Carbohydrates: Definition and Roles

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Questions and Answers

What is the significance of glucose being classified as a reducing sugar?

It can enhance the glucose oxidase method's effectiveness.

It enables the reduction of copper ions in Fehling's reaction. (correct)

It means glucose can only exist in a cyclic form.

It allows glucose to undergo oxidation without requiring an enzyme.

Which component is essential for the measurement of glucose using the Hexokinase method?

NADH

H2O2

Hexokinase (correct)

Dianisidine

In the context of glucose oxidase method, what is the final product that indicates the presence of glucose?

D-Glucano-δ-lactone

Colored oxidized dianisidine (correct)

NADPH

Glucose-6-phosphate

Why must the glycosidic hydroxyl group be free for reducing properties to be displayed?

To allow the sugar to adopt an acyclic form. Signup and view all the answers

What role does NADP+ play in the Hexokinase method?

It is the electron acceptor in the reaction. Signup and view all the answers

What is the term used for the interconversion of the two anomers of glucose?

Mutarotation Signup and view all the answers

Which form of D-glucopyranose is considered more stable?

β-D-glucopyranose Signup and view all the answers

What configuration describes the relationship between α-D-glucopyranose and β-D-glucopyranose?

Diastereomers Signup and view all the answers

At equilibrium, what is the optical rotation of the anomers of glucose?

+52.70 Signup and view all the answers

What type of interactions contribute to the stability differences between the two anomers of glucose?

1,2 dipolar repulsions Signup and view all the answers

In which form does fructose exist when in equilibrium with its anomers?

A mix of both anomers Signup and view all the answers

What is a primary reason for the greater stability of β-D-glucopyranose compared to α-D-glucopyranose?

Less steric hindrance Signup and view all the answers

Which of the following describes the structural difference between α and β D-glucopyranose?

Arrangement of hydrogen and hydroxyl groups Signup and view all the answers

What is the general formula for carbohydrates?

(CH2O)n Signup and view all the answers

Which of the following is classified as an oligosaccharide?

Sucrose Signup and view all the answers

What role do carbohydrates play in plants?

Structural components Signup and view all the answers

What is the smallest monosaccharide?

Glyceraldehyde Signup and view all the answers

Which monosaccharides are considered aldoses?

Glyceraldehyde and glucose Signup and view all the answers

How many stereoisomers can a sugar with three chiral carbons have?

6 Signup and view all the answers

What distinguishes epimers from other sugars?

They differ in the configuration of one chiral carbon atom. Signup and view all the answers

What are monosaccharides with a ketone group called?

Ketoses Signup and view all the answers

How many stereoisomers can a hexose have if it has four asymmetric carbons?

16 stereoisomers Signup and view all the answers

Which of the following correctly describes polysaccharides?

Can contain more than 100,000 monosaccharide units Signup and view all the answers

What type of monosaccharides are aldoses and ketoses?

Aldoses contain an aldehyde group while ketoses contain a ketone group. Signup and view all the answers

What type of carbohydrate serves as inactive ingredients in pharmaceutical formulations?

Excipients Signup and view all the answers

What phenomenon occurs when monosaccharides cyclize?

New chiral centers are created. Signup and view all the answers

Which configuration is predominantly found in natural sugars?

D configuration Signup and view all the answers

What are anomers?

Stereoisomers that differ in the configuration at the hemiacetalic carbon. Signup and view all the answers

Which of the following statements regarding chiral centers in D-glucose and D-galactose is true?

They differ only at the second chiral center. Signup and view all the answers

What characterizes the cyclic structure of monosaccharides?

They possess both alcohol and carbonyl groups simultaneously. Signup and view all the answers

Which of the following correctly describes ketohexoses?

They are structurally distinct from aldohexoses. Signup and view all the answers

Which of the following best describes the nature of glycosidic bonds in disaccharides?

They are created by dehydrating two monosaccharides. Signup and view all the answers

What is the source of lactose intolerance in individuals?

Deficiency in lactase enzyme (β-D-galactosidase). Signup and view all the answers

Which type of polysaccharide is starch primarily composed of?

Both amylose and amylopectin. Signup and view all the answers

What type of linkage is primarily found in cellulose?

β1→4 linkage. Signup and view all the answers

Which characteristic distinguishes chitin from cellulose?

Chitin is composed of N-acetylglucosamine. Signup and view all the answers

How does penicillin act on bacterial peptidoglycan synthesis?

