Carbohydrates Flashcards
Carbohydrates
-naturally occurring compounds that consist of carbon, hydrogen and oxygen
-ratio: Cn:H2n:On
Types of carbohydrates
-Starch
-Glycogen
-Cellulose (fibre)
Carbohydrate options
-starch
-non starch polysaccharides (NSP)
Carbohydrate digestion
-single most important source of energy in monogastrics
-linkages between the units and organization affect where or it the CHOs are degraded
Degradation of starch
-alpha-linked carbs (starch) are degraded to glucose by endogenous enzymes
Degradation of non-starch polysaccharides
-beta-linked carbohydrates are degraded by microbial enzymes (except lactose)
Carbohydrate variations
-based on chemical properties (degree of polymerization, types of linkages, individual monomers)
Carbohydrates in feed
-usually monosaccharides linked together via glycosidic bonds to form disaccharides, oligosaccharides, or polysaccharides
Glycosidic bonds
-a covalent bond that joins carbohydrate molecules to another group
How are glycosidic bonds broken?
-by hydrolysis reaction (use of one water molecule)
Monosaccharides
-absorbed from intestinal tract of nonruminants
>carbohydrates are broken down into monosaccharides by digestive enzymes BUT enzyme ability is limited and many carbohydrates escape digestion in small intestine
Disaccharides
-two monosaccharides linked by glycosidic bond
-sucrose and lactose commonly fed to livestock
>sucrose from feed of plant origin
>lactose is milk sugar (milk powder, whey powder, whey permeate-protein removed from whey, purified lactose)
Common disaccharides and different glycosidic bonds
-sucrose (alpha 1-2 bond)
-lactose (beta 1-4 bond)
-maltose (alpha 1-4 bond)
Oligosaccharides
-monosaccharides residues joined by glycosidic bonds
-some escape digestion and are fermented by microbes in either the small or large intestine resulting in an end product= short chain fatty acid
Polysaccharides
-starch and glycogen
-non-starch polysaccharides (NSP)
Non-Ruminant diets
-often contain variable amounts of both starch and non-starch polysaccharides
Starch
-major storage of carbohydrate of grains
-composed of glucose units; consists of amylose and amylopectin polymers
Cereal starch components
-25% amylose
-75% amylopectin
Amylose
-a linear chain of glucose residues linked by alpha-(1,4)-glycosidic bonds
Amylopectin
-highly branched
-composed of both alpha (1,6)- and alpha (1,4)- glycosidic bonds
Starch hydrolysis
-starch is hydrolyzed to maltose, maltotriose and isomaltose forming dextrins
-dextrins then hydrolyzed to glucose in small intestine
Dextrins
-term describing small molecular weight compounds obtained from starch hydrolysis
What effects rate and extent of starch breakdown?
-nature of the structure (crystallinity) of the starch molecule
-amylose:amylopectin ratio
-type and extent of processing
Resistant starch
-not digested in the small intestine but instead fermented in large intestine
-inaccessible to enzymes because enclosed in an indigestible matrix (some grains)
-due to the physical conformation (granules) or chemical structure
Dietary fibre
-fraction undigested by the GIT of non-ruminants
Crude fibre vs. total dietary fibre
-Crude fibre only includes insoluble dietary fiber
-Total dietary fiber includes both soluble and insoluble dietary fibre= total non-digestible material…would be better used on food bags
Trehalose
-sugar commonly found in bacteria, fungi, plants
Starch digestion steps
1.Mouth: Secretion of alpha-amylase (alpha 1,4 endoglucosidase) in the saliva (mostly omnivores) breaking starch down into dextrins
2. continues in the stomach but gastric digestion of CHO is limited
3.majority of action in the small intestine which requires a neutral pH. alpha amylase works in the centre of starch molecules to produce intermediate length polysaccharides dextrins
4.Amylase is unable to break down amylopectin alpha 1-6 bonds resulting in limit dextrins (similar structure to glycogen)
5. Membrane bound oligosaccharidases/alpha-glucosidases that are synthesized in enterocytes continues digestion.
Amylase deficiency
-rare (encoded by several genes and secreted in excess)
-some starch will not be broken down by amylase, this is due to the physical structure of the starch granule not a lack of amylase
-low activity in neonates
Membrane bound oligosaccharidases
-sucrase-isomaltase
-maltase-glucoamylase
-lactase
Maltase-glucoamylase
-has 2 active sites
1.maltase site hydrolyzes terminal alpha 1-4 linked D-glucose residues to form maltose or maltotriose
2.glucoamylase site hydrolyzes alpha 1-4 bonds, AND alpha 1-6 bonds to free beta-D-glucose bonds
Sucrase-isomaltase
-sucrase site hydrolyzes sucrose into fructose and glucose
-isomaltase site hydrolyzes the alpha 1,6 linkages of the limit dextrins
Luminal phase of starch digestion production
-produces only a limited amount of free glucose
Lactose
-hydrolyzed to glucose and galactose by lactase
-lactase activity declines by 90% by age 5 in most humans
>mutations in some populations results in persistence of lactase activity
European dogs and lactase persistence
-dogs domesticated long ago had an adaptation to agricultural practices by allowing them to have lactase persistency and allowed them to digest milk
**arose probably due to dairy industry
Cats vs dogs carbohydrate enzymes
-dogs: enzyme activity decreases down the GI tract
-cats: not adapted for carbohydrate digestion/absorption. Low numbers of enzymes compared to dogs
Absorption of monosaccharides made from digestion
*driven by ATPase on basolateral side
1.Glucose and galactose are transported across the apical membrane by the Na/glucose cotransporter (2Na with a hexose)=SGLT1 (sends Na down concentration gradient)
2. Na exits enterocyte via Na/K ATPase on basolateral side (maintains Na concentration gradient but requires energy)
3.Fructose enters enterocyte via facilitated fructose transporter (GLUT 5)
**fructose not well absorbed.. due to low affinity transporter (especially compared to glucose)
4.Hexoses exit the enterocyte via facilitated glucose transporter (GLUT 2), moving them down their concentration gradient into interstitial fluid, capillaries and then the portal vein
SGLT1
-apical membrane enterocyte of the small intestine
SGLT 5
-apical membrane of the enterocyte in the small intestine, luminal membrane of proximal renal tubules
GLUT 1
-found in all cells, provides cells with basal glucose requirement
GLUT 2
-major glucose transporter in hepatocytes, pancreatic beta cells
-found on basolateral and sometimes apical membrane of enterocytes
GLUT 5
-fructose transporter
-expressed in many tissues
