The rate of s-cis/s-trans isomerization in angiotensin II is at least 70-fold greater than in His-Pro and is not rate limiting in receptor binding.
The dipeptide L-histidyl-L-proline interconverts between the s-cis and s-trans rotational isomers of the amide bond with an average rate constant of 2.0 ks-1 at 22.5 degrees. This rate is independent of pH throughout the pH range 5.5-8.5 as determined by pH jump from low pH with potentiometric recording of the slow approach to equilibrium pH, and by integration of the histidine C-2 and C-4 proton n.m.r. resonances as a function of time following pD jump from low pD. From the temperature dependence of the n.m.r. experiments this interconversion has an Arrhenius activation energy of 20 kcal X mol-1 and a free energy of activation of 21 kcal X mol-1, similar to other simple acyl- and aminoacylprolines. The octapeptide angiotensin II containing L-histidyl-L-proline at position 6-7 interconverts at least 70-fold faster than the free dipeptide. This rate is too fast to be rate limiting in the binding of angiotensin II to its receptor in rat adrenal cortex.
D000804:Angiotensin II; D004151:Dipeptides; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D013816:Thermodynamics
D004151:Dipeptides; D000804:Angiotensin II; C035699:histidylproline