Biochemistry

Carbohydrate Digestion and Metabolism

Carbohydrates
Carbohydrates are composed of carbon and water and have a composition of (CH2O)n. The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram.
energy
Carbon dioxide Water Chlorophyll
GLUCOSE
6 CO2 + 6 H20 + energy (sun)
C6H12O6 + 6 O2

120 grams of glucose / day = 480 calories

Simple Sugars -

Disaccharides

Complex carbohydrates

Oligosaccharides Polysaccharides Starch Glycogen Dietary fiber (Dr. Firkins)

Starch

Major storage carbohydrate in higher plantsAmylose – long straight glucose chains (α1-4)Amylopectin – branched every 24-30 glc residues (α 1-6)Provides 80% of dietary calories in humans worldwide

Glycogen

a 1-4 link
G
G
G
G
G
G
G
G
G
G
G
G
G
a 1-6 link
G
G
G
G
G
G
Major storage carbohydrate in animalsLong straight glucose chains (α 1-4)Branched every 4-8 glc residues (α 1-6)More branched than starchLess osmotic pressureEasily mobilized


Digestion
Pre-stomach – Salivary amylase : a 1-4 endoglycosidase G
G
G
G
G
G
G
G
a 1-4 link
G
G
G
G
a 1-6 link
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
maltose
G
G
G
isomaltose
amylase
maltotriose
G
G
G
G
a Limit dextrins

Small Intestine

Pancreatic enzymes a-amylase
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
amylose
amylopectin
G
G
G
G
G
a amylase
+
G
G
G
G
G
maltotriose
maltose
a Limit dextrins
G

Oligosaccharide digestion..cont

G
G
G
G
G
G
G
G
G
G
G
G
Glucoamylase (maltase) or a-dextrinase
G
G
G
G
G
a-dextrinase
G
G
G
G
G
G
G
maltase
sucrase
a Limit dextrins
G

Small intestine

Portal for transport of virtually all nutrients Water and electrolyte balance
Enzymes associated with intestinal surface membranes Sucrase a dextrinase Glucoamylase (maltase) Lactase peptidases

Carbohydrate absorption

Hexose transporter
apical
basolateral

Digestion of Carbohydrates

Monosaccharides Do not need hydrolysis before absorption Very little (if any) in most feeds Di- and poly-saccharides Relatively large molecules Must be hydrolyzed prior to absorption Hydrolyzed to monosaccharides Only monosaccharides can be absorbed

Non-Ruminant Carbohydrate Digestion

Mouth
Salivary amylase
Breaks starches down to maltose Plays only a small role in breakdown because of the short time food is in the mouth Ruminants do not have this enzyme Not all monogastrics secrete it in saliva

Carbohydrate Digestion

Pancreas
Pancreatic amylase
Hydrolyzes alpha 1-4 linkages Produces monosaccharides, disaccharides, and polysaccharides Major importance in hydrolyzing starch and glycogen to maltose
Polysaccharides
Disaccharides
Amylase

Digestion in Small Intestine

Digestion mediated by enzymes synthesized by cells lining the small intestine (brush border)
Disaccharides
Monosaccharides
Brush Border Enzymes
* Exception is β-1,4 bonds in cellulose

Digestion in Small Intestine

Maltose
Glucose + Glucose
Maltase
Lactose
Lactase
Glucose + Galactose
* Poultry do not have lactase
Sucrose
Glucose + Fructose
Sucrase
* Ruminants do not have sucrase

Overview Monogastric Carbohydrate Digestion

Location Enzymes Form of Dietary CHOMouth Salivary Amylase Starch Maltose Sucrose LactoseStomach (amylase from saliva) Dextrin→MaltoseSmall Intestine Pancreatic Amylase Maltose Brush Border Enzymes Glucose Fructose Galactose + + + Glucose Glucose GlucoseLarge Intestine NoneBacterial Microflora Ferment Cellulose

Carbohydrate Absorption in Monogastrics

With exception of newborn animal (first 24 hours), no di-, tri-, or polysaccharides are absorbed Monosaccharides absorbed primarily in duodenum and jejunum
Little absorption in stomach and large intestine

Carbohydrates Monosaccharides

Small Intestine
Active Transport
Liver
Portal Vein
Distributed to tissue through circulation

Nutrient Absorption - Carbohydrate

Active transport for glucose and galactose Sodium-glucose transporter 1 (SGLT1) Dependent on Na/K ATPase pump Facilitated transport for fructose

Summary of Carbohydrate in Monogastrics

Polysaccharides broken down to monosaccharides Monosaccharides taken up by active transport or facilitated diffusion and carried to liver Glucose is transported to cells requiring energy Insulin influences rate of cellular uptake

Glucose

Glucose-6-P
Pyruvate
Hexokinase
Pentose Phosphate Shunt
glycolysis
Carbohydrates Metabolism in Monogastrics
Serve as primary source of energy in the cell Central to all metabolic processes
Glc-1- phosphate
glycogen
Cytosol - anaerobic

Pyruvate

cytosol
Aceytl CoA
mitochondria (aerobic)
Krebs cycle
Reducing equivalents
Oxidative Phosphorylation (ATP)
AMINO ACIDS
FATTY ACIDS

Control of enzyme activity

Rate limiting step

Glucose utilization

Stage 1 – postparandial All tissues utilize glucoseStage 2 – postabsorptive KEY – Maintain blood glucoseGlycogenolysisGlucogneogenesisLactatePyruvateGlycerolAAPropionateSpare glucose by metabolizing fatStage 3- Early starvationGluconeogenesisStave 4 – Intermediate starvationgluconeogenesisKetone bodiesStage 5 – Starvation

Carbohydrate Metabolism/ Utilization- Tissue Specificity

Muscle – cardiac and skeletalOxidize glucose/produce and store glycogen (fed)Breakdown glycogen (fasted state)Shift to other fuels in fasting state (fatty acids)Adipose and liverGlucose  acetyl CoAGlucose to glycerol for triglyceride synthesisLiver releases glucose for other tissuesNervous systemAlways use glucose except during extreme fastsReproductive tract/mammaryGlucose required by fetusLactose  major milk carbohydrateRed blood cellsNo mitochondriaOxidize glucose to lactateLactate returned to liver for Gluconeogenesis

Carbohydrate Digestion Rate

Composition and Digestion of Carbohydrate Fractions
___________________________________________________________
Composition Rumen Digestion (%/h)
_____________________________________________________
Sugars 200-350 Fermentation and Organic Acids 1-2 Starch 10-40 Soluble Available Fiber 40-60 Pectins B glucans Insoluble Available Fiber 2-10 Cellulose Hemicellulose Unavailable Fiber (lignin) 0
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