This paper reports on an insulin analogue with 12.5-fold
receptor affinity, the highest increase observed for a
single replacement, and on its solution structure, determined
by NMR spectroscopy.
The analogue is [d-AlaB26]des-(B27–B30)-tetrapeptide-insulin-B26-amide.
C-terminal truncation of the B-chain by four (or five)
residues is known not to affect the functional properties
of insulin, provided the new carboxylate charge is neutralized.
As opposed to the dramatic increase in receptor affinity
caused by the substitution of d-Ala for the wild-type
residue TyrB26 in the truncated molecule, this very substitution
reduces it to only 18% of that of the wild-type hormone
when the B-chain is present in full length.
The insulin molecule in solution is visualized as an ensemble
of conformers interrelated by a dynamic equilibrium. The question
is whether the “active” conformation of the hormone,
sought after in innumerable structure/function studies, is or is
not included in the accessible conformational space, so that it
could be adopted also in the absence of the receptor. If there were
any chance for the active conformation, or at least a predisposed
state to be populated to a detectable extent, this chance should
be best in the case of a superpotent analogue. This was the motivation
for the determination of the three-dimensional structure of
However, neither the NMR data nor CD spectroscopic comparison
of a number of related analogues provided a clue concerning
structural features predisposing insulin to high receptor
affinity. After the present study it seems more likely
than before that insulin will adopt its active conformation
only when exposed to the force field of the receptor surface.