CARBOHYDRATE CHEMISTRY and MTABOLISM

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CARBOHYDRATE CHEMISTRY and MTABOLISM

• By
• Mohamed Aly Abdelhafez, PhD
• Medical Biochemistry Molecular Biology
  Department
• Faculty of Medicine, Cairo University

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Definition

• Carbohydrates Chemically, they are polyhydroxy
  aldehydes or ketones.
• Carbohydrates have the following biological
  functions
• 1- Chief source of energy (50-60) of daily
  requirement.
• 2- Partners of cell organelles and intercellular
  matrix structure, in complex with protein (as
  glycoprotein or proteoglycans), or with lipids
  (as cerebrosides or gangliosides).
• 4- Interconvert with some amino acids and lipid
  derivatives in
• intermediary metabolic pathways.

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Classification

• The basic unit is monosaccharide (mono- single
  or one) or simple sugar. Structurally
  monosaccharides contain carbon and water elements
  (hydrogen and oxygen).
• Oligosaccharides (oligo- few) are containing
  2-10 monosaccharide units.
• If more than 10 monosaccharide units it is
  polysaccharide(poly- many).

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Monosaccharides

• Monosaccharides are water-soluble sweet
  carbohydrates.
• They are classified into a) aldoses e.g.glucose
  , galactose , mannose, and b) ketoses e.g.
  fructose.
• Each class is further subclassified according to
  number of carbon atoms per molecule of
  monosaccharide into Triose (C3) , tetrose (C4)
  ,pentose (C5) as ribose ,hexose (C6) as glucose
  ,galactose ,fructose ,as well as mannose and
  heptose (C7) .
• CHO CHO
  CH2OH CHO CHO
• HCOH HCOH CO
  HCOH HCH
• HOCH HOCH OHCH
  HCOH HCOH
• HCOH HOCH HCOH
  HCOH HCOH
• HCOH HCOH HCOH
  CH2OH CH2OH
• CH2OH CH2OH CH2OH
• Glucose Galactose
  Fructose Ribose Deoxyribose
• H E X O S E S
  P E N T O S E S

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Monosaccharide derivatives

• a- Deoxy-sugar, in which C-2 is devoid of oxygen,
  e.g. deoxyribose.
• b- Sugar alcohol, in which the carbonyl group
  (aldehyde or ketone) are reduced into alcoholic
  group.
• c- Amino-sugar, e.g. glucosamine.
• d- Aminosugar acid, the aminosugar is linked to
  pyruvic or lactic acid, neuraminic acid.
• e- Uronic acid, the terminal -OH group of C-6 is
  oxidized to COOH,e.g. glucuronic acid.
• f- Aldonic acid, the aldehyde group of C-1 is
  oxidized to -COOH, e.g. gluconic acid
• g- Aldaric acid, both the -OH of terminal carbon
  and -CHO of C-1 are oxidized to -COOH, e,g.
  Glucaric acid.

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Cyclic Configuration

• The cyclic configuration may be in a vertical or
  a horizontal ring. In either condition it is 5-C
  ring that is closed with oxygen bridge, called
  pyranose.
• A 4-C ring with oxygen bridge is another form
  called furanose. All forms are inter-changeable.
• Essentially the carbonyl group should be involved
  in formation of oxygen bridge. It is the anomeric
  carbon.
• As the ring is formed the anomeric carbon becomes
  asymmetric carbon.

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Cyclic Configuration

• According to the position of OH group the sugar
  is a or ß- .
• The a sugar of vertical assigned ring, the OH-
  group is oriented on the right of the anomeric
  carbon but in horizontal ring the orientation
  this OH is below the plane of the ring. Vice
  versa for ß-sugar.
• Similarly all carbons involved in the ring
  formation i.e., what's on the right of the
  straight chain is below the plane of the
  horizontal ring. An exception is 4th.C in
  furanose and 5th. C in pyranose structure
  because of rotation associated with ring
  closure.

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Properties of monosuccharides

• Optical activity
• The optically active compound contains at least
  one asymmetric carbon atom (the four valances are
  satisfied by four different atoms or groups.
• The plane-polarized light (PPL) is that passed
  through a calcite (CaCO3) prism. The vibrations
  of the electromagnetic waves are unified in one
  plane instead of being in three perpendicular
  planes.
• As PPL passes through an optically active
  material it deviates either in clock-wise
  direction (right-handed or dextrorotation, signed
  "" ) or anti-clock-wise direction (left-handed
  or levorotatory, signed "-" ) .
• Deviation of light path depends on the number of
  asymmetric carbons per molecule.

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Optical activity

• Glucose is dextrorotatory, it is called dextrose
  , whereas fructose is levorotatory , levulose.
• Specific angle of rotation considering the pH,
  concentration , temperature , and other dissolved
  materials in solution.
• The angle of rotation value is preceded by the
  symbol () or (-) depending on the direction of
  rotation. The angle is measured by an instrument
  called Polarisoscope or Polarimeter.
• D- or-L sugar depends on orientation of the -OH
  group in the penultimate carbon atom, assuming
  the aldehyde or ketonic group is oriented "up.
  Right handed position of this OH, the sugar is
  in D- form, whereas left-sided position it is in
  L- form.

