Chaper 10: Carbohydrates and Glycoproteins

carbohydate

• polyhydroxy (2 or more OH groups) aldehydes and ketones, and their derivatives

• hydrophilliv

aldehydes vs ketones

both types of carbonyls (C=O)

• aldehyde → end of molecule; R-C(=O)-H

• ketone → middle of molecules; R-C(=O)-R

monosaccharides

• simplest carbs → called sugars

• 3C→7C length

• aldehyde (aldose) or ketone (ketose) with 2 or more alchohol groups

complex carbohydrates

polysaccharides → 2 or more monosaccharides (monomers)

nomenclature of monosaccharides

• ketose → has a ketone

• aldose → has an aldhedye

• #-ose → number of carbons

  • triose (3), pentose (6)

2 main types of isomers

constitutional and stereoisomers

2 main types of isomers: constitutional

same chemical formula with different bonding order

2 main types of isomers: stereoisomers

same bonding order but different spatial arrangment

• enantiomers → non superimposable mirror images (L and D)

• diastereoisomers → non mirror images

  • epimer

  • anomer

number of possible stereoisomers for a monosaccharide

2ⁿ → n is the number of non symmetric C

D and L isomer enantiomers

• configuration of asymmetric C atom farthest from the aldehyde or keto group

• denotes OH → L for left and D for right

which enantiomer form are most monosaccharides in

D → OH on the right of the last asymmetric C

6 common monosaccharides

• all D form

  • 2 pentoses of nucleic acids → ribose and deoxyribose

  • 4 hexoses

    • 3 aldoses: glucose, mannose, galactose

    • 1 ketose: fructose

why do many monosaccharides cyclise

their OH groups react with their aldehydes/ketones → form hemiacetals and hemiketals

hemiacetals

product of OH + aldehyde → forms pyranose (6 sided ring resembling pyran)

hemiketals

product of OH + ketone → forms furanose (5 sided ring resmbling furan)

anomers

type of diasteroisomers → isomers formed by asymmetric C in cyclic form

• α → OH below plane of the ring

• β → OH above plane of the ring

• anomeric carbon atom → C where α/β OH is

which is the anomeric carbon in aldoses

C1

which is the anomeric carbon in ketoses

C2

anomer equilibrium of glucose

• 99% pyranose (forms from hemiacetal)

  • 1/3 α anomer

  • 2/3 β anomer

• <1% open chain

mutarotation

• constant interconvertion of anomers by opening to linear stucture and reclosing again

• fast

anomer equilibrium of fructose

exists as a fructofuranose (5C ring) or fructopyranose (6C ring)

• predominately pyranose → less steric clashing

• furanose only predominates if more groups attach and disfavor/prevent pyranose

reducing sugars

• sugar w free aldehyde → gets oxidixed, thus reduces another compound

• all monosaccharide that can access linear stucture → includes ketoses (can breifly become aldoses)

glycation

spantaneous addition of a carb to another molecule

glycosylation

enzymatically catalized addition of a carb to another molecule

oxidixed form of glucose

gluconic acid → aldehyde group becomes a carboxyl

reduced form of glucose

sorbitol

glucose and glycation

• unlikely to glycate proteins

• when glucose high and protein high, like with glucose and hemoglobin in blood, glycation occurs

• called glycated hemoglobin (hemoglobin A1c or A1c)

  • marker of blood glucose control

common reactants for monosaccharide modification

alcohols, amines, and phosphates

glycosidic linkage

covalent linked anomeric carbon to O of an alchohol

• specifxally called O-glycosidic linkage

N-glycosidic linkage

colavently linked anomeric carbon to to N of amine

• linkage in nucleosides

which 2 bonds link sugars to proteins

O-glycosidic and N-glycosidic links

why is phosphorylation of sugars useful

good in metabolic pathways

• turns sugar anionic → dont cross bilayer, dont get transported

• blocks alternate ring from forming → phosphylation of fructofuranose prevents both anomers of fructopyranose

