Structure, function, and regulation of insulin-degrading enzyme

Abstract

The short half-life of insulin in the human body (4-6 min) prompted the search and discovery of insulin-degrading enzyme (IDE), a 110-kDa metalloprotease that can rapidly degrade insulin into inactive fragments. Genetic and biochemical evidence accumulated in the last sixty years has implicated IDE as an important physiological contributor in the maintenance of insulin levels. Recent structural and biochemical analyses reveal the molecular basis of how IDE uses size and charge distribution of the catalytic chamber and structural flexibility of substrates to selectively recognize and degrade insulin, as well as the regulatory mechanisms of this enzyme. These studies provide a path for potential therapeutics in the control of insulin metabolism by the degradation of insulin.

Figure 1. Structural conformations of IDE and the presence of a crypt

A. IDE is a 110,000 Da metalloprotease comprised of two roughly equal-sized domains; IDE-N and IDE-C. IDE-N is the location of several regions that are important in the interaction of substrate to IDE including the exosite and the active site. The active site is also the location of the catalytic zinc ion. The presence of a loop joins the two domains together. B. When closed, IDE forms a crypt with a volume of 15,700 Ǻ³ (Malito et al., 2008)which restricts amyloidogenic peptides to less than 70 amino acids in length. In the closed state, entrance to the catalytic and active site are occluded and substrates may not enter or leave. Structure PDB 2JG4.

Figure 2. Sequence, substrate cleavage and structure of IDE substrates involved in glucose metabolism

¹ Duckworth et al., 1998, ² Bennett et al., 2003, ³ Kurochkin, 2000. PBDs; Full chain insulin 1ZNI, glucagon 1GCN, and partial amylin (residues SNNFGAILSS) 1KUW. Arrows indicate sites of cleavage (Grasso et al., 2007, Shen et al., 2006), underlined arrows correspond to P1-P1′ sites of substrate verified in structures 2G56 (Zn²⁺ free IDE bound to insulin B chain), 2G48 (amylin bound to IDE) and 2G49 (glucagon bound to IDE). Disulfide bonds are indicated by lines joining indicated cysteine residues.

Figure 3. Regulators of IDE

Multiple regulatory mechanisms control the ability of IDE to degrade insulin.