A new paper details how methicillin-resistant Staphylococcus aureus (MRSA) regulates the critical crosslinking of its cell wall in the face of beta-lactam antibiotics, the mechanistic basis for how the MRSA bacterium became such a difficult pathogen over the previous 50 years, in which time it spread rapidly across the world.

MRSA has been a difficult hospital pathogen to control and has emerged in the broader community in the past several years, especially in such places as prisons, locker rooms and nurseries. In the United States alone, the disease infects about 100,000 people and claims the lives of nearly 20,000 people annually.

Modern strains of MRSA have become broadly resistant to antibiotics, including beta-lactam antibiotics, such as penicillins. In their report, the researchers disclose the discovery of an allosteric domain in the X-ray structure of the penicillin binding protein 2a of MRSA, the enzyme that carries out the crosslinking reaction. (An allosteric site is a place on the protein where its activity is regulated by the binding of another molecule.)

The authors document that an allosteric trigger by a fragment of the cell wall at a distance of 60 Ångstroms (6 nanometers) activates a set of conformational changes that culminates in the opening of the active site from a closed conformation, enabling catalysis for the physiological role of the enzyme.

Point mutations identified in ceftaroline-resistant methicillin-resistantS. aureus(MRSA) clinical isolates. Two clinical MRSA isolates, strains 4977 and 13101 [GenBank accession nos. JQ582124 (4977) and JQ582126 (13101)] displaying increased minimum inhibitory concentration (MIC) values for ceftaroline were recently reported (1). No alterations in themecR1, mecApromoter or rbs regions were observed, but strains 4977 and 13101 had changes of N146K and E150K, and the second isolate (13101) had an additional substitution of H351N. All three mutations occur remotely from the active site. (A) The first two mutations are located within the allosteric site, and (B) the third mutation is a part of the hinge between the allosteric and transpeptidase domains. The Nterminal extension is colored in green, the allosteric domain is colored in gold, and the transpeptidase domain is colored in blue. Credit and link: 
doi:10.1073/pnas.1300118110

They also document that the new beta-lactam antibiotic ceftaroline, recently approved by the Food and Drug Administration, is able to bind to the allosteric domain and trigger the same allosteric opening of the active site.

This subversion of the allosteric control allows another molecule of ceftaroline to access the active site, which inhibits the function of the enzyme, leading to cell death by MRSA.

This mechanism of action for the antibiotic is unprecedented and offers important insights for design of future drugs to combat MRSA.

Citation: Lisandro H. Otero, Alzoray Rojas-Altuve, Leticia I. Llarrull, Cesar Carrasco-López, Malika Kumarasiri, Elena Lastochkin, Jennifer Fishovitz, Matthew Dawley, Dusan Hesek, Mijoon Lee, Jarrod W. Johnson, Jed F. Fisher, Mayland Chang, Shahriar Mobashery, and Juan A. Hermoso, '
How allosteric control of Staphylococcus aureus penicillin binding protein 2a enables methicillin resistance and physiological function ', PNAS October 1, 2013, doi:10.1073/pnas.1300118110