Week 9 Carbohydrate, Digestion and Metabolism

Catabolism

Breakdown of molecules by enzymes to release energy stored in chemical bonds

Anabolism

Use of energy to build complex molecules or power cellular processes

Carbohydrate intake

45–65% of diet should be carbohydrates

Carbohydrate storage

Stored as glycogen in liver and muscle

Animal carbohydrate source

Dairy products contain lactose

Carbohydrate composition

General formula CnH2nOn and often end in -ose

Monosaccharides composition

Simple sugars with >3 carbons, hydroxyl groups, and a ketone or aldehyde

Oligosaccharides

Chains of 3–10 monosaccharides

Polysaccharides

Chains of more than 10 monosaccharides

Monosaccharide structure

• Exists in equilibrium between ring (anomer) and linear forms

• Ring form predominates over linear form

alpha anomer

OH group at C1 is at the bottom

Beta anomer

OH group at C1 is in the top position

Ring formation step 1

the carbonyl group C=O, bonds with any hydroxyl group

Ring formation summary

Hydroxide bonds to carbon and loses its H+ to form an ether bond

Ring formation step 3

H+ binds to O on C=O to form another OH which determines the alpha or beta position

Glycosidic bond formation

The OH between two molecules undergo a condensation reaction to form a glycosidic bonds

Lactose composition

Galactose + glucose

Sucrose composition

Fructose + glucose

Glycogen and starch structure

Glucose polymers with α-1,4 and α-1,6 bonds

Cellulose structure

Glucose polymer with β-1,4 bonds

Cellulose digestion

Indigestible in humans due to lack of enzyme for β-1,4 bonds

Dietary fiber

Indigestible carbohydrates like cellulose

Soluble fiber

• Forms gel with water and reduces cholesterol by binding bile salts

• Reduce blood cholesterol by binding bile slats together which reduces cholesterol reabsorption by the intestine

Insoluble fiber function

Slows movement and absorption in GI tract

Carbohydrate digestion

Enzyme-catalyzed hydrolysis reactions which use covalent catalysis and acid-based catalysis

Salivary amylase

Works in mouth at pH ~6 and is inactivated in stomach

Brush border enzymes

in the small intestine, they catalyze the final hydrolysis reaction to produce monosaccharides that can be absorbed

Brush border enzymes location

Integral membrane proteins in small intestine

Absorption of monosaccharides

Enter capillaries and travel to liver via hepatic circulation

what happens to di/polysaccharides left unabsorbed

May be metabolized by bacteria in the colon, producing gas, short-chain fatty acids and lactate

Lactase function

Breaks lactose into glucose and galactose

Lactase activity across lifespan

Highest in infants and declines with age

Lactase persistence phenotype

Continued high lactase expression in some populations who have continued consumption of dairy into adulthood

Symptoms of lactose intolerance

Gas, lactate production, H+, diarrhea from osmotic water influx because of excess solute (lactose) concentration

Na+ glucose co-transporter

Uses secondary active transport

GLUT transporters

Use facilitated diffusion for glucose movement

SGLT1 function

move glucose and Na+ from the small intestine to the mucosal cells

SGLT2 function

move glucose and Na+ from the renal tubule into the epithelial cells

GLUT1 function

insulin independent, imports glucose from the blood into RBC and across the blood brain barrier

GLUT2 function

insulin independent, imports glucose from the mucosal cells to blood and from blood into liver and pancreas

GLUT3 function

insulin independent, imports glucose from the blood into brain, neurons, sperm

GLUT4 function

Insulin-dependent, transport into muscle, heart, adipose tissue

GLUT5 function

Transports fructose in small intestine to mucosal cells

Brain glucose dependence

Neurons rely primarily on glucose for energy

Brain glucose gradient

• Glucose is transported a little faster than it is used into the brain, but it is used VERY quickly by neurons

• This establishes a concentration gradient where glucose is always lower in the brain than the blood

Brain transporter

Uses GLUT3 with high affinity and high capacity

Hypoglycemia

Blood glucose <4 mmol, result in less glucose being transported into neural tissue and decreased energy production

Hypoglycemia effects

Less glucose being transported into neural tissue and decreased energy production

Hypoglycemia symptoms

Neurogenic (affecting sympathetic nervous system) and neuroglycopenic (direct brain dysfunction) symptoms