Glycation
Glycation (sometimes called non-enzymatic glycosylation) is the covalent attachment of a sugar to a protein or lipid. Typical sugars that participate in glycation are glucose, fructose, and their derivatives. Glycation is the non-enzymatic process responsible for many (e.g. micro and macrovascular) complications in diabetes mellitus and is implicated in some diseases and in aging. Glycation end products are believed to play a causative role in the vascular complications of diabetes mellitus.In contrast with glycation, glycosylation is the enzyme-mediated ATP-dependent attachment of sugars to protein or lipid. Glycosylation occurs at defined sites on the target molecule. It is a common form of post-translational modification of proteins and is required for the functioning of the mature protein.

Biochemistry
Glycations occur mainly in the bloodstream to a small proportion of the absorbed simple sugars: glucose, fructose, and galactose. It appears that fructose has approximately ten times the glycation activity of glucose, the primary body fuel.  Glycation can occur through Amadori reactions, Schiff base reactions, and Maillard reactions; which lead to advanced glycation end products (AGEs).

Biomedical implications
Red blood cells have a consistent lifespan of 120 days and are accessible for measurement of glycated hemoglobin. Measurement of HbA1c—the predominant form of glycated hemoglobin—enables medium-term blood sugar control to be monitored in diabetes.
Some glycation product are implicated in many age-related chronic diseases, including cardiovascular diseases (the endothelium, fibrinogen, and collagen are damaged) and Alzheimers disease (amyloid proteins are side-products of the reactions progressing to AGEs).Long-lived cells (such as nerves and different types of brain cell), long-lasting proteins (such as crystallins of the lens and cornea), and DNA can sustain substantial glycation over time.  Damage by glycation results in stiffening of the collagen in the blood vessel walls, leading to high blood pressure, especially in diabetes. Glycations also cause weakening of the collagen in the blood vessel walls, which may lead to micro- or macro-aneurysm; this may cause strokes if in the brain.

See also
Advanced glycation end-product
Alagebrium
Fructose
Galactose
Glucose
Glycosylation
Glycated hemoglobin
List of aging processes

Additional reading
Ahmed N, Furth AJ (July 1992). "Failure of common glycation assays to detect glycation by fructose". Clin. Chem. 38 (7): 1301–3. doi:10.1093/clinchem/38.7.1301. PMID 1623595.
Vlassara H (June 2005). "Advanced glycation in health and disease: role of the modern environment". Annals of the New York Academy of Sciences. 1043 (1): 452–60. Bibcode:2005NYASA1043..452V. doi:10.1196/annals.1333.051. PMID 16037266. S2CID 20952378.


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