Chapter 11 - Carbohydrates

7/23/2019 Chapter 11 - Carbohydrates

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Chapter 11 Carbohydrate

Monosaccharides are the Simplest Carbohydrates

- Carbohydrates = carbon-based molecules rich in hydroxyl groups

o Empirical formula (CH2O)n- Simple carbohydrates = monosaccharides = aldehydes or etones that ha!e 2 or more hydroxyl groups

- "ihydroxyacetone and glyceraldehyde are constitutional isomers # same molecular formula$ atoms ordered

di%erently- Stereoisomers are isomers that di%er in spatial arrangement (" or &)

o 'ost !ertebrate monosaccharides ha!e the " conguration

o " and & isomers are determined by the conguration of the asymmetric carbon atom farther from the

aldehyde or eto groupo "ihydroxyacetone is the only monosaccharide ithout at least one asymmetric carbon

*


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- 'onosaccharides made up of more than + carbon atoms ha!e multiple asymmetric carbons$ so they can exist

as enantiomers (mirror images) and diastereomers

o ,he number of possible stereoisomers is 2n$here n the number of asymmetric carbonso .-carbon aldose ith / asymmetric carbons can exist in *. possible diastereomers

Know aldehyde sugars Glyceraldehyde, Erythrose, Threose, Ribose, ylose, !llose, Glucose, Mannose,

and Galactose"

2


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Know #eto sugars $ihydro%yacetone, Erythrulose, Ribulose, ylulose, and &ructose" 'e able to draw

Sedoheptulose

+

,hese are the structures of most of the

most important sugars0

1otice that all biological sugars are "3$

meaning the last hydroxyl before the

CH2OH at the bottom goes to the right

1otice "-glucose and "-mannose di%er

only at C-20 Sugars that are

diastereomers di%ering in conguration

at only a single asymmetric center are

called epimers"


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Many common sugars e%ist in cyclic (orms

- ,he predominant forms of ribose$ glucose$ fructose$ and many other sugars in solution (inside the cell) are not

open chains4the open chain forms of these sugars cycli5e into rings

) Similarly$ a etone can react ith an alcohol to form a hemietal

/

6hen a eto sugar cycli5es you

get a hemietal ith OH and O7

here the eto carboxyl used to

be0

Hemietals and Hemiacetals are

reducing sugars3 because they

can be easily oxidi5ed0

6hen theanomeric OH is deri!ati5ed gi!ing

O7 an O7 instead of OH and O7

8ou ha!e an cetal or a 9etal (the

hemi3 goes aay) and you also

ha!e a non-reducing sugar0

6hen an aldehyde sugar

cycli5es you get a hemiacetal

ith OH and O70


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'e able to draw all assigned sugar structures either in &ischer or *aworth pro+ections"

:


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)$)sedoheptulo(uranose - .)$)%ylopyranose

'oats and Chairs

.

,he chair form$ shon here$ is the typical conformation of the

pyranose ring0

1otice that ha!ing buly groups mounted axially (straight up or

don$ perpendicular to the plane of the ring) leads to strong

steric hindrance$ so the OH and CH2OH groups should all be

e;uatorial (sharing the plane of the ring)0

,he only aldohexose that can do that$ ha!e all buly groups

e;uatorial$ is beta-"-glucose0 nd many people thin that is

hy glucose is the default sugar3 in biology0


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Reducing Sugars

>

?or the CH


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@lycoside3 has

something attached

to the nomeric

Hydroxyl (C-* for

aldehydes$ C-2 for

etones)0 ,his

stabili5es the sugarring structure0


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Sucrose, lactose, and maltose are the common disaccharides

- "isaccharide = 2 sugars oined by an O-glycosidic bond- &actose$ the disaccharide of mil$ consists of galactose oined to glucose by a beta-*$/-glycosidic linage- &actose is hydroly5ed to these monosaccharides by lactase in human beings

&actose (and Cellobiose) ha!e a beta- *$/ linage0 &actose or mil sugar is a disaccharide of @alactose beta *$/

@lucose0

Cellobiose (not shon) is @lucose beta *$/ @lucose0


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- &actose intolerance = hypolactasia # caused by deciency of the en5yme lactase$ hich clea!es lactose into

glucose and galactose- &actose is a good energy source for microorganisms in the colon$ and they ferment it to lactic acid hile

generating methane (CH/) and hydrogen gas (H2)G the gas creates the uncomfortable feeling of gut distension

and farting- ,he lactate produced by the microorganisms is osmotically acti!e and dras ater into the intestine and

undigested lactosediarrhea

*

Here is the basic structure of@lycogen0

&ong starch lie chains of alpha-

*$/ lined glucose$ but about

e!ery *thglucose there is an

alpha *$. branch point0

So the hole molecule is

branchy3 and that maes it easier

to metaboli5e ;uicly than the

linear starch molecules

Cellulose on the left

forms straight chains

and at sandich3

structures0 ,his is part

of hy it is hard to

metaboli5e (although

microorganisms can

mae Cellulase)0

Starch and glycogen

ha!e alpha *$ / lins

hich mae arc shapes

that are easier for

en5ymes to interact

ith0


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**

,he IO blood group

antigens are simply small

oligosaccharides0

,he di%erences are minimal$

but if you ha!e ,ype blood

and you get a transfusion of,ype I blood$ the response

from your immune system

could ill you0

,ype O is the uni!ersal donor0

JBotential test material this is

importantKKKL

Chapter 11 - Carbohydrates

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