Carbohydrates And Sugar Code Primer

Carbohydrates (sugars) follow the formula $C_x(H_2O)_y$.

Sugar Nomenclature

Monosaccharides are named by number of carbons. Triose: 3C, tetrose: 4C, etc.

When ending in -ranose, it is a ring named similarly. Furanose: 5C ring, pyranose: 6C ring, etc.

-aldehyde: C=O group at the end
-ketose: C=O group plus an extra item on the same carbon
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Chirality

Most monosaccharides have at least 1 chiral carbon, creating 2 stereoisomers (D and L). The number of stereoisomers = 2^chiral centers . Epimers are stereoisomers which differ at 1 chiral carbon. Most natural sugars are D.

Anomeric carbon: Bound to two oxygens. Alpha has OH group on the bottom, beta on the top.
Mutarotation: Flipping between alpha and beta configurations until equilibrium is reached.

When sugars are in open form (the anomeric carbon is available to donate electrons), it can act as a reducing agent. Benedicts test will show if there are reducing sugars in urine. Blue = negative (no reducing sugars), red = positive (yes reducing sugars).

Important Monosaccharides

There are five simple sugar structures worth knowing: Ribose, Glucose, Fructose, Galactose, and Mannose.

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Sugar Modifications

No modifications

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Amino sugar

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Deoxy sugars (deoxygenation)

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Carboxy sugars (carboxylation)

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Other modifications

Aldonic acid: Oxidating aldose carbonyl
Uronic acid: Oxidation at opposite end
Lactone: Has an ester portion

Disaccharides

2 monosaccharides linked via O-glycosidic bond, which is formed when hydroxyl of one sugar reacts with anomeric carbon of another. The non-reducing end donates its anomeric carbon while the reducing end retains its anomeric carbon.

Exception: Sucrose has both of its anomeric carbons react with each other. Thus, sucrose is non-reducing.

There are three digestible disaccharides: Sucrose, Lactose, and Maltose.

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Polysaccharides

Functions

Fuel storage (e.g. glycogen and starch)
Structure (e.g. cellulose, chitin, glycosaminoglycans)
Information transfer (e.g. sugar code)

Fuel storage

Glycogen has a chain of glucoses linked via alpha 1-4 bonds. It branches via alpha 1-6 bonds every 9-12 sugars on the chain. Glycogen only has one reducing end and several non-reducing ends. Glucose-1-P (G1P) can be released very quickly from the numerous non-reducing ends, making glycogen good for energy.

However, because glycogen is hydrophilic (will pull water into the cell), it can only be used for short term storage. Depletion of glycogen will occur within 12 hours of fasting.

Glycosaminoglycans

Makes up the EXM/ground substance of the extracellular matrix, which is a gel-like material between cells. Is has repeating saccharide pairs, an amino sugar, an acidic sugar (carboxyl), and is sometimes sulfated. It will covalently bond to proteoglycans. All of the negative charges make it play nice with water.

Sugar Code

Molecular messaging via the complexity of carbohydrates. Cells are recognized by surface glycans.

Selectins/lectins

Proteins which use the sugar code to recognize carb structures.

Selectins

Mediate inflammatory response in arthritis, asthma, psoriasis, multiple sclerosis, and transplanted organ rejection.

Influenza virus

Hemagglutinin (lectin) [H]: Antigen on influenza virus recognizes a glycan on cell surface which is essential for infection
Neuraminidase [N]: An enzyme which cleaves the glycan on the influenza virus surface to release the virus

The different H and N antigens year to year are which flu shots must be updated. Tamiflu and Relenza are sugar analogs which inhibit N

ABO blood groups

H/O antigen: Core of the glycan, Type O blood only has the H antigen (no A/B)
Type A: Enzyme adds N-acetyl galactosamine
Type B: Enzyme adds galactose