Chapter 10: Carbohydrates

structure of dihydroxyacetone

(a ketose)

Structure of D-Glyceraldehyde

(an aldose)

Structure of L-Glyceraldehyde

(an aldose)

Structure of D-Deoxyribose

*see on slide 7

Describe cellulose

-a structural component of plants, is made of chains of glucose -a unbranched homopolymer of glucose units linked by β-1, 4-glycocidic bonds.

What are monosaccharides?

-aldehydes or ketones that contain two or more alcohol groups. -they are also the simplest carbohydrates

Describe pyranose formation:

The open-chain form of glucose cyclizes when the C-5 hydroxyl group attacks carbon atom C-1 of the aldehyde group to form an intramolecular hemiacetal. Two anomeric forms, designated α and β, can result.

What is glycogen?

The polysaccharide glycogen is the storage form of glucose in animal cells.

Proteoglycans:

The protein is attached to a particular type of polysaccharide called a glycosaminoglycan. By weight, proteoglycans are mainly carbohydrate. Proteoglycans play structural roles or act as lubricants.

In the case of the ketose fructose, the intramolecular hemiketal,

a five-carbon ring (five-membered), is called a furanose because of its resemblance to furan.

Describe chitin

a glycosaminoglycan found in insect wings and the exoskeleton of insects, crustaceans, and arachnids, is one of the most abundant carbohydrates in the world.

What is amylose

a linear polymer of glucose units linked by α-1, 4-glycosidic bonds.

In the case of glucose, the resulting intramolecular hemiacetal,

a six-carbon ring, is called a pyranose because of its resemblance to pyran (six membered)

The β linkage of cellulose yields

a straight chain capable of interacting with other cellulose molecules to from strong fibrils.

The blood type O produces no

active glycosyltransferase.

Monosaccharides are joined to ______ and ______ through ______ bonds

alcohols, amines; glycosidic

Aldoses contain an ________, whereas ketoses, such as fructose, contain a

aldehyde; ketone

What are two storage forms of glucose?

glycogen and starch

Proteoglycans are proteins attached to __________ , which make up

glycosaminoglycans; 95% of the proteoglycan by weight.

The VNTR region is highly _______, forcing the molecule into

glycosylated; an extended conformation.

The human ABO blood groups reflect the specificity of

glycosyltransferases.

If all of the monosaccharides in the polysaccharide are the same, the polysaccharide is called a

homopolymer.

In A,

N-acetylgalactosamine is added to the O by a specific glycosyltransferase.

In B,

galactose is added by another transferase.

Common monosaccharides

1) D-Ribose 2) D-Deoxyribose 3) D-Glucose 4) D-Mannose 5) D-Galactose 6) D-Fructose

There are three main classes of glycoproteins:

1) Glycoproteins 2) Proteoglycans 3) Mucins or mucoproteins

( look at images of these on slides) 1. N-linked: 2. O-linked:

1. GlcNAC 2. GalNAC

1. Formation of a cyclic hemiacetal creates....... 2. The α form means 3. The β form means

1. another diastereoisomeric form called an anomer. 2. the hydroxyl at C-1 is below the plane of the ring. 3. that the hydroxyl at C-1 is above the plane of the ring.

Describe the O-glycosidic bond:

A bond formed between the anomeric carbon atom and a hydroxyl group of another molecule is called an O-glycosidic bond, and the product is called a glycoside.

Describe the N-glycosidic bond

A bond formed between the anomeric carbon atom and an amine is called an N-glycosidic bond.

The formation of a mannose 6-phosphate marker:

A glycoprotein destined for delivery to lysosomes acquires a phosphate marker in the Golgi complex in a two-step process. First, N-acetylgalactosamine phosphotransferase adds a phospho-N-acetylglucosamine unit to the 6-OH group of a mannose residue; then, N-acetylgalactosaminidase removes the added sugar to generate a mannose 6-phosphate residue in the core oligosaccharide.

Explain how specific enzymes are responsible for oligosaccharide assembly:

A large class of enzymes, glycosyltransferases, catalyze the formation of glycosidic bonds.

Describe N-linked oligosaccharides:

A pentasaccharide core is common to all N-linked oligosaccharides and serves as the foundation for a wide variety of N-linked oligosaccharides

What do organic molecules contain?

Carbon, hydrogen, and oxygen

Explain how the lack of glycosylation can result in pathological conditions?

