Chemistry of carbohydrates-2

Chemistry of Carbohydrates-2Disaccharides

Maltose

Lactose

Sucrose

Lactose intolerance Malabsorption, diarrhea and flatulence;Seen in new born infants and babies feeding on milk;Deficiency lactase in the intestinal lumen;Failure to thrive.Why Sucrose and Trehalose are non-reducing sugars?

linear form with free anomeric carbon atom is essential for reduction;

- When the anomeric carbon is involved in a glycosidic bond, that sugar cannot take a linear form;LactuloseSynthetic disaccharide of galactose and fructose;Poorly absorbed from the gastrointestinal tract;Used in the treatment of hepatic encephalopathy;Metabolized by the colonic bacteria to acidic products;Promotes the excretion of ammonia in feces as protonated ammonium ions;Also used as a laxative.

Polysaccharides More than 10 sugar units;

Types : Homopolysaccharides same type of sugar units . Ex starch, cellulose

Heteropolysaccharides different sugar units. Ex glycosaminoglycans StarchAmylose 15 20%

Small (60 kDa)

Unbranched structure

Blue color with iodine

Alpha (1,4) glycosidic linkageAmylopectin 80 85%

Big (500 kDa)

Branched structure - One branch every 24-30 glucose residues

Reddish purple

Alpha (1,4) and (1,6) linkage

Starch Vs GlycogenStarch (amylopectin)Storage form of plants

No primer molecule

Blue/reddish purple with iodine

Few branches (one branch per 24-30 sugar units)

Glycogen Storage form of animals

Glycogenin

Red colour

Branch point every 12-14 sugar unitsMuscle Glycogen

Storage of carbohydrate in a 70kg manBody contentLiver Glycogen90gmMuscle glycogen245gmExtracellular glucose 10gmOnly liver glycogen contributes to blood glucose. Why?

Answer: Glucose 6 phosphatase is absent in muscle

Structure of Glycogen

Glycogenin

Glucose

The arrow on the Right picture of the above slide points to the anomeric carbon;

Glucose is a Hemi-acetal because the CHO group (Carbon 1) reacts with the OH group on Carbon 5 to form a cyclic ring; 20

-A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This allows the sugar to act as a reducing agent; -A non-reducing sugar has its anomeric carbon locked in a ring conformation; the -OH on its end cannot form a ketone or aldehyde to allow the sugar to act as a reducing agent.Fluctuation of liver and muscle glycogenLiver glycogen increases during well-fed state and decreases during a fast;

Muscle glycogen not affected in short fasts (few days) and only moderately in long fasts (weeks);InulinPolymer of D fructose;Tubers of Dahlia, roots of the Jerusalem artichoke, Dandelion, bulbs of onion and garlic;

12 glycosidic linkages;Ideal substance for the estimation of GFR; freely secreted by the glomerulus and not acted on by the tubules;Cellulose 14 linked poly-D-glucose;

Most abundant polysaccharide;

Acid hydrolysis yields cellobiose and glucose;

Enzymes that digest Cellulose: Cellulase, cellobiase;

Most animals lack an enzyme to hydrolyze the 14 linkage;

Dextrans Bacterial and yeast polysaccharide;16 linked poly-D-glucose; all have 13 branches and some 12 and 14 branches;Dextran solution of MW: 75kDa;In hypovlemic shock, given intravenously increases blood volume;High viscosity, low osmotic effects, slow disintegration and utilization, slow elimination from the body;

Glycosaminoglycans (GAGS) Hetero polysaccharides made of repeating units of di-saccharide units;

GAGS have a gel-like matrix due to H2O binding properties;

GAGS form bodys ground substance;

GAGS have lubricating properties;

Also called Mucopolysaccharides component of mucus secretions;

Proteoglycans GAGs + protein ( 95 : 5)%

GAGS stabilize and support cellular and fibrous components of tissue;

Mainly present in extracellular matrix.GAGSGAGS are made up of repeating disaccharide unit;

[acidic sugar + amino sugar] n

Acidic sugar glucuronic acid / iduronic acid.

Amino sugar glucosamine / galactosamine with a acetyl group.

Chondroitin sulfateMost abundant;

N-acetyl galactosamine sulfate + glucuronic acid;

Found in cartilage, tendon, ligament;

provides much of the resistance of cartilage to compression.

Dermatan sulfateFound in skin, blood vessels and heart valves.

Iduronic acid + N-actyl galactosamine sulfate.

Helps in wound repair and fibrosisHeparin intracellular GAG

Glucosamine sulfate and Glucuronic acid

Found in mast cells

Acts as an anticoagulant

Hyaluronic acidN-acetyl glucosamine and glucuronic acid

Not linked to any protein

Found in synovial fluid of joints, vitreous humor of eye.

MucopolysaccharidosisHereditary disorders characterized by accumulation of GAGS in various tissues due to defective lysosomal enzymes.

All are autosomal recessive except Hunter (x-linked)

Skeletal, extracellular matrix deformities and mental retardation.

Result in presence of oligosaccharides in urine due to incomplete breakdown of GAGs.

GluosmamineWhy might an individual suffering from Osteoarthritis be tempted to consume the nutraceutical Glucosamine? Answer: Glucosamine is a building block of certain GAG components of Proteoglycans. Boosting the bodys supply of glucosamine might slow the progression of the disease Osteoarthritis, which is characterized by the degradation of proteoglycan-rich articular cartilage; Hurler syndrome Enzyme def: L Iduronidase

Accumulation : Heparan and Dermatan sulfate

Key features: corneal clouding, mental retardation, micrognathia, coarsening of facial features with macroglossia, inguinal and abdominal hernias, retinal degeneration.

Death due to accumulation in coronary arteries.

Can be treated before 18 months BM transplant.

Hunter syndromeOnly X-linked MPS.

Similar to hurler but milder, no corneal clouding

Variable mental retardation.

Enzyme defect : Iduronate sulfatase.

Accumulation Heparan and Dermatan sulfate