exam 3

most abundant biomolecule in nature

carbohydrates

cellular functions of carbs

energy, structure, communication, precursurs for other biomolecules

what types of energy do carbohydrates link

solar energy and chemical bond energy

monosaccharides

• simple sugars

• polyhydroxy aldehydes or ketones

aldoses

monosaccharides with an aldehyde functional group

ketoses

monosaccharides with a ketone functional group

how are carbohydrates classified

• aldose/ketose

• number of carbon atoms/the functional groups they contain

triose

3 carbon carbohydrate

tetrose

4 carbon carbohydrate

pentose

5 carbon carbohydrate

hexose

6 carbon carbohydrate

most abundant types of carbs in living systems

pentoses and hexoses

monosaccharide stereoisomers

• increase in number of chiral carbon increases the number of possible optical isomers

• 2^n where n is number of chiral carbons

what are all monosaccharides derived from

D-glyceraldehyde or nonchiral dihydroxyacetone

how to determine if monosaccharide is D or L

• look at chiral carbon furthest from carbonyl

• D : hydroxyl on right

  • rotates light clockwise

• L : hydroxyl on left

  • rotates light counterclockwise

what form are almost all naturally occurring monosaccharides

D

how to determine D or L of carbohydrate

reference carbon is the asymmetric carbon furthest from the carbonyl carbon

diastereomers

stereoisomers that are not enantiomers

enantiomer

nonsuperimposable mirror image

epimers

diastereomers that differ at a single chiral carbon

sugars with more than _____ carbons primarily exist in cyclic forms

4

how does ring structure occur in monosaccharides

aldehyde and ketone groups react reversibly with hydroxyl groups in an aqueous solution to form hemiacetals and hemiketals

anomers

2 possible diastereomers that form because of cyclization

glucose structure

haworth structures

more accurately depict bond angle and length in ring structures than the original Fischer structures

drawing haworth from fischer

• if OH is on the left in the fischer, it is up in the haworth

• if OH is on the right in the fischer, it is down in the haworth

• last carbon faces up

alpha vs beta forms

In the D-sugar form, when the anomer hydroxyl is up, it gives a b-anomeric form while down gives the a-anomeric form

furanoses

5-membered rings with an oxyen

pyranoses

6-membered rings with an oxygen

cyclic form of fructose

fructofuranose

glucose in the pyranos form

glucopyranose

anomeric carbon

• carbon that was the aldehyde in the fischer projection but has OH in haworth structure

• can shift back to the carbonyl and react as such

• C1 in glucose, C2 in fructose

mutarotation

• spontaneous process in which alpha and beta forms of monosaccharides are readily interconverted in aqueous environments

• ring form opens back to chain then forms ring again in different form

• produces a mixture of alpha and beta forms in both furanose and pyranose ring structures

which form can participate in redox reactions

open chain form

which form is most stable

ring form

which groups in monosaccharides can react

carbonyl and hydroxyl groups

most important reactions of monosaccharides

• oxidation

• reduction

• isomerization

• esterification

• glycoside formation

• glycosylation reactions

oxidation

monosaccharides may readily undergo several oxidation reactions in the presence of metal ions or certain enzymes

what are aldehydes oxidized to

carboxylic acid

what are primary alcohols oxidized to

aldehyde

isomerization

• switching between open chain to ring

• mutarotation

• D-glucose incubated in an alkaline solution for several hours produces 2 isomers: D-mannose and D-fructose

  • both involve enediol intermediate

what does the enediol intermediate allow for

aldose-ketose interconversion and epimerization

epimerization

glucose converted to mannose

reducing sugars

• sugars that can be oxidized by weak oxidizing agents such as Benedicts reagents

• NOT a reduced sugar

• have free anomeric carbon that can open up and react

what does a sugar need to be a reducing sugar

• open chain

• all aldoses are reducing sugars

• ketoses such as fructose due to isomerization

glycoside formation

• hemiacetals and hemiketals react with alcohols form the corresponding acetal and ketal

• when cyclic hemiacetal or hemiketal form of the monosaccharide reacts with an alcohol, the new linkage is a glycosidic linkage and the compound is a glycoside

disaccharide formation

acetal linkage formed between hemiacetal hydroxyl of one monosaccharide and the hydroxyl of another

polysaccharide formation

large numbers of monosaccharides are linked together through acetal linkages

glucose (d-glucose)

• originally called dextrose

• blood sugar

• found in large quantities throughout natural world

• primary fuel for living cells

• preferred energy source for brain cells and cells without mitochondria (erythrocytes)

fructose (d-fructose)

• fruit sugar because high content in fruit

• twice as sweet as sucrose per gram

• often used as a sweetening agent in processed foods

• sperm use fructose as an energy source

galactose

• necessary to synthesize variety of important biomolecules

  • lactose, glycolypids, phospholipids, proteoglycan, glycoproteins

• epimer of glucose

galactosemia

genetic disorder resulting from a missing enzyme in galactose metabolism

monosaccharide derivatives

• amino sugars

• deoxy sugars

amino sugars

• hydroxyl group (usually on C2) replaced with amine group

• most common are d-glucosamine and d-galactosamine and are often attached to proteins or lipids

deoxy sugars

• monosaccharides that have an OH replaced by an H or CH3

• reduced sugar

• 2-deoxy-d-ribose is pentose sugar of DNA

• fucose (6-deoxygalactose) is part of ABO blood group determination

disaccharides

two monosaccharides linked by a glycosidic bond (covalent)

how are linkages named for disaccharides

1. α and β confirmation of anomeric carbon

2. which carbons are connected

hemiacetal

hemiketal

lactose

• milk sugar

• disaccharide found in milk

• one molecule of galactose linked to one molecule of glucose

  • β(1,4) linkage

• deficiency of lactase is common

• reducing sugar

maltose

• disaccharide

• malt sugar

• intermediate product of starch hydrolysis

• α(1,4) linkage between two molecules of glucose

• does not exist freely in nature

how to determine α vs β in disaccharides

difference in free anomeric carbon

what type of sugars can be absorbed in the digestive tract?

