CARBOHYDRATES: STRUCTURE AND FUNCTION By Dr. Sumbul Fatma Objectives To understand: The structure of carbohydrates of physiological significance The main role of carbohydrates in providing and storing of energy The structure and function of
glycosaminoglycans OVERVIEW Carbohydrates: The most abundant organic molecules in nature The empiric formula is (CH2O)n, hydrates of carbon Carbohydrates: provide important part of energy in diet Act as the storage form of energy in the body are structural component of cell membranes OVERVIEW CONTD Many diseases associated with disorders of carbohydrate metabolism including: Diabetes mellitus Galactosemia
Glycogen storage diseases Lactose intolerance CLASSIFICATION Monosaccharides: Simple sugar Disaccharides: 2 monosaccharide units Oligosaccharides: 3-10 monosaccharide units Polysaccharides: more than 10 sugar units Homopolysaccharides &
heteropolysaccharides Monosaccharides Further classified based on: 1. No. of carbon atoms 2. Functional sugar group: Aldehyde group aldoses Keto group ketoses Monosaccharides CONTD Aldose Triose Pento
se Hexos e Ketose Glyceraldehy Dihydroxyace de tone Ribose Ribulose Glucose Fructose Isomeris m Isomers Compounds
having same chemical formula but different structural formula Aldo-Keto Isomers Example: Glucose (Aldose) and Fructose (Ketose) Epimers Epimers CHO dimers that differ in configuration around
only one specific carbon atom -Glucose and galactose, C4 -Glucose and Mannose, C2 Galactose and mannose are not epimers Enantiomers (D- and LForms) Structures that are mirror images of each other and are designated as D- and L- sugars based on the position of OH grp on the asymmetric carbon farthest from the carbonyl
carbon Majority of sugars in humans are D-sugars - and and and -Forms 1 H Cyclization of Monosaccharides Monosaccharides with 5 or more carbon are predominantly found in the ring form 2 HO
3 H 4 H 5 6 CHO C OH C H
C OH (linear form) C OH D-glucose CH2OH 6 CH2OH 6 CH2OH 5 H O H
OH 4 OH 1 H 2 3 H OH 5 H
H H OH 4 OH O OH H 1 H OH
H 2 3 OH -D-glucose -D-glucose CH2OH 1 The aldehyde or ketone grp reacts with the OH grp on the same sugar -
HO H H 2C O C H C OH C OH 3
4 5 6 - Cyclization creates an anomeric HOH2C 6 CH2OH D-fructose (linear) H 5 H
1 CH2OH O 4 OH HO 2 3 OH H -D-fructofuranose Mutarotation
In and solution, the cyclic and and and and anomers and of and a and sugar and are and in and equilibrium and with and each and other, and and and can and be and interconverted and spontaneously Fischer Projection Fischer Projection Haworth Projection Sugar Isomers 1. Aldo-keto 2. Epimers 3. D- and L-Forms 4. - and -anomers Disaccharides Joining of 2 monosaccharides by O-glycosidic bond: Maltose (-1, 4)= glucose + glucose
Homopolysaccharides: Branched: Glycogen and starch (-glycosidic polymer) Unbranched: Cellulose (-glycosidic polymer) Heteropolysaccharides: e.g., glycosaminoglycans (GAGs) Reducing Sugars If the O on the anomeric C of a sugar is not attached to any other structure (Free), that sugar can act as a reducing agent
Reducing sugars reduce chromogenic agents like Benedicts reagent or Fehlings solution to give a colored precipitate Urine is tested for the presence of reducing sugars using these colorimetric tests Reducing Sugars CONTD Examples: Monosaccharides Maltose and Lactose Sucrose is non-reducing, Why? Complex Carbohydrates
Carbohydrates attached to noncarbohydrate structures by glycosidic bonds (O- or N-type) e.g., 1. Purine and pyrimidine bases in nucleic acids 2. Bilirubin 3. Proteins in glycoproteins and proteoglycans 4. Lipids found in glycolipids Glycosidic Bonds N-Glycosidic O-Glycosidic
Glycosaminoglycans (GAGs) Glycosaminoglycans (GAGs) are large complexes of negatively charged heteropolysaccharide chains are associated with a small amount of protein, forming proteoglycans, which consist of over 95 percent carbohydrate bind with large amounts of water, producing the gel-like matrix that forms body's ground substance The viscous, lubricating properties of mucous secretions also result from GAGs, which led to the original naming of these compounds as mucopolysaccharides
Glycosaminoglycans (GAGs) GAGs are linear polymers of repeating disaccharide units [acidic sugar-amino sugar]n The amino sugar (usually sulfated) is either D-glucosamine or Dgalactosamine The acidic sugar is either acid D-glucuronic acid or L-iduronic
GAGs are strongly negatively-charged: carboxyl groups of acidic sugars Sulfate groups Resilience of GAGs Relationship between glycosaminoglycan structure and function Because of negative charges, the GAG chains tend to be extended in solution and repel each other and when brought together, they "slip" past each other This produces the "slippery" consistency of mucous secretions and synovial fluid When a solution of GAGs is compressed, the water is "squeezed out" and the GAGs are forced to occupy a smaller volume. When the compression is released, the GAGs spring back to their original,
hydrated volume because of the repulsion of their negative charges This property contributes to the resilience of synovial fluid and the vitreous humor of the eye Members of GAGs Examples of GAGs are: 1. Chondroitin sulfates: Most abundant GAG 2. Keratan sulfates: Most heterogeneous GAGs 3. Hyaluronic acid: Compared to other GAGs, it is unsulfated and not covalently attached to protein 4. Heparin: Unlike other GAGs, Unlike other GAGs that are extracellular, heparin is intracellular and serves as an anticoagulant Take home Message
Structure and function of carbohydrates Mono-, Di-, and Poly-saccharides Sugar Isomers: Aldo-keto, epimers, D- and L-, - and -anomers Complex carbohydrates: e.g., Glycosaminoglycans and proteoglycans Structure and function of GAGs Examples of GAGs: chondroitin sulfate, keratin sulfate, hyaluronic acid and heparin Reference
Lippincotts Illustrated reviewsBiochemistry, 6th Edition, pages- 83-86 and 157-159
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