Carbohydrates and Glycoproteins – Chapter 11

Question-and-answer flashcards covering key terms, mechanisms, structures, and physiological roles of carbohydrates, their polymers, glycosylation, and lectin interactions from Chapter 11.

What is the empirical formula that describes many carbohydrates?

(CH₂O)ₙ

How are monosaccharides formally defined?

As aldehydes or ketones containing two or more hydroxyl groups.

Which carbon-chain lengths correspond to trioses, tetroses, pentoses, hexoses, and heptoses?

3, 4, 5, 6, and 7 carbons respectively.

What is the difference between an aldose and a ketose?

An aldose contains an aldehyde group; a ketose contains a keto group.

Define constitutional isomers in carbohydrates.

Molecules with the same molecular formula but different order of atom attachment.

What are stereoisomers?

Molecules connected in the same order but differing in spatial arrangement.

How many stereoisomers are possible for a sugar with n asymmetric carbons?

2ⁿ stereoisomers.

What term describes diastereoisomers that differ at only one chiral center?

Epimers.

What is an anomer?

A diastereoisomer formed when ring closure creates a new asymmetric (anomeric) carbon.

Which anomer of D-glucose has the C-1 hydroxyl trans to C-6?

α-D-glucopyranose.

What structural feature distinguishes a pyranose from a furanose?

Pyranoses are six-membered rings; furanoses are five-membered rings.

Why does β-D-glucopyranose predominate in solution?

It adopts a chair form with all bulky groups equatorial, minimizing steric hindrance.

Define mutarotation.

The change in optical rotation as α and β anomers interconvert through the open-chain form.

Why is D-glucose called a reducing sugar?

Its open-chain aldehyde can reduce Cu²⁺ in Fehling’s solution to Cu⁺ while being oxidized.

What undesirable protein modification can reducing sugars produce?

Non-enzymatic glycation forming advanced glycation end products (AGEs).

What clinical marker is measured via glucose glycation of hemoglobin?

Hemoglobin A1c (A1C).

What percent of hemoglobin is typically glycated in uncontrolled diabetes?

~10 %.

What is an O-glycosidic linkage?

A bond between the anomeric carbon of a sugar and an alcohol’s oxygen.

What is an N-glycosidic linkage?

A bond between the anomeric carbon of a sugar and the nitrogen of an amine.

Give two reasons phosphorylation of sugars is useful in metabolism.

1) Traps sugar inside cells by giving it a negative charge. 2) Creates reactive intermediates and locks one ring form.

What defines the reducing and non-reducing ends of an oligosaccharide?

The reducing end has a free anomeric carbon able to open; the non-reducing end’s anomeric carbon is in a glycosidic bond.

Which linkage connects the two glucose units in maltose?

α-1,4-glycosidic linkage.

Why is sucrose not a reducing sugar?

Both anomeric carbons (glucose C-1 and fructose C-2) are tied up in the glycosidic bond.

Name the monosaccharide components and linkage of lactose.

Galactose-β-1,4-glucose.

What two polymers form plant starch?

Amylose (unbranched α-1,4) and amylopectin (branched α-1,4 with α-1,6 branches).

Describe the branching pattern of glycogen.

α-1,4 chains with α-1,6 branches roughly every 10 glucose residues.

Which linkage gives cellulose its rigidity?

β-1,4-glycosidic linkages between glucose units.

Why can’t most mammals digest cellulose?

They lack cellulase enzymes to hydrolyze β-1,4 linkages.

What is chitin chemically?

A β-1,4 homopolymer of N-acetylglucosamine.

Define glycoprotein.

A protein covalently linked to carbohydrate; ~50 % of human proteins are glycosylated.

List the three classes of glycoproteins and their dominant component.

Glycoproteins (protein-rich), proteoglycans (carbohydrate-rich glycosaminoglycans), and mucins (heavily O-glycosylated).

Which consensus sequence is required for N-linked glycosylation?

Asn-X-Ser/Thr where X ≠ Pro.

What common core do all N-linked oligosaccharides share?

Three mannoses and two N-acetylglucosamine residues (pentasaccharide core).

What is the biological role of erythropoietin (EPO) glycosylation?

Glycosylation enhances EPO’s stability in the bloodstream.

What enzyme catalyzes single O-GlcNAc additions to Ser/Thr?

O-GlcNAc transferase (OGT).

Name three pathologies linked to dysregulated O-GlcNAcylation.

Insulin resistance/diabetes, cancer, and neurological disorders.

What structural polysaccharide/ protein combination cushions joints?

Aggrecan proteoglycan bound to hyaluronate with water for shock absorption.

What repeating disaccharide feature characterizes glycosaminoglycans?

An amino sugar plus a sugar bearing a carboxylate or sulfate, giving negative charge.

Where do N-linked and O-linked glycosylation occur within the cell?

N-linked begins in the ER and finishes in the Golgi; O-linked occurs only in the Golgi.

What is the molecular basis of ABO blood groups?

Specific glycosyltransferases add either GalNAc (A) or Gal (B) to the O antigen; type O lacks active enzyme.

Define lectin.

A carbohydrate-binding protein that mediates specific cell–cell interactions.

How do C-type lectins bind sugars?

Via Ca²⁺ that bridges lectin glutamates to sugar hydroxyls, plus other H-bonds.

Which viral protein is a lectin that recognizes sialic acid on host cells?

Influenza hemagglutinin.