Metal Coordination as a Method for Templating Peptide Conformation

The use of a metal template was explored as a strategy for
controlling the conformation of a short peptide.
A CAVEAT search of the Cambridge Structural Database suggested
that peptide complexes of the Cu(II) ion may
adopt the appropriate conformation to mimic the Trp-Arg-Tyr β-turn
segment of tendamistat, a proteinaceous inhibitor
of α-amylase. Complexation of tetrapeptides containing this
sequence with the Cu(II) ion leads to an average
enhancement of 200-fold in their ability to inhibit the enzyme.
Whereas the free peptides Gly-Trp-Arg-Tyr (GWRY),
Gly-Trp-Arg-d-Tyr (GWRy), and Trp-Arg-Tyr-Gly (WRYG)
exhibit inhibition constants

in
the range of 680 to
750 μM, those for their Cu(II) complexes,

, were found to be 2.4−5.9
μM. Since Cu(II) ion is itself a potent
inhibitor of α-amylase (

= 1
μM), several methods were used to determine the inhibition constants
of the peptide
complexes. The most effective employed fixed concentrations of
both Cu(II) (20 μM) and tetrapeptides (0.4−2.0
mM), with variation of the ratio of the subject tetrapeptide to a
non-inhibitory tetrapeptide like tetraglycine (GGGG)
or Gly-Gly-Phe-Leu (GGFL). Under these conditions, almost all of
the copper ion is in the form of a peptide
complex, and the concentration of the inhibitory complex itself is
determined by the mole ratio of the peptides and
their complexation constants, K. Nonlinear regression
analysis of the data allowed consistent values of

as well
as K to be determined for each peptide. The large
enhancement in affinity induced by copper complexation
suggests
that the metal ion templates the peptides and increases the proportion
present in the bioactive β-turn conformation.