Carbohydrates as Energy Sources. Practical Considerations 1.Carbohydrates are consumed as cereal...
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Transcript of Carbohydrates as Energy Sources. Practical Considerations 1.Carbohydrates are consumed as cereal...
Practical Considerations1. 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 andage 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, theyare metabolized in key biochemical pathways to provide reducingequivalents 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
– Sugar alcohols (aldehyde or ketone reduced to alcohol form: maltitol, sorbitol, isomalt, and xylitol)
Chemical forms exist as aldehydes or ketones
Common aldoses and ketosesAldoses Ketoses
Trioses
C3H6O3
Glycerose (glyceraldehyde)
Dihydroxyacetone
Tetroses
C4H8O4
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)
•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)
Key features of starch, Summary
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 againstproteolytic degradation in mouth and duodenum• Salivary amylase is more significant in suckling nonruminants, as GItract is less developed
Digestion-Key Enzymes
• The process reduces complex CH2O to simple molecules that can be absorbed by the enterocyte:
“disaccharide_ases”
• 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
“brush border enzymes”
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
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.