monosaccharides

α glycosidic bond

β glycosidic bond

cellubiose

• disaccharide

• degradation product of cellulose

• composed of two molecules of glucose linked with a β(1,4) bond

• does not exist freely in nature

sucrose

• disaccharide

• table sugar (cane/beet sugar)

• produced in leaves and stems of plants

• one molecule of glucose linked to one molecule of fructose by an α,β(1,2) glycosidic bond

  • glycosidic bond between both anomeric carbons

• nonreducing sugar

polysaccharides (glycans)

composed of large numbers of monosaccharides connected by glycosidic linkages

oligosaccharides

• smaller glycans made of 10-15 monomers

• most often attached to polypeptides as glycoproteins

2 classes of oligosaccharides

N- and O-linked

classes of polysaccharides

homoglycans and heteroglycans

polysaccharides can be ___________ or __________ in shape

linear; branched

homoglycans

• have one type of monosaccharide and are found in starch, glycogen, cellulose, chitin

• no fixed molecular weight because size is reflection of the metabolic state of cell producing them

starch and glycogen are ___________ molecles while chitin and cellulose are _________

energy storage; structural

chitin

• polysaccharide

• part of cell wall of arthropod exoskeleton

• acetyl group and amine bound to glucose

cellulose

• polysaccharide

• primary component of plant cell walls

starch

• polysaccharide

• energy resevoir of plant cells and a significant source of carbohydrate in the human diet

• two polysaccharides together: amylose and amylopectin

amylose

• composed of long unbranched chains of D-glucose with α(1,4) bonds

• thousands of glucose monomers

amylopectin

• branched polymer

• α(1,6) and β(1,4) linkages between glucoses

• branch points every 20-25 residues (moderately)

glycogen

• polysaccharide

• carb storage molecule in vertebrates found in greatest abundance in liver and muscle cells

• similar in structure to amylopectin, with more branch points (every 10-highly branched)

• more compact and easily mobilized than other polysaccharides

• glucoses linked by α(1,4) and α(1,6)

  • α(1,6) is branch point

cellulose

• polymer of D-glucopyranosides linked by beta(1,4) glycosidic bonds

• unbranched

• made of 12,000 glucose units

most abundant organic substance on earth

cellulose

most important structural polysaccharide of plants

cellulose

myofibrils

• pairs of unbranched cellulose molecules held together by hydrogen bonding to form sheetlike strips

• tough and inflexible

• high tensile strength

heteroglycans

high molecular-weight carbohydrate polymers that contain more than one type of monosaccharide

major types of heteroglycans

• N and O linked glucosaminoglycans (glycans)

• glycan components of glycolipids

• GPI anchors

GPI

glycolsylphosphatidylinosotol

N and O glycans

• many proteins have N and O linked oligosaccharides

• N linked: liked via b-glycosidic bond

  • bond through amino group

• O linked: disaccharide core of galactosyl-β-(1,3)-N-acetylgalactosamine linked via a α-glycosydic bond to the hydroxyl of serine or threonine residues

  • bond through O

glycoconjugates

result from carbohydrates being linked to proteins and lipids

proteoglycans

• distinguished from other glycoproteins by high carbohydrate content (95%)

• occur on cell surfaces or are secreted to ECM

• all contain heteroglycan chain that are linked to core proteins by N and O glycosidic bonds

• have roles in organizing ECM and involved in signal transduction

• metabolism of proteoglycans involved in many genetic disorders, including Hurler’s syndrome

aggrecan

• type of proteoglycan that is found in abundance in cartilage

• core protein linked to over 100 chondroitin sulfate and 40 keratin sulfate chains

• up to 100 aggrecans are attached to hyaluronic acid to form proteoglycan aggregate

glycoproteins

• proteins that are covalently linked to carbohydrates through N and O linkages

• several addition reactions in lumen of ER and Golgi are responsible for final N linked oligosaccharide structure

• O glycan synthesis occurs later, probably in Golgi

• carb could be 1%-85% of total weight

functions of glycoproteins

• enzymes

• blood clotting

• hormone

• receptor proteins

• transport proteins

• cell adhesion

the sugar code

• sugars placed on molecules that change function, has ability to encode information in cell

• living organisms require large coding capacities for information transfer because of profound complexity of functioning systems

• to succeed as a coding mechanism, a class of molecules must have a large capacity for variation

• more possibilities with hexasaccharides than hexapeptides

most important posttranslational modification in terms of coding capacity

glycosylation

glycome

• total set of sugars and glycans in cell or organism

• constantly in flux depending on cell’s response to environment (different than genome)

• no template for glycan biosynthesis; done in a stepwise process

glycoforms

can result based on slight variations in glycan composition of each glycoprotein

catabolism

the degradation of fuel molecules which provides energy for cellular energy-requiring functions

cells use an energy conversion strategy that oxidizes glucose

• small amounts of energy are released at several points in this pathway

• energy is harvested and stored in bonds of ATP

universal energy currency

ATP (adenoside triphosphate)