Glucosepane
Encyclopedia
Glucosepane is a lysine
-arginine
protein
cross-link
ing product and advanced glycation end product (AGE) derived from D-glucose
. It is an irreversible, covalent cross-link product that has been found to make intermolecular and intramolecular cross-links in the collagen
of the extracellular matrix
(ECM) and crystallin
of the eyes. Covalent protein cross-links irreversibly link proteins together in the ECM of tissues. Glucosepane is currently considered to be the most important cross-linking AGE yet discovered because its levels are found to be 10 to 1000 times higher in human tissue than any other cross-linking AGE.
, or non-enzymatic browning. Glucosepane is found to form through a non-oxidative path. The exact mechanism leading to glucosepane has been a challenge for researchers to determine. However, it is currently well characterized up to the ring formation, for which there is a solid hypothesis as to the mechanism though no current proof.
, which then rearranges
through elimination of water to form the more stable aminoketose Amadori product. From there, the stable Amadori Product slowly degrades to form glucosepane through an α-dicarbonyl intermediate.
and the elimination of the C-4 hydroxyl. Further, evidence was given for the extent of the hypothesized carbonyl shift by using heavy hydrogen in the solvent water, D2O. Researchers found that all the H-C-OH of the carbon backbone were converted to D-C-OH after the reaction, demonstrating that all the hydrogens got transferred out through keto-enol tautomerism, and thus the carbonyl shift went all the way down the backbone, finally eliminating the C-4 hydroxy group.
6-(3,4-dihydroxy-6-oxo-3,4,5,6-tetrahydro-2H-azepinium-1-yl) norleucine. The ring is hypothesized to form by a nucleophilic attack of N on C-6 carbonyl, followed by elimination of a water (2). This then condenses with the arginine side chain to yield glucosepane in nucleophilic addition-elimination reactions of the nitrogens of arginine and the electrophilic carbonyls on the ring, eliminating two waters.
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
-arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
ing product and advanced glycation end product (AGE) derived from D-glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
. It is an irreversible, covalent cross-link product that has been found to make intermolecular and intramolecular cross-links in the collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
of the extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
(ECM) and crystallin
Crystallin
In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast cancer tumors....
of the eyes. Covalent protein cross-links irreversibly link proteins together in the ECM of tissues. Glucosepane is currently considered to be the most important cross-linking AGE yet discovered because its levels are found to be 10 to 1000 times higher in human tissue than any other cross-linking AGE.
Role in aging
Aging leads to progressive loss of elasticity and stiffening of tissues rich in the ECM such as joints, cartilage, arteries, lungs and skin. It has been shown that these effects are brought about by the accumulation of cross-links in the ECM on long-lived proteins. Studies done on glucosepane by the Monnier group have shown that the level of glucosepane cross-links in human collagen in the ECM increases progressively with age and at a more rapid pace in people with diabetes, thus suggesting the role of glucosepane in the long-term effects associated with diabetes and aging such as arteriosclerosis, joint stiffening and skin wrinkling. In fact, they report that in the ECM of the skin of a non-diabetic 90-year-old, glucosepane accounts for about 50 times the protein cross-linking as all other forms of protein cross-linking. Further, the build up of cross-links such as glucosepane within and between proteins is shown to reduce proteolytic degradation in the ECM. This leads to increased cross-link accumulation and is thought to be linked to the thickening of basement membranes in capillaries, glomeruli, lens, and lungs.Formation
As an AGE, the reaction pathway that leads to glucosepane formation is known as the Maillard ReactionMaillard reaction
The Maillard reaction is a form of nonenzymatic browning similar to caramelization. It results from a chemical reaction between an amino acid and a reducing sugar, usually requiring heat....
, or non-enzymatic browning. Glucosepane is found to form through a non-oxidative path. The exact mechanism leading to glucosepane has been a challenge for researchers to determine. However, it is currently well characterized up to the ring formation, for which there is a solid hypothesis as to the mechanism though no current proof.
Overall reaction pathway
The overall pathway of glucosepane formation starts with lysine attacking the reducing sugar D-glucose to form the unstable imine known as a Schiff baseSchiff base
A Schiff base, named after Hugo Schiff, is a compound with a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group, not hydrogen....
, which then rearranges
Amadori rearrangement
The amadori rearrangement is an organic reaction describing the acid or base catalyzed isomerization or rearrangement reaction of the N-glycoside of an aldose or the glycosylamine to the corresponding 1-amino-1-deoxy-ketose...
through elimination of water to form the more stable aminoketose Amadori product. From there, the stable Amadori Product slowly degrades to form glucosepane through an α-dicarbonyl intermediate.
