Engineering Insulin For Better Treatment of Diabetes
This Jmol Exploration was created using the Jmol Exploration Webpage Creator from the MSOE Center for BioMolecular Modeling.
Insulin consists of two chains; the A chain has 21 amino acids and the B chain has 29 amino acids. Thus, insulin is a protein with quaternary structure. The two subunits are held together by two interchain disulfide bonds; an additional intrachain disulfide bond is found on chain A. Insulin helps to regulate glucose homeostasis in the blood. The active form is a monomer, but insulin is also stored in the body in an inactive hexamer form.
In the following structures, the A chain is purple, the B chain is orange, and disulfide bonds are shown in yellow. The N terminus of each chain is blue, and the C terminus of each chain is red. Zinc atoms are light steel blue.
insulin monomer PDB ID: 1aiyInsulin is released from beta cells in the pancreas in response to increased glucose levels in the blood. In type I diabetes, the pancreas can no longer make insulin, and constant high blood glucose levels lead to serious consequences, including poor circulation and damage to many internal organs. Although porcine (pig) and bovine (cow) insulin are similar to human insulin and have been used to treat diabetes since the 1920's, human insulin was the first 'drug' to be produced in bacteria in the 1980's.
Unfortunately, injections of insulin prior to each meal aren't as finely tuned to fluctuating glucose levels as the pancreas beta cells. Glucose levels also fluctuate in response to exercise levels. Researchers have made modifications to the human insulin sequence to produce insulin molecules that act rapidly, intermediately, or over a long period of time. Combinations of these various forms of insulin can be injected or delivered with an insulin pump, improving the quality of life for diabetics. Below we explore some of these modifications and how they impact the delivery of insulin.
By inhibiting the formation of insulin hexamers, rapid acting insulin injections are able to signal glucose uptake within 10-30 min. (Normal human insulin (Humulin or Novolin) respond within 30 min to 1 hour of injection.) All the substitutions in rapid acting insulins are made on the insulin B chain.
Humalog (lispro) P28K, K29P PDB ID: 2mpgApidra (insulin glulisine) is another rapid-acting insulin with changes R3L and L29E of the insulin B chain.
Human insulin is mixed with protamine and zinc in this formulation (NPH). These additions help to maintain the hexamer structure of insulin, so it is released more slowly and acts over a longer period of time. This is useful for overnight regulation of sugar levels, or is used for about half the day.
The logic behind the design of long acting insulin derivatives is to either 1) create a microprecipitate that slowly dissolves, releasing insulin over a long period or 2) binding the insulin to other substances in the body.
Lantus (insulin glargine) has two changes to the A chain: N21G and the addition of 2 arginine residues at the C terminus. This formulation is soluble at pH 4.0, but forms tiny microprecipitates in the blood.
Levemir (insulin detemir) has myristic acid (a 14 carbon fatty acid) attached to L29 of chain B. A crystal structure of a similar insulin, with a different fatty acid bound to L29, is shown below.