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): Carbon(C)……Hydrate

4. 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

6. Common aldoses and ketoses

7. Classification • Disaccharides: monosaccharides linked together • Maltose (glucose + glucose); Isomaltose • Sucrose (glucose + fructose) • Lactose (glucose + galactose)

9. 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

10. Classification • Polysaccharides: >10 monosaccharides linked together • Starch (amylose and amylopectin) • Dextrin polymers • Glycogen Largely exist as hexose polymers (hexosans) or pentose polymers (pentosans)

11. 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)

12. Representative partial structures of amylose

13. Representative partial structure of amylopectin

14. From Dr. Yuri Kiselov, with permission

15. 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

16. 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

17. 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

18. 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”

19. From H. Dieter-Dellman and E. M. Brown, Veterinary Histology (with permission)

20. From H. Dieter-Dellman and E. M. Brown, Veterinary Histology (with permission)

21. From H. Dieter-Dellman and E. M. Brown, Veterinary Histology (with permission)

22. 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

23. 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.

24. 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.