Bio 98 - Lecture 11

Carbohydrates

a.k.a. Sugars,

Carbs…

I. Definition of a carbohydrate

General formula: (CH2O)n , “hydrated carbon”

Example: C6H12O6 is glucose

Many carbohydrates have more complex formulas & contain amino, phosphate, sulfate & other groups

II. Functions

1. Fundamental source of metabolic energy for most life forms.

2. Component of many important biomolecules.

animals, plants respiration

ATP ADP

sunlight

CO2 + H2O

plants photosynthesis

carbohydrate + O2

1. Carbohydrates as an energy source

bacteria

2. Biological molecules

Cellulose - wood, plant fiber, etc.Chitin - exoskeleton of arthropodsCell walls of bacteria & yeastGlycoproteins, glycolipids - cell membranesDNA, RNA - deoxyribose and ribose

Carbohydrates are often polymers

Monosaccharides: glucose, ribose, fructose, etc. Oligosaccharides: di-, tri-, tetra-, etc. Sucrose is a disaccharide: glucose + fructose

Polysaccharides: can be linear or branched (i.e. starch)

Monosaccharide nomenclature

1. Carbon number: triose, .., pentose, hexose, .., octose

2. Aldoses and ketoses

HC=O

HC-OH

HO-CH

H2C-OH

HC-OH

HC-OH

C#

1

2

3

4

5

6

D-glucose

an aldohexose

H2C-OH

C=O

HO-CH

H2C-OH

HC-OH

HC-OH

D-fructose

a ketohexose

Asymmetric (chiral) carbons generate many distinct monosaccharides; compare mannose, glucose, and galactose, which are all aldohexoses.

How many chiral carbons are in glucose?

O-chem terms relevant to monosaccharide structure

2. epimers - differ in chirality at only one carbon. Glucose and galactose are epimers at carbon 4.

1. diastereomers - identical structures except for configuration (chirality) at one or more carbons; e.g., all aldohexoses are diastereomers of each other.

3. enantiomers - mirror images; designated as D- & L-; no name change (like amino acids)!

How many aldohexose names are possible?

2 configuration choices at each of 4 asymmetric carbons; however, half of these represent enantiomers (3. above).

Number of unique names = 24 / 2 = 8.

Ring structures

Most pentoses and hexoses spontaneously form ring (cyclized) structures in solution.

5-member ring: furanose

6-member ring: pyranose

When forming a ring, a new chiral center is created, giving rise to 2 possible anomers.

HC=O

HC-OH

HO-CH

H2C-OHHC-OHHC-OH..

+

D-glucose(linear

form)

and anomers

hemiacetal

Linear form

Haworth projection

-D-glucopyranose

LinearD-glucose

-D-glucopyranose

-D-glucopyranose

In solution rapid mutarotation occurs

(1%)

(~66%)

(~33%)

Disaccharides

1. Mainly found in plants

2. Three common disaccharides• sucrose - sugar cane, sugar beets• lactose - milk sugar• maltose - malted (germinating) barley, wheat

3. 2 monosaccharides joined covalently by an O-glycosidic bond

Common disaccharides are produced by enzyme- catalyzed condensation/dehydration reactions

Can be either or due to mutarotation

Notice: there is nohemiacetal

Polysaccharides - aka Glycans

1. homopolysaccharides vs heteropolysaccharides

2. can be branched or unbranched

3. used by animals and plants as a compact storage form of CHOs

4. common examples• starch - plants, roots and seeds• glycogen - liver of mammals• cellulose - plant fiber, wood

Starch = amylose + amylopectin

found in corn, rice, potato, wheat and barley

a linear polysaccharide of glucose units

Amylopectin - a branched form of amylose

Structure of starch

glycogen (animal starch) is like starch, but more highly branched

High-fructose corn syrup

High-fructose corn syrup is produced by milling corn to produce corn starch, then enzymatically processing that starch to yield corn syrup, which is almost entirely glucose, followed by adding other enzymes that change a portion of the glucose into fructose.

1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides.2. Glucoamylase - which is produced by Aspergillus, a fungus, in a fermentation vat — breaks the sugar chains down even further to yield the simple monosaccharaide glucose.3. The enzyme xylose isomerase (aka glucose isomerase) then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in.

http://en.wikipedia.org/wiki/High-fructose_corn_syruphttp://en.wikipedia.org/wiki/High-fructose_corn_syrup_and_health

High-fructose corn syrup

1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides.

High-fructose corn syrup

3. The enzyme glucose isomerase then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in.

https://wiki.geneseo.edu/display/food/Group+7-+High+Fructose+Corn+Syrup+-+Societal+Beliefs+vs.+Science

• Sugars covalently attached to the polypeptide as oligosaccharide chains containing 4 to 15 sugars

• Sugars frequently comprise 50% or more of the total molecular weight of a glycoprotein

• Most glycosylated proteins are either secreted or remain membrane-bound

• Glycosylation is the most abundant form of post-translational modification

• Glycosylation confers resistance to protease digestion by steric protection

• Important in cell-cell recognition

Protein glycosylation:A post-translational modification

Blood group antigens on the erythrocyte surface

• The O substance is a tetrasaccharide which is missing the 5th residue and does not elicit an antibody response (non-antigenic).

• The A antigen and B antigen are pentasaccharides which differ in composition of the 5th sugar residue