Mechanism of α-dicarbonyl formation from the Amadori product
The particular reaction path proceeding from the Amadori product to the α-dicarbonyl intermediate that will yield glucosepane was difficult to determine. Initially, researchers hypothesized an α-dicarbonyl intermediate in which the carbonyls were located on C-2 and C-3 of D-Glucose. However, by using glucose with C-1, the carbonyl carbon, marked with the isotope 13C in the reaction, researchers found that the α-dicarbonyl formed has the carbonyls located at C-5 and C-6 of the original glucose backbone. The best mechanism proposed is that the α-dicarbonyl N 6-(2,3-dihydroxy-5,6-dioxohexyl)-L-lysinate, a key intermediate in the glucosepane reaction, forms from the Amadori product through a carbonyl shift all the way down the 6 carbon sugar backbone by keto-enol tautomerismKeto-enol tautomerism
In organic chemistry, keto-enol tautomerism refers to a chemical equilibrium between a keto form and an enol . The enol and keto forms are said to be tautomers of each other...
and the elimination of the C-4 hydroxyl. Further, evidence was given for the extent of the hypothesized carbonyl shift by using heavy hydrogen in the solvent water, D2O. Researchers found that all the H-C-OH of the carbon backbone were converted to D-C-OH after the reaction, demonstrating that all the hydrogens got transferred out through keto-enol tautomerism, and thus the carbonyl shift went all the way down the backbone, finally eliminating the C-4 hydroxy group.
Ring closure to arginine cross-linking
It is still relatively unclear how the ring is formed and when. One article suggests, and it seems the current belief, that the ring must form in the step after the α-dicarbonyl is formed. The study hypothesized, and another found correlating evidence, that the most likely mechanism of getting from the α-dicarbonyl to glucosepane is through the intramolecular aldimineAldimine
In organic chemistry, an aldimine is an imine that is an analog of an aldehyde.As such, aldimines have the general formula R–CH=N–R. Aldimines are similar to ketimines, which are analogs of ketones.An important subset of aldimines are the Schiff bases,...
6-(3,4-dihydroxy-6-oxo-3,4,5,6-tetrahydro-2H-azepinium-1-yl) norleucine. The ring is hypothesized to form by a nucleophilic attack of N on C-6 carbonyl, followed by elimination of a water (2). This then condenses with the arginine side chain to yield glucosepane in nucleophilic addition-elimination reactions of the nitrogens of arginine and the electrophilic carbonyls on the ring, eliminating two waters.
Accumulation
Glycation processes that lead to AGEs particularly affect long-lived proteins in the human body, such as collagen in the skin and crystallin in the eyes. Skin collagen, for instance, has a half-life of fifteen years. Because thee proteins do not degrade as quickly as other proteins in the body, the Amadori product, which is stable and thus transforms very slowly, has time enough to convert into glucosepane. It has been estimated that 50-60% of the steady state levels of Amadori product is converted into glucosepane in old age. A suspected reason for the prevalence of the glucosepane cross-link product as opposed to others is that the α−dicarbonyl from which it forms, N 6-(2,3-dihydroxy-5,6-dioxohexyl)-L-lysinate, is a persisting glycating agent because it is irreversibly bound through lysine to a protein. Therefore, it is not easily degraded and thus more commonly available to form a cross-links with arginine, unlike other cross-link a-dicarbonyl intermediates, which are found bound and free and thus more susceptible to being degraded by enzymes in the ECM.Prospects for inhibition or removal
Because of the important role glucosepane has been found to play in many pathologies of aging, many researchers have been investigating ways in which the levels of glucosepane could be reduced in tissues. Various methods of doing so have been examined.α-Dicarbonyl trap
One method attempted to inhibit glucosepane formation is to use an α-dicarbonyl trap molecule, aminoguanidine (AG). AG reacts with the α-dicarbonyl intermediate with a higher affinity than arginine, thus blocking the cross-link. While this method has been seen to have some success, it did not greatly interfere with the normal aging of rats.Thiazolium salts
Another method that has been investigated is the use of thiazolium salts to break the a-dicarbonyl intermediate, therefore cutting off the reaction pathway that leads to glucosepane. These compounds are thought to act as bidentate nucleophiles that attack the adjacent carbonyls in the alpha-dicarbonyl intermediate, which then leads to the cleaving of the C-C bond between the carbonyls. However, an alternate hypothesis as to how they work is that they act as chelating agents. Two thiazolium molecules, PTB (N-phenacylthiazolium bromide) and ALT-711, have demonstrated success at reducing glucosepane levels in rats.ECM turnover
A completely different approach to reducing cross-links that has been proposed is enhancing the ECM turnover processes, which would force the degradation of cross-linked proteins to replace them with new. However, a potential downside to this would be leaky blood vessels resulting from too far enhanced turnover.See also
- GlycationGlycationGlycation is the result of the bonding of a protein or lipid molecule with a sugar molecule, such as fructose or glucose, without the controlling action of an enzyme. All blood sugars are reducing molecules. Glycation may occur either inside the body or outside the body...
- GlycosylationGlycosylationGlycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
- Advanced glycation end product
- Maillard ReactionMaillard reactionThe Maillard reaction is a form of nonenzymatic browning similar to caramelization. It results from a chemical reaction between an amino acid and a reducing sugar, usually requiring heat....