Design and synthesis of conformationally constrained Grb2 SH2 domain binding peptides employing α-methylphenylalanyl based phosphotyrosyl mimetics

Previous work has shown that incorporation of either I-aminocyclohexanecarboxylic acid (Ac(6)c) or alpha-methyl-p-phosphonophenylalanine ((alpha-Me)Ppp) in the phosphotyrosyl (pTyr) C-proximal position (pY + 1 residue) of Grb2 SH2 domain binding peptides confers high affinity. The tetralin-based (S)-2-amino-6-phosphonotetralin-2-carboxylic acid (Atc(6-(POH2)-H-3)) simultaneously presents structural features of both (a-Me)Ppp and Ac(6)c residues. The current study compares the affinity of this tetralin hybrid Atc(6-PO3H2) versus Ac(6)c and (alpha-Me)Ppp residues when incorporated into the pY + I position of a high-affinity Grb2 SH2 domain binding tripeptide platform. The highest binding affinity (K-D = 14.8 nM) was exhibited by the (alpha-Me)Ppp-containing parent, with the corresponding Ac(6)c-containing peptide being nearly 2-fold less potent (K-D = 23.8 nM). The lower KD value was attributable primarily to a 50% increase in off-rate. Replacement of the Ac(6)c residue with the tetralin-based hybrid resulted in a further 4-fold decrease in binding affnity (K-D = 97.8 nM), which was the result of a further 6-fold increase in off-rate, offset by an approximate 45% increase in on-rate. Therefore, by incorporation of the key structural components found in (alpha-Me)Ppp into the Ac(6)c residue, the tetralin hybrid does enhance binding on-rate. However, net binding affinity is decreased due to an associated increase in binding off-rate. Alternatively, global conformational constraint of an ((alpha-Me)Ppp-containing peptide by beta-macrocyclization did result in pronounced elevation of binding affinity, which was achieved primarily through a decrease in the binding off-rate. Mathematical fitting using a simple model that assumed a single binding site yielded an effective KD of 2.28 nM. However this did not closely approximate the data obtained. Rather, use of a complex model that assumed two binding sites resulted in a very close fit of data and provided KD values of 97 pM and 72 nM for the separate sites, respectively. Therefore, although local conformational constraint in the pY + 1 residue proved to be deleterious, global conformational constraint through beta-macrocyclization achieved higher affinity. Similar beta-macrocyclization may potentially be extended to SH2 domain systems other than Grb2, where bend geometries are required.