Carbohydrates: Structure, Functions, and Composition

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Questions and Answers

What are examples of proteoglycans mentioned in the text?

hyaluronic acid, chondroitin sulfate, heparin

What is the main characteristic of glycoproteins?

Have structurally complex oligosaccharides (correct)

Main components of the extracellular matrix

Contain many GAG chains

Are proteins found in interstitial connective tissues

What are glycolipids carbohydrate-lipid conjugates where the oligosaccharide is attached to the lipid moiety through a ______ residue or an alcohol?

hexosamine

Match the following blood types with their corresponding transferase enzymes:

A type = Add an extra N-acetyl galactosamine B type = Add an extra galactose AB type = Both A and B type transferases O type = Defective A and B type transferases Signup and view all the answers

What are the functions of carbohydrates?

Energy/storage, structural and protective elements, other carbohydrate conjugates such as lubricating skeletal joints and recognition/adhesion between cells. Signup and view all the answers

How many major classes of carbohydrates are there?

Three Signup and view all the answers

Glucose is the most important monosaccharide in nature.

True Signup and view all the answers

Aldose monosaccharides contain an ________ group.

aldehyde Signup and view all the answers

Match the following ring structures with the correct monosaccharide:

Glucopyranose = D-Glucose Fructofuranose = D-Fructose Signup and view all the answers

Which sugar alcohol can accumulate in tissues such as the lens, sciatic nerve, and renal papillae in disorders like diabetes mellitus and galactosemia?

D-Sorbitol Signup and view all the answers

What is the main clinical use of Mannitol in patients?

Mannitol is administered intravenously as an osmotic diuretic in patients with acute renal failure. Signup and view all the answers

Humans can digest cellulose due to the presence of specific enzymes.

False Signup and view all the answers

What kind of link is present between alpha-glucose units in Dextran? Dextran consists of α-glucose units linked by ______________ bonds.

α-1,4, α-1,3, and α-1,6 glycosidic Signup and view all the answers

Match the following sugar derivatives with their roles:

Glucosamine, Galactosamine, Mannosamine = Components of Glycoproteins, Gangliosides, and Glycolipids Erythromycin = Antibiotic containing amino sugar Glucosamine derivative = Part of many structural polymers including bacterial cell walls Signup and view all the answers

Study Notes

Carbohydrates

General characteristics:
- Many have the empirical formula (C(H2O)n, n ≥ 3
- Some contain N, P, or S
- Are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis
Functions:
- Energy/storage
- Structural and protective elements
- Lubricate skeletal joints
- Recognition and adhesion between cells

Classification of Carbohydrates

Three major classes:
1. Monosaccharides: Consist of a single polyhydroxyaldehyde or ketone unit
2. Oligosaccharides: Consist of 2-20 monosaccharides covalently linked by glycosidic bonds
3. Polysaccharides: Polymers consisting of chains of monosaccharide or disaccharide units (>20)

Monosaccharides

Characteristics:
- Simplest of the carbohydrates
- Either aldehydes or ketones, with two or more -OH groups
- Colorless and crystalline solids; freely soluble in water but insoluble in non-polar solvents
- Most have a sweet taste
- Most monosaccharides of >4 C's tend to have cyclic structures
Examples:
- Glucose, Fructose, Galactose, Mannose, and Ribose

Glucose

Most abundant monosaccharide in nature
Mainly absorbed from the diet; converted to other carbohydrates having specific functions
Major fuel for certain mammalian tissues
Converted to other carbohydrates, such as glycogen, nucleic acids, and galactose

Other Monosaccharides

Mannose: A subunit in glycoproteins and neuraminic acid
Galactose: A subunit of the milk sugar, lactose
Fructose: The sweetest sugar, found in honey and fruits
D-Ribose: A component of RNA, DNA, ATP, GTP, and coenzymes
D-glyceraldehyde and dihydroxyacetone: Aldehydo- and keto-trioses, respectively, that are intermediary compounds in carbohydrate and lipid metabolism

Isomerism in Carbohydrates

D- and L-isomerism: Determined by the orientation of the —H and —OH groups around the carbon atom adjacent to the terminal alcohol carbon
- D-sugars are metabolizable, while L-sugars are not
Optical isomerism: Refers to the ability of a sugar solution to rotate plane-polarized light
- Fructose is the L(-) isomer, while Glucose and Galactose are the D(+) isomers

Anomers

α-anomer: The -OH on C1 is on the opposite side of the ring from the -CH2OH
β-anomer: The -OH on C1 is on the same side of the ring as the -CH2OH

Pyranose and Furanose Ring Structures

Pyranose: A six-membered ring
Furanose: A five-membered ring

Chemical Properties of Monosaccharides

Susceptibility to oxidation: Can be oxidized to sugar acids
Susceptibility to reduction: Can be reduced to sugar alcohols

Sugar Alcohols (Polyols)

Formed when the carbonyl group of the monosaccharide is reduced to a hydroxyl group
Examples:
- Sorbitol (D-glucitol)
- Mannitol
- Inositol (a hexahydric cyclic sugar alcohol)
Uses:
- As a dietary substitute for sugar
- In the manufacture of diabetic-friendly foods
- As a sweetener and flavoring agent

