cis/trans Proline

[moreaut at univ-tours.fr]

dear netters,

I have a peptide substrate, the sequence of which is 
Ala-Gly-Pro-Arg-Ser-Leu
This peptide and a second one 4 residues longer at the N-ter side have
been synthezised in order to study the specificity of a serine proteinase.
The longest peptide is rapidly cleaved at the Arg/Ser bond as expected
but the efficiency of cleavage (i.e the kcat/Km ratio) is reduced for
the short peptide. My questions are:

1/ which conformation of the Pro residue can we expect for such a short 
peptide i.e cis, trans or do both forms equilibrate?
I know that cis forms of Pro sometimes occur in proteins but I do not
know what happens for short peptides.

2/ if we have 80% of cis form in the short peptide, should we still have
80% of cis form in the long peptide?

3/ is it possible that one form (i.e cis or trans) could be a better 
substrate than the other one for the protease?

Thanks in advance for your help and for any comments

*------------------------------------------------------------------*	
|  Dr. Thierry MOREAU                                              |
|                                                                  |
|  Laboratoire d'Enzymologie et Chimie des Proteines               |
|                                                                  |
|  URA CNRS 1334	           Phone: (33) 46 36 62 06         |
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|      FRANCE                                                      |
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