when is a protein not a peptide?

PandaGroup pandagroup at aol.com
Sat Dec 9 13:24:44 EST 1995

Which came first , the chicken or the peptide?  I think you'll find some
differing opinions as to peptide/protein criteria.  I ran into it again
when writing a report on biopharmaceutical technology this past month.  

When I was an undergrad, my biochem prof set a cut-off of MW=10,000 for
proteins versus peptides ... this is about 80 or so aa.  But insulin has a
MW~6,000 and has two chains ... so ... peptide or protein?  I vote protein
... but this is based on (a) structure and (b) definite function.  Want to
really muddy the waters?  Add the term "polypeptide."  

Over the years I've shifted toward structural/functional definitions for
the most part, except where really small (10mers, 20mers) come into play
... then they're all peptides.  On the other hand, construct a
non-biological chain of aa's to a weight of 50,000 ... with no defined
function... and that's a polypeptide.  A biological combination of aa's of
that length is a protein ... by definition, if it's produced in a
biological system it has to have function ... even if we can recognize it.

Maybe I should also add "origin" as a criterion.  Synthetic stuff
classifies as peptides .. no matter what.  Biological stuff ... even if
manufactured in a lab .. gets the peptide/protein option.

Anyway, those are my thoughts ... so long as I leave it clear with you
that many times I'm left making judgement calls .. that somebody else may
disagree with.  That's fine ... it's not worth arguing over ... a rose is
a rose.
Dr. Ken Krul              "Cole's Law - Thinly sliced cabbage"
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