Carbohydrates and Monosaccharides

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Questions and Answers

Which of the following statements about reducing sugars is correct?

Reducing sugars do not include any polysaccharides.

Sugars with a free anomeric carbon are reducing sugars. (correct)

Only oligosaccharides can be reducing sugars.

Disaccharides are nonreducing if one sugar contains a free anomeric carbon.

Which of the following correctly describes the structures of starch?

Starch is not digestible by animals.

Starch has a linear structure with branched chains every 5 to 10 residues.

Starch is composed of α(1→4) D-glucose chains and α(1→6) D-glucose branches. (correct)

Starch consists solely of β(1→4) D-glucose chains.

What characterizes the structure of cellulose?

It is a linear polymer of β(1→4) D-glucose. (correct)

Cellulose is exclusively made from N-acetyl-D-glucosamine.

It has both α(1→4) and β(1→4) linkages.

It consists of branched α(1→6) D-glucose.

Which statement regarding glycogen is accurate?

Glycogen is made of α(1→4) and has branches every 8 to 12 residues. Signup and view all the answers

Which type of linkage is associated with N-linked glycoproteins?

The linking amino acid is asparagine. Signup and view all the answers

What distinguishes Gram-negative bacterial cell walls?

They have NAM-NAG repeats crosslinked with tetrapeptides. Signup and view all the answers

What is the primary function of glycosaminoglycans?

They provide structural support and are negatively charged. Signup and view all the answers

Which of the following correctly describes peptidoglycan in bacterial cell walls?

It contains NAM-NAG repeats crosslinked with tetrapeptides. Signup and view all the answers

Which of the following represents a characteristic of diastereomers?

They differ at one or more chiral carbons but are not mirror images. Signup and view all the answers

In the context of carbohydrates, what defines an epimer?

Two molecules that differ at one asymmetric carbon. Signup and view all the answers

What configuration of an anomeric carbon indicates an alpha (α) configuration?

The OH of the anomeric carbon is on the opposite side from the highest numbered asymmetric carbon. Signup and view all the answers

Which of these statements best describes furanose structures in carbohydrates?

They are named after the five-membered, oxygen-containing ring. Signup and view all the answers

Which carbohydrate classification refers to a sugar with four carbon atoms?

Tetrose Signup and view all the answers

What is the primary function of the anomeric carbon in carbohydrate chemistry?

It becomes chiral upon ring formation. Signup and view all the answers

Which of the following best describes the D- and L- configuration of carbohydrates?

They refer to the configuration of the highest numbered asymmetric carbon. Signup and view all the answers

Which of the following describes glycosides?

Products of the reaction between an anomeric carbon and an alcohol. Signup and view all the answers

Study Notes

Carbohydrates

Carbohydrates are based on the formula (CH2O)n, where n is greater than or equal to 3
Simple sugars are called monosaccharides
Polymers of simple sugars are called oligo- and polysaccharides

Monosaccharide Classification

Monosaccharides are classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group)
They are also classified based on the number of carbon atoms they contain: triose (3 carbons), tetrose (4 carbons), pentose (5 carbons), hexose (6 carbons), etc.

Stereochemistry

Monosaccharides with 3 or more carbons (aldoses) and 4 or more carbons (ketoses) have asymmetric carbons (chiral centers)
D- and L- configurations refer to the configuration of the highest numbered asymmetric carbon
D- and L- forms are mirror images and are called enantiomers
Molecules with more than one asymmetric carbon can be diastereomers: they differ in configuration at one or more chiral carbons but are not mirror images
Epimers are two molecules that differ in configuration about a single asymmetric carbon

Ring Structures

Pyranoses are six-membered, oxygen-containing rings
Furanoses are five-membered, oxygen-containing rings
The ketone or aldehyde carbon that becomes chiral upon ring formation is called the anomeric carbon
Anomers (α and β) differ in configuration about the anomeric carbon
- In the α-configuration, the OH of the anomeric carbon is on the same side as the OH of the highest numbered asymmetric carbon in a Fischer projection
- In the β-configuration, the OH of the anomeric carbon is on the opposite side as the OH of the highest numbered asymmetric carbon in a Fischer projection
Haworth projections provide a three-dimensional representation of ring structures: groups to the right in a Fischer projection are drawn down in the Haworth projection
Chair and boat conformations are possible due to ring pucker, with axial and equatorial orientation of groups attached to the ring

Sugar Derivatives

Oxidation of the free anomeric carbon to carboxylate results in acids
Other derivatives include alcohols, deoxysugars, esters (like ATP and GTP), amino sugars (like glucosamine), muramic acid, and glycosides (anomeric carbon reacted with an alcoholic function)
Sugars with a free anomeric carbon are reducing sugars
The end of a sugar polymer with a free anomeric carbon is called the reducing end; the opposite end is the nonreducing end

Oligosaccharides

Disaccharides include:
- Maltose (diglucose)
- Lactose (galactose and glucose)
- Sucrose (fructose and glucose) - a nonreducing sugar

Storage Polysaccharides

Starch
- Composed of α-amylose and amylopectin
- α-amylose is a linear chain of α(1→4)D-glucose
- Amylopectin is also a linear chain of α(1→4)D-glucose but with branches every 12 to 30 residues, created by α(1→6)D-glucose linkages
- In animals, α-amylase hydrolyzes internal α(1→4) glycosidic linkages in starch
- Microorganisms use α-amylase as an exoamylase, releasing the disaccharide maltose
Glycogen, similar to amylopectin, is composed of α(1→4) D-glucose chains with α(1→6) D-glucose branches every 8 to 12 residues
Dextrans, produced by bacteria, are α(1→6) D-glucose polymers

Structural Polymers

Cellulose: a linear polymer of β(1→4) D-glucose, forming the plant cell wall
Chitin: a β(1→4) N-acetyl-D-glucosamine polymer, found in exoskeletons and fungal cell walls

Glycosaminoglycans

These are polymers with repeating disaccharide units and are negatively charged

Peptidoglycan of Bacterial Cell Walls

Gram-negative bacterial cell walls
- Contain NAM-NAG repeats crosslinked with a tetrapeptide
- The tetrapeptide is attached to an outer cell wall via a 57-amino acid protein
- This protein is linked to aminopimelic acid (which replaces 10% of D-alanines)
- The outer cell wall contains lipopolysaccharides, which are antigenic
Gram-positive bacterial cell walls
- Contain NAM-NAG repeats crosslinked with tetrapeptides and pentaglycines
- These bacteria lack an outer cell wall
- They contain teichoic acids: polymers of ribitol phosphate or glycerol phosphate

Glycoproteins

Functions include structural support, enzyme activity, receptors, transport, and immunoglobulins
Linked by:
- O-linked: the amino acids serine, threonine, or hydroxylysine are linked to sugars, with N-acetylgalactosamine being the most common
- N-linked: asparagine is linked to sugars, playing roles in protein folding and turnover

Proteoglycans

These are glycoproteins
Contain glycosaminoglycans as their sugar component
Interact with a variety of molecules

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Description

Explore the fascinating world of carbohydrates, including their structures and classifications. This quiz covers monosaccharides, their types, and stereochemistry, focusing on concepts like aldoses, ketoses, enantiomers, and diastereomers. Test your knowledge and understanding of these essential biomolecules.