Chapter 4: Carbohydrate Structure and Function Flashcards
What is the empirical formula to define monosaccharides?
Cn(H2O)n
What is the empirical formula to define complex sugars?
Cn(H2O)m
What are monosaccharides with three, four, five and six carbon atoms?
3: triose
4: tetrose
5: pentose
6: hexose
What are aldoses?
Carbohydrates that contain an aldehyde group as their most oxidized functional group
What are ketoses?
Carbohydrates that contain an ketone group as their most oxidized functional group
What would a six-carbon sugar with an aldehyde group be called?
Aldohexose
What is the simplest aldehyde and ketone sugars?
Aldehyde: glyceraldehyde
Ketone: dihydroxyacetone
What is the number of most carbonyl carbons in ketoses?
C-2
Carbonyl carbons in glyceraldehyde and dihydroxyacetone can participate in _____ linkages.
Glycosidic linkages
What are sugars acting as substituents via glycosidic linkages called?
Glycosyl residues
What are the common names of the four most important monosaccharides?
- Fructose
- Glucose
- Galactose
- Mannose
Learn the Fischer projections of the important monosaccharides.
Learn it
Differentiate D and L sugars.
D-sugars: highest-numbered chiral carbon with the OH group on the RIGHT
L-sugars: highest-numbered chiral carbon with the OH group on the LEFT
Define stereoisomers. What are they also called?
- Compounds that have the same chemical formula
- Differ form one another only in terms of the spatial arrangement of their component atoms
- Optical isomers
Define enantiomers.
Stereoisomers that are nonidentical, nonsuperimposable mirror images of each other
Any molecule that contains _____ and no _______ has an enantiomer
- chiral carbons
- internal planes of symmetry
What is absolute configuration?
The nomenclature system used for three-dimensional arrangement of atoms in isomers; the most common systems are D/L and (R)/(S)
What is the equation that allows us to calculate the number of stereoisomers with common backbone?
2^n
where n is the number of chiral carbons in the molecule
What is the optical rotation of D-glyceraldehyde? What is the optical rotation of L-glyceraldehyde? Do these optical rotations apply to all sugars?
D: +
L: -
No, it needs to be determined experimentally for every type of sugar
In the Fischer projection, what do the horizontal lines (wedges) designate? What about the vertical lines (dashes)?
Horizontal (wedges): out of the page
Vertical (dashes): into the page
How many chiral centers in D-glucose have the opposite configuration of L-glucose?
EVERY chiral center is opposite
The same sugars, in different optical families, are ________
enantiomers
Two sugars that are in the same family (both are either ketoses or aldoses, and have the same number of carbons) that are not identical and are not mirror images of each other are ______
diastereoisomers
Define epimers.
Diasteroisomers that differ in configuration at exactly one chiral center
What are the three main types of stereoisomers?
- Enantiomers
- Diastereoisomers
- Epimers
Learn the steroisomers on page 109.
Learn it
How many enantiomers can a compound have? How many diastereoimers?
Enantiomers: ONE
Diastereoisomers: multiple
What is the nucleophile and electrophile found in monosaccharides?
Nucleophile: hydroxyl group
Electrophile: carbonyl group
Define cyclization.
Describes the ring formation of carbohydrates from their straight-chained forms
Monosaccharides can undergo intramolecular reactions to form cyclic _______ and ________
- hemiacetals (aldoses)
- hemiketals (ketoses)
What are the two only cyclic molecules that are stable in solution? How many members do their rings contain?
- Pyranose (6)
- Furanose (5)
Do sugars tend to exist predominantly in their cyclic or linear form?
Cyclic
How does a ring form in a monosaccharide?
Hydroxyl group acts as a nucleophile, so oxygen becomes a member of the ring structure
In a hemiacetal or hemiketal, the carbonyl carbon becomes chiral in the ring-formation process. What is it now referred to?
Anomeric carbon
Define anomeric carbon.
New chiral center formed in ring closure; it was the carbon containing the carbonyl in the straight-chain form
One of two rings can form during cyclization of a sugar molecule: a or B. What are they termed in reference to one another?
Anomers
What is an a-anomer?
-OH group of C-1 trans to the -CH2OH substituent (axial and down)
What is a B-anomer?
-OH group of C-1 cis to the -CH2OH substituent (equatorial and up)
Are furanoses or pyranoses planar? Which adopts a chair-like configuration?
Furanose: planar
Pyranose: chair-like configuration
When we convert the monosaccharide from its straight-chain Fischer projection to the Haworth projection, any group on the right in the Fischer projection will point _____
down
What happens when you expose hemiacetal rings to water?
Cause them to spontaneously cycle between the open and closed form
What is mutarotation?
Cyclic compounds shift from one anomeric form to another (a and B) with the straight-chain form as an intermediate
When does mutarotation occur more rapidly?
When the reaction is catalyzed with an acid or a base
Which configuration is less favorable in cyclic compounds: a or B? Why?
- Alpha
- Hydroxyl group of the anomeric carbon is axial, adding to the steric strain of the molecule
Mutarotation results in a mixture that contains both a- and B-anomers at ________ concentrations. Are they equal in terms of percentage?
- equilibrium
- No
What three reactions can monosaccharides undergo?
- Oxidation-reduction
- Esterification
- Glycoside formation from nucleophilic attack
Are monosaccharides oxidized or reduced to produce energy?