It inhibits cross-linking of polysaccharide chains. Signup and view all the answers

What type of carbohydrate structure is common to glycosaminoglycans?

Linear heteropolysaccharides of disaccharide units. Signup and view all the answers

What is the primary function of heparin in medicine?

To act as an anticoagulant. Signup and view all the answers

What feature do lectins possess that allows them to bind carbohydrates?

High affinity and specificity for oligosaccharides. Signup and view all the answers

What does the cluster structure of starch granules consist of?

Both amylose and amylopectin molecules. Signup and view all the answers

How do β-lactamases affect antibiotic treatment?

They cleave β-lactam antibiotics, rendering them inactive. Signup and view all the answers

Which of these sugars could be classified as a non-reducing sugar?

Sucrose. Signup and view all the answers

Study Notes

Carbohydrate Definition & Classification

Polyhydroxyaldehydes or polyhydroxyketones with general formula (CH2O)n.
May contain nitrogen, sulfur, or phosphorus.
Exist in linear or cyclic forms.
Include oligomers and polymers that can form these compounds upon hydrolysis.
Also known as saccharides, with names ending in "-ose".
Classified by their monosaccharide units:
- Monosaccharides: Single unit of polyhydroxyaldehydes or ketones (e.g., glucose, fructose, erythrose, etc.)
- Oligosaccharides: Few units linked by ether bonds (e.g., sucrose, a disaccharide containing glucose and fructose).
- Polysaccharides (Glycans): More than 20 monosaccharide units (some have 100,000+ units). They can be linear (e.g., amylose) or branched (e.g., amylopectin, glycogen).

Carbohydrate Roles

Energy generation and storage: The primary energy source in metabolism through oxidation.
Structural role: Found in plants (cellulose), bacterial cell walls, and insect exoskeletons.
Inactive ingredients (excipients): Used in tablets, capsules, and other pharmaceuticals.
Osmolites: Contribute to osmotic pressure inside and outside of cells.
Recognition elements: Involved in immunity, immune responses, and cell identity/recognition.

Monosaccharide Types & Nomenclature

Two main types: polyhydroxyaldehydes (aldoses) and polyhydroxyketones (ketoses).
Classified by the number of carbon atoms:
- Trioses
- Tetroses
- Pentoses
- Hexoses

Monosaccharide Chirality

Smallest monosaccharide is glyceraldehyde, which has two stereoisomers.
Most natural sugars have a D relative configuration.
Dihydroxyacetone is a ketotriose with no stereoisomers.

Monosaccharide Series: Aldoses

Most natural sugars have a D relative configuration.
Each chiral carbon generates two enantiomers.
A sugar with 'n' chiral carbons has 2n stereoisomers.
Only half of these stereoisomers will have the D-relative configuration.

Monosaccharide Series: Ketoses

Refer to provided image (Figure 8 in the original text).

Monosaccharide Series: Epimers

Two sugars that differ only in the configuration of one chiral carbon atom.

Hexose

Includes aldohexose and ketohexose examples.
See figure 10 for structural representations of D-glucose, D-galactose, D-fructose, and L-galactose.

Monosaccharides: Cyclic Structure

Monosaccharides exhibit special properties due to the presence of both alcohol and carbonyl functionalities in their molecules.
Leads to high reactivity.

Monosaccharides: Cyclic Structure - Anomers

Cyclization creates a new chiral center.
Two stereoisomers, differing only in stereochemistry at the hemiacetalic carbon, are called anomers.
Alpha and Beta anomers are shown in the figure 12.

Monosaccharides: Mutarotation

Glucose and fructose have anomers, these are stereoisomers that differ in their configuration at the anomeric carbon.
α-D-glucopyranose is 36.4% at equilibrium, while β-D-glucopyranose is 63.5%.
The interconversion of the two anomers via the acyclic form is called mutarotation.
The stability of the anomers is dependent on the position of the hydroxyl groups.
α-D-glucopyranose has an axial hydroxyl group, while β-D-glucopyranose has an equatorial hydroxyl group.
Equatorial hydroxyl groups are more stable due to less steric hindrance.
α and β are different configurations, not conformations, and are diastereomers.

Reducing Properties of Monosaccharides

Glucose is a reducing sugar because it can be oxidized to D-gluconate.
Copper is reduced from +2 to +1 in the Fehling's reaction, which is a test for reducing sugars.
Cyclic forms need a free glycosidic OH to display reducing properties.

