Biomedical Engineering Reference
In-Depth Information
Although a high degree of homology is evident between insulins from various species, the same
is not true for proinsulins, as the C peptide sequence can vary considerably. This has therapeutic
implications, as the presence of proinsulin in animal-derived insulin preparations can potentially
elicit an immune response in humans.
11.2.3 The insulin receptor and signal transduction
The insulin receptor is a tetrameric integral membrane glycoprotein consisting of two 735 amino
acid
-chains. These are held together by disulfi de linkages
(Figure 11.2). The α-chain resides entirely on the extracellular side of the plasma membrane and
contains the cysteine-rich insulin-binding domain.
Each β-subunit is composed of three regions: the extracellular domain, the transmembrane
domain and a large cytoplasmic domain that displays tyrosine kinase activity. In the absence of
bovine insulin, tyrosine kinase activity is very weak. Proteolytic digestion of the α-subunit results
in activation of this kinase activity. It is believed that the intact
α
-chains and two 620 amino acid
β
-subunit exerts a negative infl u-
ence on the endogenous kinase of the β-subunit and that binding of insulin, by causing a confor-
mational shift in
α
-subunits, relieves this negative infl uence.
The cytoplasmic domain of the β-subunit displays three distinct sub-domains: (a) the 'juxtam-
embrane domain', implicated in recognition/binding of intracellular substrate molecules; (b) the
tyrosine kinase domain, which (upon receptor activation) displays tyrosine kinase activity; (c) the
C-terminal domain, whose exact function is less clear, although site-directed mutagenesis studies
implicate it promoting insulin's mitogenic effects.
The molecular mechanisms central to insulin signal transduction are complex and have yet to
be fully elucidated. However, considerable progress in this regard has been made over the last
decade. Binding of insulin to its receptor promotes the autophosphorylation of three specifi c ty-
rosine residues in the tyrosine kinase domain (Figure 11.2b). This, in turn, promotes an alteration
in the conformational state of the entire
α
-subunit, unmasking adenosine triphosphate (the phos-
phate donor) binding sites and substrate docking sites and activating its tyrosine kinase activity.
Depending upon which specifi c intracellular substrates are then phosphorylated, at least two
different signal transduction pathways are initiated (Figure 11.2c). Activation of the 'mitogen-
activated protein kinase' pathway is ultimately responsible for triggering insulin's mitogenic ef-
fects, whereas activation of the PI-3 kinase pathway apparently mediates the majority of insulin's
metabolic effects. Many of these effects, in particular the mitogenic effects, are promoted via
transcriptional regulation of insulin-sensitive genes, of which there are probably in excess of 100
(Table 11.2).
β
11.2.4 Insulin production
Traditionally, commercial insulin preparations were produced by direct extraction from pan-
creatic tissue of slaughterhouse pigs and cattle, followed by multistep chromatographic purifi -
cation. However, the use of animal-derived product had a number of potential disadvantages,
including:
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