A strategy is developed to analyze steady-state kinetics for the hydrolysis of a soluble substrate partitioned into the interface by an enzyme at the interface. The feasibility of this approach to obtain interfacial primary kinetic and equilibrium parameters is demonstrated for a triglyceride lipase. Analysis for phospholipase A2 catalyzed hydrolysis of rapidly exchanging micellar (Berg et al. (1997) Biochemistry 36, 14512-14530) and nonexchangeable vesicular (Berg et al., (1991) Biochemistry 30, 7283-7291) phospholipids is extended to include the case of a substrate that does not form the interface. The triglyceride lipase (tlTGL) from Thermomyces (formerly Humicola) lanuginosa hydrolyzes p-nitrophenylbutyrate or tributyrin partitioned in the interface of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) vesicles at a rate that is more than 100-fold higher than that for the monodispersed substrate or for the substrate partitioned into zwitterionic vesicles. Catalysis and activation is not seen with the S146A mutant without the catalytic serine-146; however, it binds to the POPG interface with the same affinity as the WT. Thus POPG acts as a diluent surface to which the lipase binds in an active, or "open", form for the catalytic turnover; however, the diluent molecules have poor affinity for the active site. Analysis of the substrate and the diluent concentration dependence of the rate of hydrolysis provides a basis for the determination of the primary interfacial catalytic parameters. As a competitive substrate, tributyrin provided a check for the apparent affinity parameters. Nonidealities from the fractional difference in the molecular areas in interfaces are expressed as the area correction factor and can be interpreted as a first-order approximation for the interfacial activity coefficient. The basis for the interfacial activation of tlTGL on anionic interface is attributed to cationic R81, R84, and K98 in the "hinge" around the 86-93 "lid" segment of tlTGL.

Most Gram-negative bacteria are susceptible to polymyxin B (PxB), and development of resistance to this cationic lipopeptide is very rare. PxB mechanism of action involves interaction with both the outer membrane (OM) and the inner membrane (IM) of bacteria. For the design of new antibiotics based on the structure of PxB and with improved therapeutic indexes, it is essential to establish the key features of PxB that are important for activity. We have used an approach based on mimicking the outer layers of the OM and the IM of Gram-negative bacteria using monolayers of lipopolysaccharide (LPS) or anionic 1-palmitoyl-2-oleoylglycero-sn-3-phosphoglycerol (POPG), respectively, and using a combination of penetration assay, analysis of pressure/area curves, and Brewster angle microscopy to monitor surface morphology changes. Synthetic analogue sp-B maintains the basic structural characteristics of the natural compound and interacts with the OM and the IM in a similar way. Analogue sp-C, with a mutation of the sequence [d-Phe6-Leu7] into [d-Phe6-Dab7], shows that this hydrophobic domain is involved in LPS binding. The significant role of the positive charges is demonstrated with sp-Dap analogue, where l-alpha,gamma-diaminobutyric acid residues Dab1 and Dab8 are replaced by l-alpha,gamma-diaminopropionic acid (Dap), resulting in lower degrees of insertion in both LPS and PG monolayers. The importance of the N-terminal acyl chain is demonstrated with polymyxin B nonapeptide (PxB-np). PxB-np shows lower affinity for LPS compared to PxB, sp-B, or sp-C, but it does not insert into PG monolayers, although it binds superficially to the anionic film. Since PxB microbial killing appears to be mediated by osmotic instability due to OM-IM phospholipid exchange, the ability of the different peptides to induce membrane-membrane lipid exchange has been studied by use of phospholipid unilamellar vesicles. Results indicate that cationic amphipathicity determines peptide activity.

Covering: 1947-early 2017, particularly from 2005-early 2017The rise of bacterial pathogens with acquired resistance to almost all available antibiotics is becoming a serious public health issue. Polymyxins, antibiotics that were mostly abandoned a few decades ago because of toxicity concerns, are ultimately considered as a last-line therapy to treat infections caused by multi-drug resistant Gram-negative bacteria. This review surveys the progress in understanding polymyxin structure, and their chemistry, mechanisms of antibacterial activity and nephrotoxicity, biomarkers, synergy and combination with other antimicrobial agents and antibiofilm properties. An update of recent efforts in the design and development of a new generation of polymyxin drugs is also discussed. A novel approach considering the modification of the scaffold of polymyxins to integrate metabolism and detoxification issues into the drug design process is a promising new line to potentially prevent accumulation in the kidneys and reduce nephrotoxicity.