Insulin is a hormone that is secreted by the pancreas in response to an increased level of glucose in the blood, usually after a meal. Insulin stimulates the muscles and adipose tissue to take up and convert it to energy or to store the excess glucose. is a dipeptide that contains a A and B chain. The A chain has an N-terminal helix linked to an anti-parallel C-terminal helix. The B chain has a central helical segment. The two chains are connected by 3 di-sulfide bonds that join the N- and C-terminal helices of the A chain to the central helix of the B chain [1]. When the concentration of glucose in the blood drops, insulin is no longer needed and an insulin-degrading enzyme is produced in order to reduce the amount of insulin in the body.
The insulin-degrading enzyme (IDE) is a highly conserved protease that uses as a cofactor in breaking down insulin and amyloid beta-proteins [2]. IDE can be found predominantly in the cytosol, however it is also located in the cell membrane, secreted into the extracellular regions and is present at the cell surfaces of neuron cells in the brain. Insulin-degrading enzyme is also known as insulysin or insulinase and is active at neutral pH. It can be located in red blood cells, skeletal muscle, liver and brain.
Structure
The structure of IDE is a with two N-terminal domains, which forms the catalytic site and two C-terminal domains that facilitates the substrate binding [3]. The N-terminal domains are connected to the C-terminal domains via a 28-residue loop that forms a chamber that is shaped like a triangular prism.
Domain 1 houses the with two histidine's and one glutamine(his 108, his 112 and glu 198), the of a glutamine (Glu 111), ATP binding site (Arg 429) and the Zn2+ ion cofactor. Several residues of domains 1 & 4 create a polar area of the triangular cavity, while residues of domains 2 & 3 create a nonpolar region of the cavity.
There are two conformations for the enzyme, open and closed. In the open conformation, the insulin protein enters the enzyme opening causing a conformational change that allows the enzyme to fully recognize the protein and catalyzes protein degradation.
Function
Once the insulin molecule enters the active site and is recognized, ATP binds to the appropriate site and the enzyme changes conformation from the open state to the closed state and begins to unfold the insulin and makes two initial cleavages, one each in the middle of both the A and B chains. The enzyme then makes six more cleavages. One cleavage site right next to the first one on the A chain and 5 more on the B chain [4]. Three near the middle and two near the C-terminus. There are no cleavage sites that are near the N-terminus of either chain.
Disease
A mutation of Glu 111 in the active site will render the enzyme inactive or a mutation at Pro 286 will slow down the enzymatic activity. A low functioning or inactive insulin degrading enzyme will allow the buildup of both insulin and amyloid beta-proteins.
For insulin, if it is allowed to build up, insulin resistance can occur and contribute to the development of type II diabetes. A mutation at Asp 34 will cause Hyperproinsulinemia [5], a disease where the body secretes insulin before it has been fully processed (proinsulin) and so does not function properly.
Several different mutations at birth or a young age can contribute to the onset of neonatal diabetes or type I diabetes. The locations are: Asp 24, Arg 32, Ser 32, Gly 43, Val 47, Cys 48, Cys 89, Cys 90, Tyr 96 and Cys 108.
The buildup of amyloid beta-proteins in the brain has been determined to cause the onset of Alzheimer’s disease [6][7]. The onset of Alzheimer’s can also be contributed to the mutation of Ile 714.
