Carbohydrates as Energy
Sources
Practical Considerations
1. 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
3. Consumed (fed) as simple to complex molecules, depending on species and
age of animal, commercial production or research
4. Enterocyte absorbs simple sugars; common feed ingredients must be
“processed” as a prerequisite to turning carbohydrates into energy
5. Carbohydrates in and of themselves do not constitute energy; rather, they
are metabolized in key biochemical pathways to provide reducing
equivalents and ATP
6. 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
Chemical and Structural Features
• Hydrogen and oxygen in same proportion
as water (H2O): Carbon(C)……Hydrate
Classification
• Monosaccharides: simple sugars
–
–
–
–
Trioses
Tetroses
Pentoses
Hexoses
Chemical forms exist as aldehydes or ketones
– Sugar alcohols (aldehyde or ketone
reduced to alcohol form: maltitol,
sorbitol, isomalt, and xylitol)
Common aldoses and ketoses
Aldoses
Ketoses
Trioses
C3H6O3
Glycerose (glyceraldehyde) Dihydroxyacetone
Tetroses
C 4 H 8 O4
Erythrose
Erythrulose
Pentoses
C5H10O5
Ribose
Ribulose
Hexoses
C6H12O6
Glucose
Fructose
Heptoses
C7H14O7
Sedoheptulose
Classification
• Disaccharides: monosaccharides linked
together
– Maltose (glucose + glucose); Isomaltose
– Sucrose (glucose + fructose)
– Lactose (glucose + galactose)
Classification
• Oligosaccharides: 3-10 monosaccharides
linked together
– Maltotriose (3 glucose units, α1,4 linkage)
– Limit Dextrins (6-8 glucose units: α-(1,4)linked D-glucose polymers starting with an α(1,6) bond )
– Fructo-oligosaccharides
– Galacto-oligosaccharides
– Mannan-oligosaccharides
Classification
• Polysaccharides: >10 monosaccharides
linked together
– Starch (amylose and amylopectin)
– Dextrin polymers
– Glycogen
Largely exist as hexose polymers (hexosans) or pentose polymers (pentosans)
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 α-D-glucose units;
probably about 600 glycosyl units per molecule
• In amylose these are α 1,4 linkages, whereas in
amylopectin, about one residue in every twenty to
thirty units has an α1,6 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)
Representative partial structures of amylose
Representative partial structure of amylopectin
From Dr. Yuri Kiselov, with permission
Classification
• Non-starch polysaccharides:
– Not digested by avian and mammalian
enzymes
– Make up large portion of dietary fiber (e.g.,
cellulose, hemicellulose, pectin)
– Fermented by intestinal microflora, particularly
in the hind gut
– Some implications in immune modulation
Digestion-Key Enzymes
• The process reduces complex CH2O to simple
molecules that can be absorbed by the enterocyte
• α-Amylase: salivary gland and pancreas
– (1,4-α-D-glucan glucanohydrolase; glycogenase)
– The α-amylases are calcium metalloenzymes and
endoglucosidases, 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, α-amylase
breaks down long-chain carbs: maltotriose, maltose from
amylose; maltose, glucose, limit dextrin from amylopectin
Starch Molecule
• CCK targets the exocrine pancreas and salivary glands directly to
stimulate release in parallel with feed intake
• Substrate (starch) sensing also triggers release and protects against
proteolytic degradation in mouth and duodenum
• Salivary amylase is more significant in suckling nonruminants, as GI
tract is less developed
Digestion-Key Enzymes
• The process reduces complex CH2O to simple
molecules that can be absorbed by the enterocyte:
“disaccharide_ases” “brush border enzymes”
• Lactase: lactose to glucose and galactose
• Maltase: maltose/maltotriose to two or three glucose
units
• Sucrase: sucrose to glucose and fructose (also has
maltase activity)
• Trehalase: trehalose to two glucose units (α-1,1)
• Isomaltase (oligo α-1,6 glucosidase, α-dextrinase):
unique because it has high affinity for and activity on the
1,6 glycosidic bond
From H. Dieter-Dellman and E. M. Brown, Veterinary Histology (with permission)
From H. Dieter-Dellman and E. M. Brown, Veterinary Histology (with permission)
From H. Dieter-Dellman and E. M. Brown, Veterinary Histology (with permission)
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 glucosebinding 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
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.
Now, our critters have consumed, digested, absorbed and released carbohydrate
sugars into the blood…………liver, muscle and adipose tissue play key
roles in intermediary metabolism.