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Optical activity

• D- and L- nomination does not indicate the
  direction of rotation of the PPL.
• a- and ß- are so.
• Mutarotation, (change of the specific angle of
  rotation of a recently prepared sugar solution).
  Although both a- and ß- are in the same direction
  of rotation of PPL, yet they have different
  values.
• The D- sugar is a mirror image of the L-form of
  the same sugar , both are called enantiomorphs ,
  antimers , or stereo-isomers. Both have the same
  specific angle of rotation but on reversed
  direction.

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Optical activity

• Other form of isomerism is the similarity in
  number and distribution of groups around the
  asymmetric carbon except one carbon e.g.,
  glucose and galactose are aldohexoses identical
  in structure except the -OH attached to C-4 is on
  the right-side in glucose and on the left-side in
  galactose. Both sugars are epimers in C-4.
• If an equimolar amount of the D-form and L-form
  of the same sugar are mixed together, they will
  lose their optical activity, i.e. no deviation of
  PPL as it passes through this mixture. The
  mixture of this character is called racemic
  mixture. Deviation of light path to either side
  by the D- form is corrected by the other form by
  the same angle but on the other side.

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Other monosacchride properties

• monosaccharides are reducing agents in alkaline
  media .Cupric ions of Fehling or Benedicts
  solutions
• Monosacchrides can be oxidized into the
  corresponding aldonic, uronic or aldaric acids
  (in vitro ) depending on the strength
• Glucose is oxidized to gluconic acid, glucuronic
  acid
• or to glucaric (saccharic acid). Galactose is
  similarly oxidized to mucic acid.
• Monosaccharides are reduced to the corresponding
  alcohol i.e. glucose to sorbitol.
• The primary alcoholic group of C1 or terminal
  carbon can be esterified by phosphorylation.
  Glucose-6-phosphate, fructose-1-phosphate and
  ribulose-5-phosphate are examples.

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Oligosaccharides

• Oligosaccharides are two-monosaccharide units
  per molecule. The units are joined together with
  a glycosidic bond.
• Glycosidic bond binds the carbonyl group of one
  molecular unit with either an alcoholic group of
  C-4 of the other unit (as in lactose and
  maltose), or with the carbonyl group of the other
  unit (as in sucrose). In the latter occasion the
  sugar loses the properties due to the free
  carbonyl group as reduction of metallic ions and
  interchange of the a- and ß- forms.
• Disaccharides are oligosaccharides containing two
  monosaccharide residues.
• Lactose is the milk sugar. It is composed of
  glucose and galactose moles linked by
  ß-glycosidic bond .

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Oligosaccharides

• Maltose is barely sugar made of two moles of
  glucose linked by a-glucosidic bond. Sucrose is
  cane sugar that is made of one mole of glucose
  and another of fructose linked by ß-glycosidic
  linkage.
• Hydrolysis of the disaccharide to the parent
  monosaccharides changes the angle of rotation of
  PPL. In case of sucrose the change affects the
  angle magnitude and direction. Sucrose is
  dextrorotatory, on hydrolysis whether enzymatic
  or acidic, the hydolysate is inverted into strong
  levoroitatory mixture of glucose and fructose.
  Hence, sucrose is known as invert sugar and
  sucrase, the enzyme hydrlysing sucrose, is called
  invertase.

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Polysaccharides

• Carbohydrates containing 10 or more
  monosacchride units per molecule.
• If the monosaccharide unit is of one-type, it is
  homopolysaccharide (both starch and glycogen are
  made of hundreds of glucose units, glucosans or
  glucans) . Dextrin, is similar to starch but of
  fewer glucosyl units, lower molecular weight and
  it is less branched. Cellulose is another
  homopolysaccharide but cannot be digested in
  human gastrointestinal tract (GIT) due to absence
  of the specific digesting enzyme (ß-glucosidase).
  
• Starch is plant in origin. It is a mixture of
  amylopectin (branched-chain glucosan, represents
  80-90 of starch molecule) and amylose
  (straight-chain glucosan represents 1020 of
  starch mole).
• Acid hydrolysis of starch results in mixture of
  dextrins of different molecular weights, maltose
  and free glucose depending on the pH,
  temperature, and duration of hydrolysis process.

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Polysaccarides

• Glycogen is of animal origin. It is similar in
  structure to amylopectin but more highly
  branched.
• Inulin is a plant fructosan that is not digested
  and thus unutilizable. It is present in garlic
  ,onion and artichoke.
• Agar is sulfated galactosan present in sea-weeds.
  It makes gels and sols depending on the
  prevailing temperature and concentration. It is
  used in preparation of bacterial cultures in the
  clinical laboratory.
• Pectins are sugar-acid polymers. They are present
  in citrus fruits, apple, carrots and beets. They
  make a jelly-like substance when soluble in
  water. They have pharmaceutical and food
  industrial applications.
• Gum acacia and gum Arabic are homo-polysaccharides
  that, also have pharmaceutical and industrial
  uses.

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Hetero-polysaccarides

• Heteropolysaccharides are glucosaminoglycans
  (GAGS), or mucopolysaccharides.
• complex macromolecules, repetitive units of
  disaccharides
• A monomer consists of uronic acid (D-glucuronic
  acid or its C-5 epimer L-iduronic acid) and amino
  sugar(hexosamines as glucosamine or
  galactosamine).
• They are either sulfate-free as hyaluronic acid
  (N-acetylglucosamine glucuronic acid) or
• sulfated glucosaminoglycans as heparine ,
  chondroitin sulfate , dermatan sulfate and
  heparan sulfate.

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Krebs Cycle (Tricarboxylic acid cycle,TCA)
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Glycogen metabolism
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Lipogenesis
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