Carbohydrates as Energy Sources

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

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 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”

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 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.

Now, our critters have consumed, digested, absorbed and released carbohydratesugars into the blood…………liver, muscle and adipose tissue play keyroles in intermediary metabolism.