Hemoglobin is the protein that transports oxygen in our blood. In the , all four heme groups bind to oxygen. Two amino acid side chains are covalently bound to a sugar molecule (they are glycated), indicating that this protein experienced high sugar levels for a long time. The sugar is is labeled fructose, but it originated from glucose. Glucose forms a glycosidic bond with the nitrogen atom of the sidechain of lysine. Then, it dehydrates to a Schiff base and undergoes Amadori rearrangement[3] (a shift in the double bond) to fructose. In hemoglobin from patients with untreated diabetes, the modifications include glucose linked to the amino terminal groups of chain A and chain B as well as modification of lysine side chains by glucose. The HbA1c test [4]specifically tests for the modification of the (not modified in this structure).
