nutr 455 exam 2 (

Carbohydrates:

major source of energy fuel

supply \>50% of total caloric intake

Simple carbohydrates include

monosaccharides and disaccharides

Complex carbohydrates include

oligosaccharides (3-10 units) and polysaccharides

Monosaccharides

Glucose, Fructose, Galactose

Disaccharides

Lactose, Sucrose, Maltose, Trehalose

important polysaccharides

starch and glycogen

Structural features on monosaccharides

stereoisomerism: chiral carbons

ring structure

Stereoisomerism

Compounds with the same structural formula but a different arrangement of atoms in space

Contains one or more asymmetrical or chiral carbon atoms.

chiral carbons

have four different groups attached

chiral molecules

-a molecule that is non super-imposable upon its mirror image or lacks an internal plane of symmetry

enantiomers

molecules that are mirror images of each other

D enantiomer

OH group on the right

L enantiomer

OH group on the left

Only \_____ isomers serve as a substrate for digestive enzymes (meaning they are metabolically active)

D

anomeric carbon

the new chiral center formed in ring closure

the carbon containing the carbonyl in the straight-chain form

OH group oriented up

beta

OH group oriented down

alpha

The \_________ form of monosaccharides is dominant in nature

beta

reducing sugars

cyclized monosaccharides

capable of reducing other substances

useful to identify which end of the polysaccharide has monosaccharide unit that can open and close

reducing end

the end of a carbohydrate chain that has a free anomeric carbon

alpha-D-glucose structure

D-glucose structure

beta-D-galactose structure

D-galactose structure

beta-D-fructose structure

D-fructose structure

pentose monosaccharides

Components of DNA, RNA

Components of key nucleotides (ATP, ADP, AMP, NAD, NDAP)

pentose monosaccharide examples

ribose, deoxyribose, xylose, arabinose, etc.

\_______________ is a reduction product of ribose

ribitol

amino sugars

monosaccharide derivatives that contain an amino group in place of a hydroxyl group on carbon 2

two examples of amino sugars

glucosamine and galactosamine

\________________ is a constituent of follicle stimulating hormone and is a hepatotoxic

galactosamine

\__________________ can be used as therapy for osteoarthritis

glucosamine

sugar phosphates

make up the backbone of DNA

monosaccharide derivatives

three examples of sugar phosphates

glucose-1-phosphate

fructose-1,6-biphosphate

adenosine-5'-triphosphate (ATP)

disaccharides

carbohydrates made up of two monosaccharide units attached to one another

in a disaccharide, the glycosidic bond is formed between:

the -OH of one unit and -OH of a second unit, with the elimination of H2O

usually between the anomeric carbon of one unit and C4 or C6 of a second unit

the glycosidic bond can be either

alpha (down) or beta (up)

disaccharide examples

maltose, sucrose, trehalose, lactose

Maltose:

formed primarily from partial hydrolysis of starch

found in malt beverages (beer or malt liquors)

Maltose structure

glucose + glucose

alpha(1--\>4) glycosidic bond

alpha-maltose

OH on the free anomeric carbon oriented down

beta-maltose

OH on the free anomeric carbon oriented up

Sucrose:

Most commonly used natural sweetener

Glycosidic bond between the anomeric hydroxyl of both units

Not a reducing sugar (no free anomeric carbon)

Sucrose structure

glucose + fructose

alpha(1--\>2) glycosidic bond

Lactose

Galactose B1-\>4 Glucose

Trahalose:

Found naturally in fungi (mushroom), plants.

Glycosidic bond is 𝛂(1--\>1) between 2 D-glucose molecules

Not a reducing sugar (no free anomeric carbon)

Rank simple sugars in order of increasing relative sweetness

lactose < maltose < glucose < sucrose < fructose

Oligosaccharides

3-10 monosaccharides linked together by glycosidic bonds

oligosaccharide examples

raffinose (trisaccharide)

stachyoses (tetrasaccharide)

verbascose (pentasaccharide)

dextrin

raffinose (trisaccharide), stachyoses (tetrasaccharide), and verbascose (pentasaccharide)

Composed of: Glucose, Galactose, Fructose

Found in: beans, peas, bran, whole grains

Human digestive enzymes do not hydrolyze them but bacteria in the colon do

dextrin

Hydrolyzed from starch

Composed of glucose

Addictive in foods and pharmaceuticals.

Polysaccharides

Carbohydrates that are made up hundreds to thousands of monosaccharides linked by glycosidic bonds

Homopolysaccharides \-- more important in nutrition (made of glucose)

heteropolysaccharides

Have one reducing end and one non-reducing end

Starch

A storage polysaccharide in plants consisting entirely of glucose

Enriched in cereal grains, potatoes, legumes

Two forms of starch

amylose - linear

amylopectin - branched

amylose

a linear chain of glucose molecules bonded together by alpha(1--\>4) glycosidic bonds

unbranched

helical conformation

makes up 15-20% of total starch

amylopectin

consists of glucose molecules bonded together in a highly branched arrangement

has both alpha(1--\>4) and alpha(1--\>6) glycosidic bonds

alpha(1--\>6) bonds occur at branch points

makes up 80-85% of total starch

glycogen

Analogue of starch (sometimes referred to as "animal starch") having a similar structure to amylopectin but more extensively branched

has both alpha(1--\>4) and alpha(1--\>6) glycosidic bonds

alpha(1--\>6) bonds occur at branch points - enzymes can hydrolyze many glucose molecules simultaneously for quick release of glucose

found in the form of granules in the cytoplasm in many cell types

can amount to 8% of the liver's wet weight soon after a meal (100-120 g)

cellulose

Major component of plant cell walls.