• makes reactive interediates

oligosaccharide

2 or more monosaccharides linked by O-glycosidic bond

directionality of an oligosaccharide

• reducing end → end with the free anomeric carbon

• still called reducing end even if it is bound to another molecule

common disaccharides

sucrose, maltose, lactose

monomers of sucrose and its enzyme

• a-gluctose + b-fructose

• sucrase

why isn’t sucrose a reducing sugar

both anomeric carbons are involved in the glycosidic linkage

maltose and its enzyme

• α-glucose + α-glucose

• maltase

lactose and its enzyme

• α-glucose + b-galactose

• lactase

polysaccharide

aka glycans. polymers of monosaccharides

carb homopolymer

all the same monomers

glycogen

• homopolymer of glucose in animals

• large and branched

• glucose links by α-1,4-glycosidic linkages

• branching links by α-1,6-glycosidic linkages

starch

• homopolymer of glucose in plants

• amylose (unbranched) or amylopectin (branched)

starch: amylose

unbranched → glucose bonds by α-1,4 glycosidic linkages

starch: amylopectin

branched → one α-1,6 linkage per 30 α-1,4 linkages; less branched version of glycogen

what hydrolyses amylopectin, amylose and glycogen

α-amylase

celluloe

• polysaccharide of glucose in plants for structure

• unbranched

• β-1,4 linkages → long straight chains, forms rigid H-bonded fibrils

• insoluable and indigestable

chitin

• homopolymer of β-1,4 linked N-acetylglucosamine

• fibers cross link → rigid and strong

• cell wall of fungi and exoskeletons of animals

glycoprotein

carbohydrate covalently attached to protein

• 3 clasess: glycoproteins, preoteoglycans, mucins

glycoprotein: class glycoprotein

• cell membrane components or secreted

• protein part heavier than carb part

• eg erythropoietin (EPO)

glycoprotein: class proteoglycan

• polysaccharide is glycosamineglycan

• carb is heavier than the protein

• structural and lubricant capabilities

glycoprotein: class mucins/mucoproteins

• carb is heavier than the protein

  • Acetylgalactosamine is usually the carb

• component of mucus

• lubricants

linkages in mucins

protein glycosylated at serine or threonine with by N-acetylgalactosamine

variable number of tandem repeats (VNTR) in mucins

• rich in O-glycosylated ser and thr residues

erythropoietin

(EPO) glycoprotein hormone in blood serum → stimulates rbc production in bone marrow

where is erythropoietin produced

kidneys

linkages in EPO

• glycosylation enhaces stability in blood → doesnt get filtered by kidneys like unglycosylated EPO

  • N-glycosylated at three asparagine residues

  • O-glycosylated on a serine residue

• 40% carb by weight

why is the function of the carbs attacthed on EPO

glycosylation (enzymed mediated)

how is EPO used as a drug

increases RBC count → increases oxygen-carrying capacity

• athletes use it

• treats anemia

glycosaminoglycan

repeating unit of disaccharides with an amino sugar (glucosamine or galactosamine → at least 1 of 2 sugars is -

aggrecan

• proteoglycan → contains charged glycosaminoglycan

• shock absorber for cartilage → water binds to glycosaminoglycan and allows “spring back” effect on impact

lectin

• glycan binding protein → recognize specific carbs on neighboring cells

• facilitate cell-cell contact → 2 or more carb binding sites, carb and lectin interact by weak bonds

which cell-surface carb structure does the influenza virus

sialic acid on galactose residues

which viral surface protein of influenza recognize and bind host cell

hemagglutinin → acts as a lectin to bind sailic acid-galactose complex

what happens when influenza’s hemagglutinin binds to target cell surface

virus englufed → virus replicates → viral particle buds from the cell

what happens when influenza is finished spreading its virus on a target cell

neuraminidase (another viral protein) hydrolyses glcosidic bonds of sialic acid to galactose → new virons are free to leave without getting stuck

anti influenza agents

tamiflu and relenza