Download Lippincott Illustrated Biochemistry 3 rd Edition, 2005

Carbohydrate
Structure and
metabolism
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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.
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Functional groups
• C-OH, hydroxyl group
• C=O, carbonyl group
• OH
• C=O, carboxyl group
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
• Glucose
Figure 4.6 Mannose and galactose are epimers of glucose
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Monosaccharides (simplest unit)
•
A . Aldose
•
•
•
•
•
•
Trioses
Tetroses
Pentoses
Hexoses
Heptoses
Nonoses
b. Ketose
[3]
[4]
[5]
[6]
[7]
[9]
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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)
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Enantiomers have identical chemical and physical
properties except for their ability to rotate planepolarized light by equal amounts but in opposite
directions.
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Figure 4.7 Formation of ring structures in sugars
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
• 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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Figure 4.6 Mannose and galactose are epimers of glucose
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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 Nglycosidic bond
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Disaccharides
Name
Source
Disaccharide (glycoside) Abbreviation
Lactose
Milk
galactose β(1,4) glucose
Gal β(1,4) Glc
Sucrose*
Sugar cane
glucose α(1,2) β-fructose
Glc α(1,2) β-Fru
Trehalose*
Insects
glucose α(1,1) α-glucose
Glc α(1,1) α-Glc
Maltose
Starch
glucose α(1,4) glucose
Glc α(1,4) Glc
Glucose β(1,4) glucose
Glc β(1,4) Glc
Cellobiose* Cellulose
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Cellulose Structure
Figure 4.9 The β1,4 glycosidic bonds in cellulose
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Figure 4.10 The structures of amylose and
Amylopectin
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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 nonreducing ends for quick release of glucose
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Glycoproteins and proteoglycans
Glycoproteins
• Oligosaccharides or small polysacch.
covalently linked to protein via N- or Oglycosidic bonds
• Examples: blood groups, signal
molecules
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Glycoproteins and proteoglycans
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)
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Digestion & absorption
• Digest.: Break of glycosidic bonds of dioligo- 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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
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.
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Glucose transporters
Transporter Tissue location
GLUT1
GLUT2
GLUT3
GLUT4
GLUT5
Brain, kidney, colon, RBC,
placenta
Liver, pancreatic B cell, small
intestine, kidney
Brain, kidney, placenta
Heart, skeletal muscles,
adipose tissue
Small intestine
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Figure 4.14 How a glucose transporter (GLUT) works.
Note the conformational change upon binding glucose
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005
Harvey RA, Champe PC. Lippincott Illustrated Biochemistry 3rd Edition, 2005