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Glycated Proteins and Hb A1C

There are many possible post-translational modifications to proteins after ribosomal synthesis. Glycated proteins are examples of modified proteins and are formed by the addition of glucose molecules to amino acid chains. Hb A1C is an important glycated protein assayed to diagnose and monitor diabetes. Fructosamine is another less assayed modified protein.
Hb A1C
Hb A comprises the majority of normal adult hemoglobin (Hb) and includes the minor hemoglobins, Hb A1a, Hb A1b, and Hb A1c. Sometimes these three are referred to as Hb A1, but A1C is the major fraction and composes 80% of Hb A1. Following synthesis of Hb A, a nonenzymatic reaction adds glucose to the N-terminal valine on either beta chain forming glycated Hb. 
As the red blood cells circulate, an irreversible rearrangement of the pre-A1C base occurs forming a stable ketoamine, A1C. Over the life span of the red blood cells (120 days) this process continues and the concentration of A1C is proportional to the concentration of the blood glucose. The concentration of A1C then relates to an individual's average glucose over time and can be used as an index relating to the extent of carbohydrate control during a 2 - 3 month period.
There is also a direct relationship between the concentration of Hb A1C and risk of complications in diabetic patients. Therefore, the ADA has recommended using Hb A1C measurements to monitor glycemic control. 
The Hb A1C assay is currently considered the standard biomarker for glycemic management. In the past, there was a lack of standardization, however, most current Hb A1C assays are now highly standardized. ADA practice recommendations clearly indicate that the Hb A1C measurement be a National Glycohemoglobin Standardization Program (NGSP) method and traceable to the Diabetes Control and Complications Trial (DCCT) reference assay.