Certain types of muscular dystrophy can be traced to improper glycosylation of dystroglycan, a membrane protein that links the extracellular matrix with the cytoskeleton. These pathological conditions reveal the importance of proper modification of proteins by carbohydrates and their derivatives. An especially clear example of the role of glycosylation is provided by I-cell disease (also called mucolipidosis II), a lysosomal storage disease. Lysosomes are organelles that degrade and recycle damaged cellular components or material brought into the cell by endocytosis. In I-cell disease, lysosomal enzymes are secreted into the blood rather than directed to the lysosome. The defect results from a mutation that disrupts the oligosaccharide signal on the enzymes that normally directs the enzymes to the lysosome. Normally, a carbohydrate marker directs certain digestive enzymes from the Golgi complex to lysosomes where they function. In patients with I-cell disease, lysosomes contain large inclusions of undigested glycosaminoglycans and glycolipids, the "I" in the name of the disease. These inclusions are present because the enzymes normally responsible for the degradation of glycosaminoglycans are missing from affected lysosomes. Remarkably, the enzymes are present at very high levels in the blood and urine. Thus, active enzymes are synthesized, but, in the absence of appropriate glycosylation, they are exported instead of being sequestered in lysosomes. In other words, in I-cell disease, a whole series of enzymes is incorrectly addressed and delivered to the wrong location. Normally, these enzymes contain a mannose 6-phosphate residue as a component of an N-oligosaccharide that serves as the marker directing the enzymes from the Golgi complex to lysosomes. In I-cell disease, however, the attached mannose lacks a phosphate. I-cell patients are deficient in the N-acetylgalactosamine phosphotransferase catalyzing the first step in the addition of the phosphoryl group; the consequence is the mistargeting of eight essential enzymes (Figure 10.25). I-cell disease causes the patient to suffer severe psychomotor retardation and skeletal deformities, similar to those in Hurler disease.

Pyranose and Furanose Rings Can Assumer Different

Conformations

What allows monitoring the long-term control of blood glucose levels in diabetics?

Determining the amount of HbA1c in the blood

Two types of stereoisomers

Enantiomers and Diastereoisomers

What cleave common disaccharides?

Enzymes on the outer surface of intestinal epithelium

Describe the hormone erythropoietin

Erythropoietin is a glycoprotein secreted by the kidney into the blood that stimulates the production of red blood cells.

Describe Hurler disease

Formerly called gargoylism, Hurler disease is a mucopolysaccharidosis having symptoms that include wide nostrils, a depressed nasal bridge, thick lips and earlobes, and irregular teeth. In Hurler disease, glycosaminoglycans cannot be degraded. The excess of these molecules is stored in the soft tissue of the facial regions, resulting in the characteristic facial features.

Ring structures of fructose:

Fructose can form both five-membered furanose and six-membered pyranose rings. In each case, both a and B anomers are possible.

Describe glucosinolates

Glucosinolates protect plants and add flavor to our diets. They are a special class of glycosides that protect some plants from herbivory.

What are isomers

Isomers have the same molecular formula but different structures

What determine polysaccharide structure and how?

Glycosidic bonds (The β-1,4 linkages favor straight chains, which are optimal for structural purposes. The α-1,4 linkages favor bent structures, which are more suitable for storage)

What of erythropoietin enhances the stability of the protein in the blood?

Glycosylation

What are polysaccharides?

Large polymeric oligosaccharides are called polysaccharides.

Mucins or mucoproteins:

Mucins, a key component of mucus, serve as lubricants. Like proteoglycans, mucins are predominantly carbohydrate. The protein is characteristically attached to the carbohydrate by N-acetylgalactosamine. N-Acetylgalactosamine is an example of an amino sugar, so named because an amino group replaces a hydroxyl group. Mucins are often lubricants.

Is carbon dioxide organic? Why or why not?

No because it doesn't contain hydrogen

All of the blood groups share the oligosaccharide foundation called

O.

What is a common source of starch?

Pasta is a common source of starch. Pasta, thin pieces of unleavened dough molded into regular shapes, is made with flour, which has a high starch content.

What make compounds very active?

Phosphates

What are glycoproteins?

Proteins with carbohydrates attached

Examples of sugars that cyclize in solution

Ribose, glucose, fructose

What are selectins?

Selectins are a class of lectins. Some selectins bind immune cells to sites of injury. Other selectins allow attachment of an embryo to the mother's uterus.