Sugar Derivatives

Amino sugars: Contain an amino group instead of a hydroxyl group
Deoxy sugars: Contain a hydrogen instead of a hydroxyl group
N-acetyl derivatives: Contain an N-acetyl group instead of a hydroxyl group

Glycosides

N- and O-glycosidic bonds: Formed when the hydroxyl group on the anomeric carbon of a monosaccharide reacts with an –OH or an –NH group of another compound
Examples:
- Adenosine (a nucleoside)
- Glycoproteins

Disaccharides

Formed from two monosaccharides units covalently bonded together by glycosidic linkages
Examples:
- Maltose (α(1→4) glycosidic linkage between two Glc residues)
- Lactose (Gal(β1→4)Glc)
- Sucrose### Disaccharides
Sucrose is a disaccharide of glucose and fructose
It is a non-reducing sugar with an α1 → 2β glycosidic link between the anomeric carbon hydroxyls of both glucose and fructose
Its chemical structure is α-D-Glucopyranosyl-(1→2)-β-D-fructofuranoside

Polysaccharides

Polysaccharides differ from each other in the identity of their recurring monosaccharide units, length of their chains, types of bonds linking the units, and degree of branching

Homopolysaccharides

Starch is a homopolymer of glucose connected by alpha-glycosidic bonds
It is the most common ingested dietary polysaccharide, found in vegetables and cereals
Starch is 80% amylopectin (branched) and 20% amylose (non-branched)
It is a storage polysaccharide in plant cells
Glycogen is a homopolymer of glucose, more highly branched and with shorter branches
It is the main storage polysaccharide of animal cells
Cellulose is a homopolysaccharide of β-glucose in β 1, 4 linkage
It is the chief constituent of plant cell walls, insoluble, and a major component of dietary fiber
Humans lack enzymes that hydrolyze β 1, 4 glycosidic bonds, so they cannot digest cellulose
Dextran is a highly branched meshwork glucosan composed of α-glucose units linked by α-1,4, α-1,3, and α-1,6-glucosidic linkages
Therapeutic applications of dextran include:
- Forming a colloidal solution in water, used as a plasma substitute to restore blood pressure in cases of hemorrhagic shock
- Dextran ferrous sulfate, used for intramuscular injection of iron for treatment of iron deficiency anemia
- Sodium dextran sulfate, an anticoagulant
Inulin is a homopolysaccharide of fructose in β 2 → 1 linkage
It is found in onions and garlic, belongs to a class of fibers, and is readily soluble in water
Inulin is not hydrolyzed by human digestive enzymes and is used in physiological investigations for determining glomerular filtration rate (Inulin clearance test) and estimating total body water

Heteropolysaccharides

Heteropolysaccharides have repeating units of two or three monosaccharides
They generally contain an amino-sugar and a uronic acid
They are also known as mucopolysaccharides
Functions of heteropolysaccharides include:
- Providing shape and extracellular support for cells and tissues
- Acting as lubricants
- Mediating cell-cell interactions
- Acting as biological anti-coagulants and anti-freeze agents
- Serving as receptors for hormones

Glycosaminoglycans

Glycosaminoglycans are unbranched heteropolysaccharide chains composed of disaccharide repeat units
One sugar of the disaccharide is either N-acetylglucosamine or N-acetylgalactosamine, and the second is usually acidic (either glucuronic acid or uronic acid)
These sugars are modified by the addition of sulfate groups to the parent sugar
Glycosaminoglycans are major components of the extracellular matrix
Examples include hyaluronic acid, chondroitin sulfate, and heparin

Proteoglycans

Proteoglycans are proteins that contain many chains of glycosaminoglycans
They occur by a core trisaccharide, Gal-Gal-Xyl
Proteoglycans are found in interstitial connective tissues, such as the synovial fluid of joints, the vitreous humor of the eye, arterial walls, bone, cartilage, and cornea
They are major components of the extracellular matrix in these tissues

Glycoproteins

Glycoproteins contain covalently linked oligosaccharides that are smaller but more structurally complex than glycosaminoglycans
Many cell surface or extracellular proteins are glycoproteins, as are most secreted proteins
The covalently attached oligosaccharides influence the folding and stability of the proteins, provide critical information about the targeting of newly synthesized proteins, and allow for specific recognition by other proteins

Glycolipids

Glycolipids are carbohydrate-lipid conjugates where the oligosaccharide is attached to the lipid moiety through a hexosamine residue or an alcohol, such as sphingosine
Bacterial cell wall lipopolysaccharides and the human ABO blood group cell membrane antigens are important examples
The ABO blood group antigens are formed of 3-4 sugar residues branched-side-chain connected to the RBCs surface ceramide or connected to a plasma secretory glycoprotein in some individuals
Both the RBCs membrane-bound and the serum glycoprotein-bound ABO blood antigens are used as individual-specific fingerprints in forensic criminal investigations

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Description

This quiz covers the basics of carbohydrates, including their empirical formula, composition, and various functions in the body.