Homopolysaccharide of glucose

linked by beta(1--\>4) glycosidic bonds

Indigestible by mammalian digestive enzymes

A dietary fiber

Can be digested by colonic bacteria and generate short-chain fatty acids

\_________________ and \_________________ are the most abundant carbohydrates in the food

polysaccharides, disaccharides

All the carbohydrates need to be hydrolyzed into \___________________ in the digestive system

monosaccharides

the GI tract only absorbs \__________________

monosaccharides

Partial digestion of polysaccharides occurs in \________________

the mouth

\____________________ in the mouth cleaves alpha(1--\>4) glycosidic bonds

After the bonds are cleaved, what is produced?

salivary alpha-amylase

dextrin --\> maltose

Why does carbohydrate digestion NOT occur in the stomach?

the acidic environment causes amylase to lose its activity

Major digestion of polysaccharides occurs in:

the lumen of small intestines

what type of bonds does pancreatic alpha-amylase cleave?

After the bonds are cleaved, what is produced?

alpha(1--\>4)

dextrins --\> limit dextrins, moltotriose, and maltose

what is the only enzyme that can cleave alpha(1--\>6) bonds?

alpha-dextrinase

What are the 5 brush border enzymes?

alpha-dextrinase
lactase
sucrase
maltase
trehalase

Maltase function:

hydrolyzes maltose to two glucose molecules

cleaves alpha(1--\>4) bonds

Alpha-dextrinase function:

hydrolyzes limit dextrins to three glucose molecules

cleaves alpha(1--\>4) bonds and alpha(1--\>6) bonds

Lactase function:

hydrolyzes lactose to galactose and glucose

cleaves beta(1--\>4) bonds

Sucrase function:

hydrolyzes sucrose to fructose and glucose

cleaves alpha(1--\>2) bonds

Trehalase function:

hydrolyzes trehalose to two glucose molecules

cleaves alpha(1--\>1) bonds

What mechanisms are used to absorb monosaccharides into enterocytes?

Sodium glucose transporter 1 (SGLT1)

Na+/K+-ATPase

Where is SGLT1 located?

on the lumen side (microvillus) of the membrane

How does SGLT1 function?

simultaneously transports one molecule of glucose (or galactose) and two molecules of Na+ in the same direction (symporter)

has two binding sites (one that binds Na+ and the other binds glucose)

the glucose binding site is not available unless the transport protein has already bound Na+

Where is Na+/K+-ATPase located?

the basolateral membrane

How does Na+/K+-ATPase function?

Na+ that enters the cell through SGLT1 is "pumped" back out by Na+/K+-ATPase

Na+/K+-ATPase combines ATP in the presence of Na+ on the inner surface of the cell membrane \-- the enzyme is then phosphorylated by the breakdown of ATP to ADP and is able to move three Na+ out of the enterocyte

the enzyme becomes dephosphorylated on the outer membrane in the presence of K+ and is able to transport two K+ into the cell

Na+ and K+ are both transported against their concentration gradients

What mechanism is used to absorb monosaccharides into the blood?

GLUT2

Where is GLUT2 located?

the basolateral membrane

What is GLUT2?

a low-affinity, high-capacity transporter that facilitates the exit of glucose, galactose, and fructose from the enterocyte into the underlying capillaries for delivery into the hepatic portal vein

as the glucose concentration increases inside the cell, it is able to bind to GLUT2

Besides transport of glucose and galactose into the blood, what is another function of GLUT2?

If the concentration of glucose/galactose in the intestinal lumen are high, GLUT2 moves to the lumen side of the intestinal membrane to facilitate the transport of glucose and galactose into the enterocyte (low-affinity, high-capacity)

What mechanism is used to transport fructose into enterocytes?

GLUT5

Where is GLUT5 located?

on the lumen side of the membrane

How is GLUT5 unique?

No energy is require - facilitated diffusion only
(this means that absorption is slower than for glucose and galactose since they use active transport)

Has a high affinity for fructose (can bind even when concentration is low)

typically no fructose exits into circulation because it is trapped in the liver cells

What happens to galactose after it is absorbed in the small intestine?

it is converted into glucose derivatives (100% liver glycogen)

What happens to fructose after it is absorbed in the small intestine?

it is catabolized for energy (100% triaclyglycerol)

What happens to glucose after it is absorbed in the small intestine?

it is catabolized for energy
(30-40% liver glycogen, 60-70% returns to circulation and is distributed to other organs)

High intake of fructose could lead to

fatty liver

There are a total of \_____ GLUTs but only \_____ are involved in glucose transport

14, 5

All cells in our body express at least \______ GLUT isoform(s)

1

All GLUTs are

transmembrane proteins

GLUT1

Substrates:

Affinity:

Most ubiquitously expressed

glucose, galactose, mannose, and glucosamine

high

GLUT2

Substrates:

Affinity:

Transports from enterocytes into the blood

glucose, galactose, fructose, mannonse, and glucosamine

low

GLUT3

Substrates:

Affinity:

Main GLUT in the brain

Glucose, galactose, mannose, xylose, and dehydroascorbic acid

high

GLUT4

Substrates:

Affinity:

Regulates hyperglycemia

Glucose, glucosamine, and dehydroascorbic acid

low

GLUT5

Substrates:

Affinity:

Specific for fructose

fructose only

high

Insulin

is secreted by 𝛃 cells in the pancreas upon increase of blood glucose

Plays a role in glucose uptake in muscle and adipose tissue and maintenance of blood glucose levels

GLUT4 and insulin

insulin signals the cell to insert GLUT4 transporters into the membrane, allowing glucose to enter the cell

in the absence of insulin, glucose cannot enter the cell