Hughes, PJ; (2016) Multivalent ligand recognition by Pentraxins. Doctoral thesis , UCL (University College London).

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Abstract

The pentraxins, serum amyloid P component (SAP) and C-reactive protein (CRP) are target proteins for the development of treatments for amyloidosis and ischaemic injury, respectively, in humans. This study reports the first multivalent ligands capable of targeting all five SAP binding sites simultaneously. Ligands presenting five or ten D-proline headgroups and composed of five peptideglycol linkers emanating from ε-N-substituted lysine residues on a central cyclic peptide core were synthesised by solid phase peptide synthesis. The sub-nanomolar, ~250pM, binding affinity approximated by Isothermal Titration Calorimetry (ITC) for one decavalent ligand is the strongest affinity for an SAP binding ligand currently known and stronger than the affinity of SAP binding to amyloid deposits. X-ray crystallography and mass spectrometry shows the decavalent ligands noncovalently cross-linking two SAP pentamers, in the same manner observed for lead drug candidate CPHPC, but with increased affinity. In addition, the binding of SAP with N-acetyl D-proline has been investigated by x-ray crystallography. Using a 1.5Å resolution structure the exact interaction of the headgroup used in CPHPC, penta- and decavalent ligands, was investigated. The results show potential for an electrostatic interaction between the carbonyl oxygen of acetyl from the ligand and the side chain amide of Gln148, which has not previously been considered. Applications of multivalent binding are still emerging; in this study, bivalent ligand BPC8 was used as an additive to crystallise CRP from the Rat (rCRP) in non-covalently cross-linked decameric complexes. Previous x-ray crystallography studies have failed due to extreme radiation sensitivity of the crystals produced. This problem has been overcome with a complete dataset obtained from a single crystal at 3.2Å. No inter-protein contacts are seen between pentamers in the decamer complex, therefore the use of bivalent ligands has facilitated the observed crystal packing. Multivalent ligands are suggested as tools for overcoming difficult crystallisation issues.

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