Structure and metabolism Carbohydrate Structure and metabolism

Definition CHO (Hydrated carbon) CHO are aldehyde or ketone (=O) compounds with multiple hydroxy (-OH) groups General formula (CH2O)n Monosaccharides and Disaccharides are called sugars, they end by – OSE e.g. glucose, lactose etc.

Functional groups C-OH, hydroxyl group C=O, carbonyl group OH C=O, carboxyl group

Functions Provision of energy Storage of energy Major component in nucleic acid structure Structural functions, cell wall (bacteria and plants) Cell membrane (Signal transduction – adhesion, cell-cell interaction and etc) Others e.g. mucin

Glucose Figure 4.6 Mannose and galactose are epimers of glucose

Classification Monosaccharides (simplest unit) Disaccharides: 2 monosaccharides linked by covalent glycosidic bond e.g. sucrose Oligosaccharides: 3 – 10 monomers Polysaccharides: more than 10 could be linear or branched

Monosaccharides (simplest unit) A . Aldose b. Ketose Trioses [3] Tetroses [4] Pentoses [5] Hexoses [6] Heptoses [7] Nonoses [9]

Hexoses [C6] (Isomers) Isomers; are sugars have the same chemical formula (CH2O)n, but have different structures Enantiomers (mirror images; L or D) (position of OH on asymmetric carbons furthest from =O group) Anomers: OH above () or below () plane around anomeric C (C to which attached the =O) e.g. C1 in glucose, C2 in fructose Epimers: they differ in orientation (right or left) of one OH around asymmetric C (C2 to C5)

Enantiomers have identical chemical and physical properties except for their ability to rotate plane-polarized light by equal amounts but in opposite directions.

Figure 4.7 Formation of ring structures in sugars

Monosacchrides form cyclic structure (ring) in solution (=O react with -OH in the same molecule) Formed Glu ring is called pyranose (pyran like) In this case the C1 is called anomeric C If the OH attached to C1 above the ring plane, we say -anomer, if below is -anomer In solution -and - interchange (mutarotation) Keto sugars form (furan like) furanoses

Figure 4.6 Mannose and galactose are epimers of glucose

Bonds (linkage) Monosaccharides interact with each other or other compound through glycosidic bonds Glycosidic bond is covalent bond between the anomeric C (in glucose C1) and C of another compound The glycosidic bond can be O- or N- glycosidic bond

Disaccharides

Polysaccharides 1. Cellulose: Found in plant cell walls Composed of glucose units linear structure 1-4 linkages Insoluble Humans can not hydrolyze the 1-4 bonds, so not digestible in humans Important in our diet – decrease constipation and colon cancer

Cellulose Structure Figure 4.9 The β1,4 glycosidic bonds in cellulose

Polysaccharides 2. Starches Plant origin Polysaccharides of long chain polymers of - D-glucose Have un-branched chains (amylose; 20%); glucose units linked by -1-4 links In branched chains part (amylopectin; 80%); in addition to -1-4 links there are -1-6 links at branch points

Figure 4.10 The structures of amylose and Amylopectin

Polysaccharides 3. Glycogen Animal origin (animal starch) Storage form of CHO in human cells, found as glycogen granules (also contain enzymes of synthesis & degradation) Polysaccharides of long chain polymers of - D-glucose Similar to amylopectin of starch but much more branched Multiple Branches; to provide many non-reducing ends for quick release of glucose

Polysaccharides Most tissues contain glycogen, But liver (10%wt), and muscles (2%wt), store most of body glycogen The muscle glycogen is for local use of the muscles, it CAN NOT be released in blood Liver glycogen is to maintain blood glucose

Polysaccharides 4. Inulin Is plant starch, found in tuber and root of certain plants A polymer of fructose The linkage is (2-1) Soluble in warm water Has been used to measure renal glomerular filtration rate

Reducing sugars The free anomeric C (aldhyde or keto group ) of the open-chain form of sugar can reduce Cu2+ (cupric) to Cu+ (cuprous) in alkaline solutions (Fehling or Benedict test) Examples of reducing sugars are: glucose, galactose, fructose, maltose, and lactose Non-reducing sugars: sucrose, cellobiose

Glycoproteins and proteoglycans Oligosaccharides or small polysacch. covalently linked to protein via N- or O- glycosidic bonds Examples: blood groups, signal molecules

Glycoproteins and proteoglycans Large protein polysaccharides complex (ground substance of connective tissue) The polysaccharides: are high MW, polyanionic glycosaminoglycans (repeated units of disaccharide, one of them is always amino sugar – glucosamine or galactosamine and the other is uronic acid)

Digestion & absorption Digest.: Break of glycosidic bonds of di- oligo- and polysaccharides by different enzymes Enzymes are glycosidases In mouth: salivary -amylase, act on - 1,4 glycosidic bonds in starch/glycogen In intestine: pancreatic -amylase, like salivary but work at lower pH (stomach acids) Both produce disaccharides e.g. maltose, isomaltose

Absorption Small intestine mucosal brush border secret disacchridases (break disaccharides), releasing monosaccharides, which are absorbed Deficiency e.g. lactase, lead to lactose intolerance; lactose acted upon by bacteria causing diarrhea, distension and dehydration

Transport of glucose in the cell A. Na+-dependent: - Needs energy (Na+/K+ ATPase) - Carrier binds both Glu and Na+ transport them inside cells (against concentration gradient), then pump Na+ out in exchange with K+, using ATP as source for energy. B. Na+-independent facilitated transport: mediated by glucose transporters (GLUT 1 to 14) located in cell membrane.

Glucose transporters Transporter Tissue location GLUT1 Brain, kidney, colon, RBC, placenta GLUT2 Liver, pancreatic B cell, small intestine, kidney GLUT3 Brain, kidney, placenta GLUT4 Heart, skeletal muscles, adipose tissue GLUT5 Small intestine

Figure 4. 14 How a glucose transporter (GLUT) works Figure 4.14 How a glucose transporter (GLUT) works. Note the conformational change upon binding glucose