Addressing the Glycine-Rich Loop of Protein Kinases by a Multi-Facetted Interaction Network: Inhibition of PKA and a PKB Mimic.

Protein kinases continue to be hot targets in drug discovery research, as they are involved in many essential cellular processes and their deregulation can lead to a variety of diseases. A series of 32 enantiomerically pure inhibitors was synthesized and tested towards protein kinase A (PKA) and protein kinase B mimic PKAB3 (PKA triple mutant). The ligands bind to the hinge region, ribose pocket, and glycine-rich loop at the ATP site. Biological assays showed high potency against PKA, with Ki values in the low nanomolar range. The investigation demonstrates the significance of targeting the often neglected glycine-rich loop for gaining high binding potency. X-ray co-crystal structures revealed a multi-facetted network of ligand-loop interactions for the tightest binders, involving orthogonal dipolar contacts, sulfur and other dispersive contacts, amide-π stacking, and H-bonding to organofluorine, besides efficient water replacement. The network was analyzed in a computational approach.

Multiple-turnover isotopic labeling of Fmoc- and Boc-protected amino acids with oxygen isotopes.

An efficient method for the selective isotopic labeling of carboxylic acids is reported. By reacting an amino acid with excess carbodiimide and (18)OH(2), a kinetically enhanced multiple turnover reaction provides the (18)O-labeled product in high yield and excellent isotopic enrichment. This reaction is fully compatible with standard Fmoc, Boc, Trt, and OtBu protecting groups and provides a means to selectively label the alpha-carboxylic acids of functionalized amino acids with stable oxygen isotopes.