James K. Kranz

Water magnetic relaxation dispersion in biological systems: The contribution of proton exchange and implications for the noninvasive detection of cartilage degradation

Magnetic relaxation has been used extensively to study and characterize biological tissues. In particular, spin-lattice relaxation in the rotating frame (T1ρ) of water in protein solutions has been demonstrated to be sensitive to macromolecular weight and composition. However, the nature of the contribution from low frequency processes to water relaxation remains unclear. We have examined this problem by studying the water T1ρ dispersion in peptide solutions (14N- and 15N-labeled), glycosaminoglycan solutions, and samples of bovine articular cartilage before and after proteoglycan degradation. We find in model systems and tissue that hydrogen exchange from NH and OH groups to water dominates the low frequency water T1ρ dispersion, in the context of the model used to interpret the relaxation data. Further, low frequency dispersion changes are correlated with loss of proteoglycan from the extra-cellular matrix of articular cartilage. This finding has significance for the noninvasive detection of matrix degradation.

Complexation of peptidoglycan intermediates by the lipoglycodepsipeptide antibiotic ramoplanin: Minimal structural requirements for intermolecular complexation and fibril formation

The peptide antibiotic ramoplanin inhibits bacterial peptidoglycan (PG) biosynthesis by interrupting late-stage membrane-associated glycosyltransferase reactions catalyzed by the transglycosylase and MurG enzymes. The mechanism of ramoplanin involves sequestration of lipid-anchored PG biosynthesis intermediates, physically occluding these substrates from proper utilization by these enzymes. In this report, we describe the first molecular-level details of the interaction of ramoplanin with PG biosynthesis intermediates. NMR analysis in conjunction with chemical dissection of the PG monomer revealed that the ramoplanin octapeptide d-Hpg-d-Orn-d-alloThr-Hpg-d-Hpg-alloThr-Phe-d-Orn recognizes MurNAc-Ala-γ-d-Glu pyrophosphate, the minimum component of PG capable of high-affinity complexation and fibril formation. Ramoplanin therefore recognizes a PG binding locus different from the N-acyl-d-Ala-d-Ala moiety targeted by vancomycin. Because ramoplanin is structurally less complex than glycopeptide antibiotics such as vancomycin, peptidomimetic chemotherapeutics derived from this recognition sequence may find future use as antibiotics against vancomycin-resistant Enterococcus faecium, methicillin-resistant Staphylococcus aureus, and related pathogens.

Functional Analysis of the Lipoglycodepsipeptide Antibiotic Ramoplanin

The peptide antibiotic ramoplanin is highly effective against several drug-resistant gram-positive bacteria, including vancomycin-resistant Enterococcus faecium (VRE) and methicillin-resistant Staphylococcus aureus (MRSA), two important opportunistic human pathogens. Ramoplanin inhibits bacterial peptidoglycan (PG) biosynthesis by binding to Lipid intermediates I and II at a location different than the N-acyl-D-Ala-D-Ala dipeptide site targeted by vancomycin. Lipid I/II capture physically occludes these substrates from proper utilization by the late-stage PG biosynthesis enzymes MurG and the transglycosylases. Key structural features of ramoplanin responsible for antibiotic activity and PG molecular recognition have been discovered by antibiotic semisynthetic modification in conjunction with NMR analyses. These results help define a minimalist ramoplanin pharmacophore and introduce the possibility of generating ramoplanin-derived peptide or peptidomimetic antibiotics for use against VRE, MRSA, and related pathogens.