Glycation and Monitoring

What is glycation?

The non-enzymatic reaction of sugars (glucose) with nucleophilic groups.

What are advanced glycation end-products (AGEs)?

AGEs are harmful compounds formed when sugars react with proteins or other molecules through glycation and undergo further chemical reactions.

Why is glycation harmful in diabetes?

High blood glucose increases protein glycation and AGE formation, causing protein dysfunction, inflammation and blood vessel damage that contribute to diabetic complications.

What is the general structure of a monosaccharide?

A monosaccharide is a single sugar unit with a carbon chain containing a carbonyl group (aldehyde or ketone) and multiple hydroxyl (−OH) groups.

How do various structural isomers of monosaccharides arise?

Structural isomers arise when monosaccharides have the same molecular formula but differ in the arrangement of atoms, such as the position of the carbonyl group or the arrangement of the carbon chain.

How do monosaccharides form rings and produce α- and β-anomers?

The aldehyde group reacts with an OH group on carbon-5 to form a hemiacetal, producing a favourable six-membered ring.

This creates a new chiral centre, which can have two configurations: α (OH points down) or β (OH points up/out to the side).

These are called anomers. The ring-opened and ring-closed forms exist in dynamic equilibrium, allowing them to interconvert.

What is mutarotation?

Mutarotation is the interconversion of α- and β-glucopyranose through ring opening and closing.

• This equilibrium means a small amount of the open-chain aldehyde form is always present in solution.

• Glycation happens when glucose reacts with proteins, and the open-chain form of glucose contains the reactive aldehyde group needed for this reaction. The aldehyde group reacts with an amino group on the protein, often on lysine residues. A bond forms, and this produces an initial product that can rearrange into a more stable product, eventually potentially forming AGEs.

What are reducing sugars, and how are they identified?

A reducing sugar is any carbohydrate that has a free aldehyde or ketone group, allowing it to act as a reducing agent in chemical reactions.

• Glycation can only occur with reducing sugars

What is the Schiff base formation in glycation?

The reactive carbonyl group of glucose reacts with an amino group on a protein, forming an unstable Schiff base. A mixture of anomers are produced.

What is the Amadori reaction in glycation?

It is the rearrangement of the unstable Schiff base mediated by protonation / deprotonation.

• Further oxidative and non-oxidative reactions can occur to give Advanced Glycation Products (AGPs)

What are Advanced Glycation Products (AGPs)?

A group of compounds formed when initial sugar-protein glycation products undergo further chemical reactions. They can alter proteins and cause cross-linking.

• AGP = A broad term for products formed when initial glycation products undergo further reactions

• AGE = Compounds formed in later stages of glycation, often associated with tissue damage and diabetic complications

Which proteins are commonly affected by glycation?

Glycation can affect serum proteins such as albumin and structural proteins (such as α-crystallins in the eye lens)

Which amino acid is the most commonly glycated residue?

Lysine

What are the consequences of glycation?

• Glycation is recognised as a major cause of diabetic secondary complications, including nephropathy (damage to kidneys), retinopathy, diabetic cataracts, and atherosclerosis (hardening of arteries)

• Glycated proteins are difficult to degrade, so there is a build up of damaged proteins in the cell

• Damage results in production of auto-antibodies, resulting in increased levels of inflammation. It also results in the production of free radical ROS

Why are free radical ROS bad?

Free radical reactive oxygen species lead to an oxidising environment, resulting in further accumulation of AGPs.

There is also peroxidation of unsaturated lipids in cell membranes.

Also leads to further damage to tissues

How can glycation status be measured?

Through analytical techniques that detect glycated proteins or identify modified amino acid residues.

There are techniques such as:

• Chromatography

• Gel electrophoresis

• Isoelectric focusing

• ELISA

• Mass spectrometry

How can mass spectrometry be used to investigate glycation, and what are its limitations?

• Mass spec can analyse glycated proteins, but mixed protein populations can make results difficult to interpret.

• Protein sequencing by mass spectrometry can identify which amino acid residues have been modified and how, but requires a relatively pure sample and is technically difficult.

How can we prevent glycation?

There is no proven therapy to reduce glycation, but there is potential:

• Small molecule amines (e.g. pyridoxamine) could compete with proteins for glycation.

• Reducing agents (e.g. vitamin C) may reduce reactive oxygen species (ROS).

• Other nucleophiles (e.g. penicillamine) could react with reducing sugars and prevent Amadori product formation.

• Plant extracts with potential antioxidant properties, such as caffeic acid, chlorogenic acid, rutin and cranberry extracts. However, we need to be careful as many foods may contain sugars, i.e. cranberries contain reducing sugars