan.
One of the most problematic aspects of the use phage lysins as antimicrobial agents is the potential immunogenicity of these enzymes. Unlike most antibiotics, proteins are prone to antibody recognition and binding, which means that lysins could be ineffective when treating bacterial infections or even dangerous, potentially leading to a systemic immune response or a cytokine storm. Nonetheless, experimental data from immunologically-rich rabbit serum showed that hyperimmune serum slows down but does not block the activity of pneumococcal lysin Cpl-1.
Phage lysins have been successfully tested in animal models to control pathogenic antibiotic-resistant bacteria found on mucous membranes and in blood. The main advantage of lysins compared to antibiotics is not only the low bacterial resistance but also the high specificity towards the target pathogen, and low activity towards the host's normal bacterial flora.
Lysins were first used therapeutically in animals in 2001, in a publication in which mice orally colonized with Streptococcus pyogenes were decolonized with a single dose of PlyC lysin delivered orally.
- Pérez-Dorado I, Campillo NE, Monterroso B, Hesek D, Lee M, Páez JA, García P, Martínez-Ripoll M, García JL, Mobashery S, Menéndez M, Hermoso JA (August 2007). "Elucidation of the molecular recognition of bacterial cell wall by modular pneumococcal phage endolysin CPL-1". J. Biol. Chem. 282 (34): 24990–9. doi:10.1074/jbc.M704317200. PMID 17581815.
- Fischetti VA (Oct 2008). "Bacteriophage lysins as effective antibacterials". Current Opinion in Microbiology 11 (5): 393–400. doi:10.1016/j.mib.2008.09.012. PMC 2597892. PMID 18824123.
- Young R (Sep 1992). "Bacteriophage lysis: mechanism and regulation". Microbiological Reviews 56 (3): 430–81. PMC 372879. PMID 1406491.
- McGowan S, Buckle AM, Mitchell MS, Hoopes JT, Gallagher DT, Heselpoth RD, Shen Y, Reboul CF, Law RH, Fischetti VA, Whisstock JC, Nelson DC (Jul 2012). "X-ray crystal structure of the streptococcal specific phage lysin PlyC". Proceedings of the National Academy of Sciences of the United States of America 109 (31): 12752–7. Bibcode:2012PNAS..10912752M. doi:10.1073/pnas.1208424109. PMC 3412044. PMID 22807482.
- García E, García JL, García P, Arrarás A, Sánchez-Puelles JM, López R (Feb 1988). "Molecular evolution of lytic enzymes of Streptococcus pneumoniae and its bacteriophages". Proceedings of the National Academy of Sciences of the United States of America 85 (3): 914–8. Bibcode:1988PNAS...85..914G. doi:10.1073/pnas.85.3.914. JSTOR 31364. PMC 279667. PMID 3422470.
- Loessner MJ, Kramer K, Ebel F, Scherer S (Apr 2002). "C-terminal domains of Listeria monocytogenes bacteriophage murein hydrolases determine specific recognition and high-affinity binding to bacterial cell wall carbohydrates". Molecular Microbiology 44 (2): 335–49. doi:10.1046/j.1365-2958.2002.02889.x. PMID 11972774.
- García P, García JL, García E, Sánchez-Puelles JM, López R (Jan 1990). "Modular organization of the lytic enzymes of Streptococcus pneumoniae and its bacteriophages". Gene 86 (1): 81–8. doi:10.1016/0378-1119(90)90116-9. PMID 2311937.
- Wang IN, Smith DL, Young R (2000). "Holins: the protein clocks of bacteriophage infections". Annual Review of Microbiology 54: 799–825. doi:10.1146/annurev.micro.54.1.799. PMID 11018145.
- Yoong P, Schuch R, Nelson D, Fischetti VA (Jul 2004). "Identification of a broadly active phage lytic enzyme with lethal activity against antibiotic-resistant Enterococcus faecalis and Enterococcus faecium". Journal of Bacteriology 186 (14): 4808–12. doi:10.1128/JB.186.14.4808-4812.2004. PMC 438584. PMID 15231813.
- Plotka M, Kaczorowska AK, Stefanska A, Morzywolek A, Fridjonsson OH, Dunin-Horkawicz S, Kozlowski L, Hreggvidsson GO, Kristjansson JK, Dabrowski S, Bujnicki JM, Kaczorowski T (Feb 2014). "Novel highly thermostable endolysin from Thermus scotoductus MAT2119 bacteriophage Ph2119 with amino acid sequence similarity to eukaryotic peptidoglycan recognition proteins". Applied and Environmental Microbiology 80 (3): 886–95. doi:10.1128/AEM.03074-13. PMC 3911187. PMID 24271162.
- Loeffler JM, Djurkovic S, Fischetti VA (Nov 2003). "Phage lytic enzyme Cpl-1 as a novel antimicrobial for pneumococcal bacteremia". Infection and Immunity 71 (11): 6199–204. doi:10.1128/IAI.71.11.6199-6204.2003. PMC 219578. PMID 14573637.
- Nelson D, Loomis L, Fischetti VA (Mar 2001). "Prevention and elimination of upper respiratory colonization of mice by group A streptococci by using a bacteriophage lytic enzyme". Proceedings of the National Academy of Sciences of the United States of America 98 (7): 4107–12. doi:10.1073/pnas.061038398. PMID 11259652.