Cathepsin B & Synthetic Substrates

[Louis Hom]

So I've been looking at a viral protease that is homologous to the
cathepsins, and has the substrate specificity of cathepsin B (i.e., it
prefers an Arg 2 aa's N-terminal to the site of cleavage).  The researchers
used Z-Arg-Arg-MCA and some other synthetic substrates to demonstrate this.
What sort of puzzles me is why these experiments almost always use
Z-Arg-X-MCA rather than Z-Lys-X-MCA.  After all, the crystal structure
suggests a preference for the positive charge of Arg more than, say, its
shape.  Any ideas?  Is there some esoteric quirk involving difficulty in
synthesis?  
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_______________________________________________________________________________
Lou Hom >K '93		       	    	"If brevity be the soul of wit, 
lhom at nature.berkeley.edu  			   play on!" 
http://www.ocf.berkeley.edu/~lhom