Glucose Measurement in Diagnosis and Treatment of Diabetes

Glucose oxidase method is used in testing strips, and the reaction produces a colored product.
Hexokinase method uses the enzyme hexokinase to convert glucose to glucose-6-phosphate.
The amount of NADPH produced is proportional to the amount of glucose.

Disaccharides

Disaccharides contain a glycosidic bond which is formed by dehydration.
Reducing disaccharides have one free anomeric OH, while non-reducing disaccharides have both anomeric OHs linked.
Lactose is a reducing disaccharide found in milk, and it is broken down by lactase.
Sucrose is a non-reducing disaccharide commonly known as table sugar.
Sucrose is the main circulating sugar in plants.
Trehalose is a non-reducing disaccharide found in insects.

Oligosaccharides: Cyclodextrins

Cyclodextrins are cyclic oligomers of D-glucose linked α1→4.
The most common cyclodextrins have 6, 7, or 8 glucose units (α, β, γ-cyclodextrins).
They have a toroidal structure with a hydrophilic exterior and a hydrophobic interior.
They can form host-guest complexes with lipophilic drugs.

Polysaccharides: Constitution and Topology

Polysaccharides are polymers of monosaccharides.
Homopolysaccharides are made up of the same type of monosaccharide, while heteropolysaccharides are made up of different types of monosaccharides.

Fuel-Storage Homopolysaccharides

Starch is a mixture of amylose and amylopectin, which are polymers of D-glucose.
Amylose is a linear polymer linked α1→4, while amylopectin is a branched polymer linked α1→4 and α1→6.
Glycogen is a branched polymer of D-glucose similar to amylopectin but with more branching.
Starch and glycogen are storage polysaccharides.

Cellulose

Cellulose is a linear polymer of D-glucose linked β1→4.
It is highly insoluble due to inter- and intra-chain hydrogen bonding.
Cellulose can be used in pharmaceutical formulations as an adsorbent, glidant, suspending agent, and diluent.

Chitin

Chitin is a polymer of N-acetylglucosamine linked β1→4.
It is found in the exoskeletons of insects.
Chitin is similar in structure to cellulose.

Glycoconjugates: Peptidoglycans

Peptidoglycans are a major component of bacterial cell walls.
They are made up of repeating disaccharide units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
Peptidoglycans are cross-linked by short peptides.
Peptidoglycans are essential for bacterial cell wall integrity and are targeted by antibiotics like penicillin.

Synthesis of Bacterial Peptidoglycan

Penicillin inhibits the transpeptidase enzyme, which is responsible for cross-linking peptidoglycans.
This weakens the bacterial cell wall and can lead to bacterial cell lysis.

Penicillins: General Structure and Representatives

Penicillins have a β-lactam ring and a thiazolidine ring.
Examples of penicillins include ampicillin and amoxicillin.

Penicillins: Mechanism of Action

Penicillins bind to the active site of transpeptidase, preventing the formation of cross-links in peptidoglycans.

Penicillins: Mechanism of Inactivation

Bacteria have evolved to produce β-lactamases, which cleave the β-lactam ring of penicillins, rendering them inactive.
Clavulanic acid is a β-lactamase inhibitor that can be used to overcome bacterial resistance to penicillins.

Glucosaminoglycans

Glucosaminoglycans are linear heteropolysaccharides composed of repeating disaccharide units.
They are found in extracellular matrices and play important roles in cell signaling and tissue structure.

Heparin and Congeners

Heparin is a highly sulfated glucosaminoglycan that is used as an anticoagulant.
Heparin has the highest negative charge density of any known biological macromolecule.
Low molecular weight heparin (LMWH) is a shorter version of heparin.

Oligosaccharide Links in Glycoproteins

Oligosaccharides are found covalently linked to proteins in glycoproteins.
They are involved in cell recognition and signaling.

Other Glycoconjugates

Digoxin is a cardiac-active glycoside extracted from foxglove.
It is used in the treatment of heart failure and atrial fibrillation.
It contains a tri-digitoxose sugar molecule linked to an aglycone steroid.

Lipopolysaccharides

Lipopolysaccharides are found on the surface of gram-negative bacteria.
They are recognized by vertebrate immune systems.

Oligosaccharide Recognition: Lectins

Lectins are proteins that bind carbohydrates with high affinity and specificity.
They play important roles in cell-cell interactions and immune recognition.

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Description

This quiz explores the definition and classification of carbohydrates, including monosaccharides, oligosaccharides, and polysaccharides. It also examines their roles in energy generation, storage, and structural functions in living organisms.