Speciality Chemicals Magazine JAN / FEB 2021 | Page 45

PEPTIDES & PROTEINS

Figure 1 – Head-to-tail , iactam & staple constraints b Placement cyclising residues for helical stabilisation in + 4 and + 7 spacing
Side chain lactam bridges i + 1 + 2 + 3 + 4 + 5 + 6 + 7 Orn-X- Y- Z -Asp Lys-X- Y- Z- A- B- C- Glu
Side chain staples ( ring-closing metathesis )
J- X- Y- Z- J U- X -Y -Z- A-B- C- U J = 2-4 ’ -Pentenyl-Ala U = 2-7 ’ -Octenyl-Ala a – Positioning of constraint
Triazole linkages J- X- Y- Z- U J- X -Y -Z- A-B- C- U ’ J = PropargylGly ( alkyne component ) U = Azido-Abu U ’= Azido-Nle
Note – Residues ( X , Y , Z , A , B , C ) inserted between cyclising residues should be favourable to forming helices such as those predicted by Chou-Fasman , e . g . Ala , Arg , Lys , Glu , Leu .
include side chain to side chain lactam bridges , staples via ringclosing metathesis , reduced alkane staples , click chemistry cyclisation and thioether ( s ). Each is discussed very briefly below .
Lactam bridges
Lactam bridges were developed in particular to help stabilise turns of α-helices . The placement of the lactam bridge is typically designed by placement at positions i and i + 4 residues ( Figure 1 ). This requires a side chain acid residue ( e . g . Glu or Asp ) at position i and a primary amine constituent ( e . g . Lys , Orn , hLys ) at position 4 . Orthogonal protection of these side chains allows for selective removal of these protecting groups ( such as Alloc , Allyl ( Pd catalyst removal ) and or ivDde and Dmab ( 2 % hydrazine removal )). 5 The positioning of these residues can also be performed in a reciprocal manner , with the amine in the i position and the acid in the i + 4 positions .
Following the selective removal of these side chain-protecting group , an amide bond is subsequently introduced by adding an appropriate coupling agent , such as PyBop or BOP reagent . This crosslink helps to maintain α-helical configurations by providing a stable helix-forcing turn . This technique has been used on several variants of glucagonlike peptide , PTH and PTH-related peptide . Positioning lactam bridges at i and i + 3 , as well as i and i + 7 , results in the stabilisation of β-sheet and α-helices , respectively . 5 Zilucoplan ( formerly RA-101495 , Figure 2 ) is a synthetic , macrocyclic lactam-constrained peptide . It is currently in late Phase III clinical trials with Ra Pharmaceuticals for generalised myasthenia gravis , immune-mediated necrotising myopathy , and other tissue-based complement-mediated disorders with an unmet medical need . It binds complement component 5 ( C5 ) with sub-nanomolar affinity and allosterically inhibits its cleavage into C5a and C5b upon activation of the classical , alternative or lectin pathways . 6 Balixafortide ( POL-6326 , Figure 3 ), which is being developed by Polyphor in Switzerland , is a macrocyclic peptide with a head-to-tail cyclisation that has a secondary constraint with a disulfide bond . It was designed using the horseshoe crab antimicrobial peptide polyphemusin as the starting scaffold . 7 Through a series of design iterations , Balixafortide was created , resulting in a potent and highly selective blocker of CXCR4 . CXCR4 belongs to the transmembrane receptor class of G-coupled protein receptors ( GPCR ). It is expressed in a variety of cancer types and in the local tumour microenvironment by cells of the immune system . It is also currently in late Phase III clinical trials . 8
Stapled peptides
Stapling technology , or ring-closing metathesis , evolved from the Verdine
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