Oxidized
When monosaccharides switch between anomeric configurations, the hemiacetal rings spend a short period of time in the open-chain aldehyde form. They can be oxidized to __________; these oxidized aldoses are called _______.
- carboxylic acids
- aldonic acids
Aldoses can be oxidized to _____ and reduced to ______
Oxidized: aldonic acids
Reduced: alditols
Any monosaccharide with a hemiacetal ring is considered a ____________
reducing sugar
When an aldose is in its ring form, oxidation yields what?
Lactone
What type of compound is a lactone?
Cyclic ester with a carbonyl group persisting on the anomeric carbon
What are the two standard reagents used to detect the presence of reducing sugars?
- Tollen’s reagent
- Benedict’s reagent
How does Tollen’s reagent test work? What happens in a positive test?
- Utilizes Ag(NH3)2+ as an oxidizing agent
- Aldehydes reduce Ag+ to metallic silver
How does Benedict’s reagent test work?
- Aldehyde group of an aldose is readily oxidized, indicated by a red precipitate of Cu2O
How can we detect the presence of reducing sugars in a glucose sample?
Utilize the enzyme glucose oxidase, which does not react with other reducing sugars
Define tautomerization.
Refers to the rearrangement of bonds in a compound, usually by moving a hydrogen and forming a double bond
Can ketones be oxidized to carboxylic acids?
No
How do ketones form aldoses under basic conditions?
Tautomerize to form aldozes, via ketonol shifts
Why do ketone sugars give positive Tollen’s and Benedict’s tests?
Since they can form aldoses under basic conditions
What does the tautomerization of a ketone result in?
An enol
What is an enol?
Compound with a double bond and an alcohol group
What is an alditol?
When the aldehyde group of an aldose is reduced to an alcohol, the compound is considered an alditol
What is a deoxy sugar?
Compound that contains a hydrogen that replaces a hydroxyl group on the sugar
How can carbohydrates form esters?
Their hydroxyl groups are able to participate in reactions with carboxylic acids and carboxylic acid derivatives
What is formed in the phosphorylation of glucose?
Phosphate ester
What is a similar reaction to esterification?
Phosphorylation
What do hemiacetals form when they react with alcohol? What do hemiketals form?
- Acetals
- Ketals
What is the basis for building complex carbohydrates? What does it require?
- Glycoside formation
- Requires the anomeric carbon to link to another sugar
What happens to do anomeric hydroxyl group when hemiacetals react with alcohols to form acetals?
Anomeric hydroxyl group is transformed into an alkoxy group, yielding a mixture of a- and B-acetals
The resulting C-O bonds following hemiacetals reaction with alcohol are called glycosidic bonds, what are the acetals formed called?
Glycosides
What are glycosides derived from furanose rings called? What are glycosides derived from pyranose rings called?
- Furanoside
- Pyranoside
Monosaccharides react with _______ to for acetals.
alcohols
Name the three important disaccharides.
- Sucrose
- Lactose
- Maltose
What is the chemical name for sucrose?
glucose-a-1,2-fructose
What is the chemical name for lactose?
galactose-B-1,4-glucose
What is the chemical name for maltose?
glucose-a-1,4-glucose
Differentiate homopolysaccharides and heteropolysaccharides.
Homopolysaccharide: polysaccharide composed entirely of a single monosaccharide
Heteropolysaccharide: polymer made up of more than one type of monosaccharide
What are the three most important polysaccharides? What are they all composed of?
- Cellulose
- Starch
- Glycogen
- D-glucose
When does branching in a polysaccharide happen?
When an internal monosaccharide in a polymer chain forms at least two glycosidic bonds, allowing for branch formation
Which polysaccharide is the main structural component of plants?
Cellulose
Is cellulose made of a-D-glucose or B-D-glucose? How are their glycosidic bonds linked?
- B-D-glucose
- B-1,4 glycosidic bonds
Which sugars can humans not digest?
B-linked sugars
Are starches made up of a-D-glucose of B-D-glucose?
a-D-glucose; they are digestible
What are the two types of starch?
- Amylose: linear
- Amylopectin: branched
What are the linkages for amylose and amylopectin?
a-1,4 glycosidic bonds
Where are the branches linked in amylopectin?
a-1,6 glycosidic bonds
What is amylose degraded by? (2) Where does it cleave? What does it yield?
- B-amylase: cleaves amylose at the reducing end of the polymer (the one with the free anomeric carbon) to yield maltose
- a-amylase: cleaves randomly along the chain to shorten it to maltose and glucose
How is amylopectin degraded?
Debranching enzymes
What is the carbohydrate storage unit in animals?
Glycogen
How does glycogen differ from starch?
It has more a-1,6 glycosidic bonds, which makes it highly branched and optimizes the energy efficiency of glycogen and makes it more soluble in solution
What degrades glycogen?
- Glycogen phosphorylase
- Cleaving GLUCOSE from the nonreducing end of a glycogen branch (no free anomeric carbon) and phosphorylating it, producing G-1-phosphate
Which of the two forms of starch is more soluble in solution?
Amylopectin because of its branched structure
Explain the difference between esterification and glycoside formation.
- Esterification is the reaction by which a hydroxyl group reacts with either a carboxylic acid or a carboxylic acid derivative to form an ester
- Glycoside formation refers to the reaction between an alcohol and a hemiacetal (or hemiketal) group on a sugar to yield an alkoxy group
