The role of the 8-18 helix of CGRP8-37 in mediating high affinity binding to CGRP receptors; coulombic and steric interactions.
1. The role of individual residues in the 8-18 helix of CGRP(8-37) in promoting high-affinity binding to CGRP(1) receptors expressed on rat L6 and human SK-N-MC cells has been examined. The relative potencies of various derivatives were estimated from their ability to inhibit the human alphaCGRP-mediated increase in cyclic AMP production and the binding of [(125)I]-human alphaCGRP. 2. Arg(11) and Arg(18) were replaced by serines to give [Ser(11,18)]CGRP(8-37). These bound with pKi values <6 to SK-N-MC cells and had apparent pA(2) values of 5.81+/-0.04 and 5.31+/-0.11 on SK-N-MC and L6 cells. CGRP(8-37) had a pKi of 8.22 on SK-N-MC cells and pK(b) values on the above cell lines of 8.95+/-0.04 and 8.76+/-0.04. 3. The arginines were replaced with glutamic acid residues. [Glu(11)]CGRP(8-37) had a pK(b) of 7.14+/-0.14 on SK-N-MC cells (pKi=7.05+/-0.05) and 6.99+/-0.08 on L6 cells. [Glu(18)]CGRP(8-37) had a pK(b) of 7.10+/-0.0.08 on SK-N-MC cells (pKi=6.91+/-0.23) and 7.12+/-0.09 on L6 cells. 4. Leu(12), Leu(15) and Leu(16) were replaced by benzoyl-phenylalanine (bpa) residues. On SK-N-MC cells, the apparent pA(2) values of [bpa(12)]-, [bpa(15)]- and [bpa(16)]CGRP(8-37) were respectively 7.43+/-0.23, 8.34+/-0.11 and 5.66+/-0.16 (pKi values of 7.14+/-0.17, 7.66+/-0.21 and <6): on L6 cells they were 7.96+/-0.36, 8.28+/-0.21 and 6.09+/-0.04 (all n=3). 5. It is concluded that the Arg(11) and Arg(18) are involved in specific electrostatic interactions with other residues, either on the CGRP(1) receptors or elsewhere on CGRP(8-37). Leu(16) is in a conformationally restricted site when CGRP(8-37) binds to CGRP(1) receptors, unlike Leu(12) and Leu(15).
Howitt
Stephen G
SG
Pharmaceutical Sciences Research Institute, Aston University, Birmingham, B4 7ET.
Kilk
Kalle
K
Wang
Yang
Y
Smith
David M
DM
Langel
Ulo
U
Poyner
David R
DR
D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000378:metabolism / Q000502:physiology*; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006801:Humans; D006860:Hydrogen Bonding; D010446:Peptide Fragments / Q000378:metabolism / Q000502:physiology*; D011485:Protein Binding / Q000502:physiology; D017433:Protein Structure, Secondary / Q000502:physiology; D051381:Rats; D018015:Receptors, Calcitonin Gene-Related Peptide / Q000378:metabolism*; D055672:Static Electricity; D013237:Stereoisomerism; D014407:Tumor Cells, Cultured
D010446:Peptide Fragments; D018015:Receptors, Calcitonin Gene-Related Peptide; C058636:calcitonin gene-related peptide (8-37); D000242:Cyclic AMP; D015740:Calcitonin Gene-Related Peptide