The helix nucleation site and propensity of the synthetic mitochondrial presequence of ornithine carbamoyltransferase

This study describes the helix nucleation site and helix propagation of the amphiphilic helical structure of the mitochondrial presequence of rat orni thine carbamoyltransferase. We investigated this property of the 32-residue synthetic presequence using CD and 2D-HR NMR techniques by determining the structure as a function of the concentration of trifluoroethanol. It was f ound that the hydrophobic cluster Ile7-Leu8-Leu9 forms the helix nucleation site, expanding to include residues Asn4 to Lys16 when the concentration o f trifluoroethanol is increased from 10 to 30%. At higher trifluoroethanol concentrations an increased 'stiffening' of the polypeptide backbone (to Ar g26) is observed. In addition, by recording CD spectra at different trifluo roethanol concentrations as a function of temperature, it was found that th e equilibrium constant between helix and random coil formation for this pep tide exhibits a strong temperature dependence with maximum values between 2 0 and 30 degrees C. Comparison of these equilibrium constants with those of homopolymers stressed the unique character of the mitochondrial presequenc e. The findings are discussed in relation to the molecular recognition even ts at different stages of the transport process of this protein into mitoch ondria.