Mouth-watering brown color of barbecue and baked goods is due to the non-enzymatic production of advanced glycation end products (AGEs).

Functional Glycomics (09 June 2011) | doi:10.1038/fg.2011.22

The solution structure of the RAGE V domain in complex with a glycated peptide indicates how this receptor can recognize a large variety of AGE ligands.

The mouth-watering brown color of barbecue and baked goods is due to the non-enzymatic production of advanced glycation end products (AGEs). Under stress conditions, these compounds are also produced endogenously on lysine and arginine side chains in proteins.

In particular, compounds such as N_ε-carboxy-methyl-lysine (CML) and N_ε-carboxy-ethyl-lysine (CEL) can cause inflammatory responses via a receptor called RAGE.

In fact, RAGE signaling has been linked to diseases such as chronic inflammation, diabetes, Alzheimer's and cancer. RAGE is a member of the immunoglobulin superfamily, with three extracellular immunoglobulin domains (V, C1 and C2), a transmembrane helix and a short cytoplasmic tail. AGE products bind exclusively to the V domain, and now Alexander Shekhtman and colleagues have made an important step in understanding this interaction at an atomic level.

The structural basis of AGE recognition by RAGE is still unclear. One obstacle for such studies has been the heterogeneity of AGE products, as glycation reactions can occur on almost any arginine or lysine residue in a protein, generating ligands with a huge diversity of primary, secondary and tertiary structures. On the other hand, it is known that free CEL and CML compounds cannot bind or activate RAGE on their own.

In a recent paper published in the journalStructure, Shekhtman and co-workers first characterize the binding of short peptides representing major glycation sites in human serum albumin, unmodified or modified with CML and CEL, to the V domain of RAGE, by monitoring the native tryptophan fluorescence. These binding experiments revealed that the CEL- or CML-containing peptides had overall similar affinities to RAGE. The authors went on to map the residues from RAGE V domain that are affected by interaction with the modified peptides, doing NMR chemical shift titrations. This analysis showed that the same residues are involved in binding to CEL and CML peptides.

The authors solved the solution structure of the V domain in complex with a peptide containing CEL. The structure revealed that the CEL peptide–binding site is relatively flat, which explains the ability of RAGE to recognize AGEs in different amino acid sequences, and it sits on a positively charged groove, consistent with its binding to negatively charged CEL or CML. The CEL moiety appears in an extended conformation and makes the most contacts with the receptor. The methyl group of CEL does not seem to contribute to the binding, as it points away from the binding site; CML lacks this methyl group and, indeed, exhibits a similar binding affinity for the V domain. The structure also shows that V domain residues Lys52 and Arg98 have an important role in ligand binding, an observation confirmed by mutagenesis. Finally, the authors show that the peptide backbone is necessary to provide stable binding with the V domain, with contacts made with the residue right beside CEL.

In summary, the functional and structural data presented provide insight into how RAGE can recognize such a wide array of modified proteins. In addition, the work opens a potential avenue for therapeutic intervention, as blocking ligand binding may be used to reduce or prevent RAGE signaling and inflammation.

Author:Inês Chen

Original research paper

1. Xue, J. et al. Advanced glycation end product recognition by the receptor for AGEs. Structure 19, 722–732 (2011). | Article