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Insulin is the major anabolic hormone in all 'higher' organisms involved in regulating the
uptake of glucose by cells of the body, amino acid synthesis and the conversion of
carbohydrate into triacylglycerols.
In diabetes mellitus, due to failure of insulin secretion or action, patients are unable to
utilise glucose properly and also to synthesise triacylglycerols from carbohydrates or
amino acids. Symptoms include excessive thirst (caused by the changed osmotic potential of
the blood), excretion of glucose in the urine (makes it taste sweet) and loss of weight
(as body energy reserves are depleted).
Demonstration of the role of the pancreas in diabetes mellitus
was first provided by Mering and Minkowski in 1889 when they
produced the condition in a dog by removal of the pancreas.
However, attempts to extract the implicated hormone did not
succeed until Banting and Best prepared an active extract from
the Islets of Langerhans which they named insulin. The discovery
of insulin dramatically changed the outlook for millions of
diabetics worldwide, who previously would have died a few years
after the onset of the illness but now could be stabilized on
daily injections of insulin purified from pig or cow pancreas
(porcine and bovine insulin respectively).
The complete amino acid sequence of porcine insulin was
determined in a herculean feat by Sanger and co-workers in 1955.
Porcine insulin was shown to contain 51
amino acids, arranged in two chains (an acidic A-chain of 21
residues and a basic B-chain of 30 residues).
Here are the amino acid sequences of the A and B chains of porcine insulin:
Circular dichroism (CD) has been used widely in the study of
insulin conformation in solution.
Here is the far UV spectrum of insulin:
Although insulin was first crystallized and made available for
clinical use by Abel and co-workers in 1925, it was not until
1934 that Scott discovered that the rhombohedral crystals were
a zinc-insulin complex. Together with Fischer, Scott showed that
the zinc might be replaced by other divalent ions. In time, this
opened up the possibility that the crystal structure might be
solved by isomorphous replacement. An initial X-ray analysis of
porcine zinc insulin at 2.8Å was completed in 1969 by Dorothy
Hodgkin and co-workers in Oxford.
for the obituary of Dorothy Hodgkin by Max Perutz, at the
Crystallography section of the World-Wide-Web Virtual Library.)
The insulin structure was refined to 2.5Å by the Peking Insulin
Structure Research Group in 1971. Much of the recent
work on insulin structure has been carried out by Professor Guy Dodson and colleagues
at York University in collaboration with Novo Nordisk of Denmark, by Professor
Michael Weiss and colleagues in Chicago in collaboration with Eli Lilly and by
Professor Axel Wollmer and colleagues in Germany.
2-zinc insulin has now been refined to 1.1Å resolution; in this
structure, the positions of water molecules determined from
neutron scattering are included.
The 2-Zinc Insulin Crystal (Porcine Insulin)
Click on one of the following to obtain the PDB structure file 4ins from the copy of the PDB nearest to you:
In this crystal form, the unit cell contains 2 zinc ions and 6 insulin molecules.
The asymmetric unit consists of 2 insulin
molecules, (designated 'molecule 1' and 'molecule 2'), which are different.
The asymmetric unit is presented and the hexamer can be generated by applying
a three-fold symmetry operation through the crystallographic axis on which
the two zinc ions lie.
Discussion Topic 1
Does this crystal structure contain
We are going to study the secondary structure of molecule 1. Download the
following script file (4ins_sc1.txt), and
input into RasMol, with the command:
Discussion Topic 2
Use the RasMol command 'structure' to calculate the secondary
structure according to the Kabsch and Sander's hydrogen bond-based
DSSP algorithm. How does this compare with the secondary structure designation
in the PDB file?
Many references use a colour scheme of blue for the A-chain and red for the
B-chain. Use the following commands to achieve this (assuming you have
already run 4ins_sc1.txt, and to remove the labels
(you can copy and paste these into your RasMol command line window, or
download and then input to RasMol the file
select cys*a, cys*b
Discussion Topic 3
Why are the three glycine residues with positive phi dihedral angles important
for the insulin fold?
A6 Cys, A11 Cys, A16 Leu, A20 Cys, B15 Leu, B19 Cys
select sidechain and 24-26b
select sidechain and (13-14a,1-2b,14b,18b)
How could insulin delivery by infusion pumps be improved?
Now look at the dimer in the direction perpendicular to the
approximate two-fold axis (Figure 1).
Note that the residues involved in dimerization are in the beta-
stranded (B24-B30) structure of the monomer.
In the presence of zinc ions, three identical insulin dimers
assemble into a hexamer in which two zinc ions lie on the central
three-fold axis. The
approximate two-fold axis within each dimer is perpendicular to
and intersects the three-fold axis.
The hexamer has been generated for you by applying a rotational symmetry
operation about the three-fold axis to the dimer (the asymmetric unit); the
solvent molecules are not included.
Obtain the hexamer structure file from either
which will be served with chemical MIME, or from the
(not chemical MIME-stamped).
The chains have been labelled as follows:
The following commands (script file 4ins_sc6.txt)
display the 6 monomers as ribbons (A-chains in blue,
B-chains in red). The 6 B10 His side chains are shown and
the 2 zinc ions are coloured green.
select *a, *c, *g, *i, *m, *o
select *b, *d, *h, *j, *n, *p
Last updated 11th April '96