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Carbohydrates as Energy Sources

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In amylose these are a 1,4 linkages, whereas in amylopectin, about one residue ... The relative proportions of amylose to amylopectin and -(1 6)- branch-points ... – PowerPoint PPT presentation

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Title: Carbohydrates as Energy Sources


1
Carbohydrates as Energy Sources
2
Practical Considerations
  • Carbohydrates are consumed as cereal grains, by
    products, milk products
  • 2. Provide considerable portion (majority) of
    energy for meat, milk, and egg production, and
    pet/horse feeds
  • Consumed (fed) as simple to complex molecules,
    depending on species and age of animal,
    commercial production or research
  • Enterocyte absorbs simple sugars common feed
    ingredients must be
  • processed as a prerequisite to turning
    carbohydrates into energy
  • Carbohydrates in and of themselves do not
    constitute energy rather, they are metabolized
    in key biochemical pathways to provide reducing
  • equivalents and ATP
  • Carbohydrates are stored in minimal capacity
    (glycogen) in animals but, biochemically,
    mammalian and avian species can capture carbon
    and hydrogen from carbohydrate as fatty acids

3
Chemical and Structural Features
  • Hydrogen and oxygen in same proportion as water
    (H2O) Carbo(C)Hydrate

4
Classification
  • Simple Carbohydrates
  • Monosaccharides
  • Disaccharides
  • Complex Carbohydrates
  • Oligosaccharides
  • Polysaccharides

5
Classification
  • Monosaccharides simple sugars that cannot be
    reduced to smaller carbohydrate units by
    hydrolysis
  • Arise in nature or as intermediates of digestion
  • Contain 3 7 carbon atoms
  • Trioses
  • Tetroses
  • Pentoses
  • Hexoses
  • Heptoses
  • Possess a hydroxyl (OH) and carbonyl (CO) group,
    that can be either an aldehyde or ketone
  • Aldoses aldehyde group
  • Ketoses ketone group
  • Sugar alcohols (aldehyde or ketone reduced to
    alcohol form maltitol, sorbitol, isomalt, and
    xylitol)

6
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7
Common aldoses and ketoses
8
Classification
  • Disaccharides monosaccharides linked together
  • Maltose (glucose glucose)
  • Sucrose (glucose fructose)
  • Lactose (glucose galactose)

9
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10
Classification
  • Oligosaccharides 3-10 monosaccharides linked
    together
  • Maltotriose (3 glucose units, a1,4 linkage)
  • Dextrin (6-8 glucose units a-(1,4)-linked
    D-glucose polymers starting with an a-(1,6) bond
    )
  • Fructo-oligosaccharides
  • Galacto-oligosaccharides
  • Mannan-oligosaccharides

11
Classification
  • Polysaccharides gt10 monosaccharides linked
    together
  • Starch (amylose and amylopectin)
  • Dextrin polymers
  • Glycogen

Largely exist as hexose polymers (hexosans) or
pentose polymers (pentosans)
12
Representative partial structures of amylose
13
Representative partial structure of amylopectin
14
Key features of starch, Summary
  • Starch consists of two types of molecules,
    amylose (normally 20-30) and amylopectin
    (normally 70-80)
  • Both consist of polymers of a-D-glucose units
  • In amylose these are a 1,4 linkages, whereas in
    amylopectin, about one residue in every twenty to
    thirty units has an a1,6 linkage to form a
    branch-points
  • The relative proportions of amylose to
    amylopectin and -(1 6)- branch-points both depend
    on the source of the starch, for example,
    amylomaizes contain over 50 amylose whereas
    'waxy' maize has almost none (3 or less)

15
Classification
  • Non-starch polysaccharides
  • Not digested by avian and mammalian enzymes
  • Make up large portion of dietary fiber (e.g.,
    cellulose, hemicellulose, pectin)
  • Glycosidic bonds connecting the redsidues are ß
    1-4 rendering the molecule resistnat to a-amylase
  • Fermented by intestinal microflora, particularly
    in the hind gut

16
Digestion-Key Enzymes
  • The process reduces complex CH2O to simple
    molecules that can be absorbed by the enterocyte
  • a-Amylase salivary gland and pancreas
  • (1,4-a-D-glucan glucanohydrolase glycogenase)
  • The a-amylases are calcium metalloenzymes,
    completely unable to function in the absence of
    calcium optimum pH of about 6.7-7.0
  • By acting at random locations along the starch
    chain, a-amylase breaks down long-chain carbs
    maltotriose, maltose from amylose maltose,
    glucose, limit dextrin from amylopectin

17
Starch Molecule
18
Digestion-Key Enzymes
  • The process reduces complex CH2O to simple
    molecules that can be absorbed by the enterocyte
  • disaccharideases
  • Lactase lactose to glucose and galactose
  • Maltase maltose to two glucose unites
  • Sucrase sucrose to glucose and fructose
  • Trehalase trehalose to two glucose units

19
Carbohydrate Absorption
  • Carbohydrate absorption is very efficient
  • Nearly all monosaccharides are usually absorbed
    by the end of the jejunum
  • Absorption occurs via active transport, a process
    that requires energy, passively (non-ion) and by
    specific receptors
  • SGLT1
  • GLUT (1-12)

20
Carbohydrate Absorption
  • The glucose/galactose transport by the
    sodium-dependent hexose transporter (SGLT 1)
    involves a series of conformational changes
    induced by binding and release of sodium and
    glucose
  • The transporter is initially oriented facing into
    the lumen - at this point it is capable of
    binding sodium, but not glucose
  • Sodium binds, inducing a conformational change
    that opens the glucose-binding pocket
  • Glucose binds and the transporter, reorients in
    the membrane such that the pockets holding sodium
    and glucose are moved inside the cell
  • Sodium dissociates into the cytoplasm, causing
    glucose binding to destabilize
  • Glucose dissociates into the cytoplasm and the
    unloaded transporter reorients back to its
    original (luminal) orientation

21
  • Other key features
  • non-ion dependence of GLUT5
  • Na/K ATPase generates the electrochemical
    gradient necessary
  • non-specificity of GLUT2 for delivering
    absorbed sugars into blood
  • The hexose transporters are large integral
    membrane proteins they have similar structures,
    consisting of 12 membrane-spanning regions with
    cytoplasmic C-terminal and N-terminal tails. All
    appear to be glycosylated on one of the
    extracellular loops.
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