Common disaccharides:

Sucrose, lactose, and maltose are common dietary components. (The angles in the bonds to the central oxygen atoms do not denote carbon atoms.)

What facilitate the polymerization of many mucins?

The Cys-rich domains and the D domain

What designates the structures as being in the d configuration? (look at slide 7 for clarification)

The asymmetric carbon atom farthest from the aldehyde or ketone designates the structures as being in the d configuration.

Explain how cartilage acts as a shock absorber:

The cartilage of a runner's foot cushions the impact of each step that she takes

Chair and boat forms of β-d-glucopyranose:

The chair form is more stable owing to less steric hindrance because the axial positions are occupied by hydrogen atoms. Abbreviations: a, axial; e, equatorial.

Explain Furanose formation:

The open-chain form of fructose cyclizes to a five-membered ring when the C-5 hydroxyl group attacks carbon C-2 of the ketone to form an intramolecular hemiketal. Two anomers are possible, but only the α anomer is shown.

Describe glycoproteins

The protein is the largest component by weight. Glycoproteins play a variety of roles including as membrane proteins. Many glycoproteins are components of cell membranes, where they take part in processes such as cell adhesion and the binding of sperm to eggs. Other glycoproteins are formed by linking carbohydrates to soluble proteins. Many of the proteins secreted from cells are glycosylated, or modified by the attachment of carbohydrates, including most proteins present in the serum component of blood.

General form of a glycosyltransferase reaction:

The sugar to be added comes from a sugar nucleotide—in this case, UDP-glucose. The acceptor, designated X in this illustration, can be a simple monosaccharide, a complex polysaccharide, or a serine or threonine residue belonging to a protein.

Epimers

They differ at one of several asymmetric carbon atoms

How branch point in glycogen is formed:

Two chains of glucose molecules joined by α-1,4-glycosidic bonds are linked by an α-1,6-glycosidic bond to create a branch point. Such an α-1,6-glycosidic bond forms at approximately every 10 glucose units, making glycogen a highly branched molecule.

What is amylopectin?

a branched polymer, with an α-1, 6-glycosidic bond for every 30 α-1, 4-glycosidic bonds.

Glycosidic bonds between proteins and carbohydrates: A glycosidic bond links

a carbohydrate to the side chain of asparagine (N-linked) or to the side chain of serine or threonine (O-linked)

All N-linked polysaccharides consist of

a common pentasaccharide core, consisting of three mannoses, a six-carbon sugar, and two N-acetylgalactosamine units.

Sugars as cyclic forms: The chemical basis for ring formation is that

an aldehyde can react with an alcohol to form a hemiacetal, while a ketone can react with an alcohol to form a hemiketal. (Practice writing out these equations found on slide 9)

The furanose form of fructose can also exist in

anomeric forms in which the α and β forms refer to the orientation of the hydroxyl at C-2.

Lectins are a particular class of glycan-binding protein that

are important for cell signaling. The lectins on one cell recognize and bind to carbohydrates on another cell with multiple weak interactions. Such binding facilitates cell-cell interaction.

Carbohydrates may be linked to what residues of proteins?

asparagine, serine, or threonine

Which amino acids are used for the attachment of carbohydrates to proteins?

asparagine, serine, or threonine

The monosaccharide substrates for glycosyltransferases are activated by

attachment to uridine diphosphate (UDP).

In the chair form, substituents on the carbon ring atoms have two orientations:

axial and equatorial.

Why some people hate vegetables:

because of the sharp taste of isothiocynate

Viral receptors: Influenza virus targets cells by The other major viral-surface protein,

binding to sialic acid residues located at the termini of oligosaccharides present on cell-surface glycoproteins and glycolipids. These carbohydrates are bound by hemagglutinin, one of the major proteins expressed on the surface of the virus.

_______ groups are based on

blood; protein glycosylation patterns

Pyranose rings can adopt two types of conformation, called

boat and chair.

Glucose-_____ form predominated

chair

neuraminidase, is an enzyme that

cleaves oligosaccharide chains to release the viral particle at a later stage of the viral life cycle.

The α linkages of starch and glycogen form

compact hollow cylinders suitable for accessible storage.

Monosaccharides are linked to form

complex carbohydrates

Two types of isomers

constitutional isomers and stereoismers

Oligosaccharides

contain two or more monosaccharides linked by O-glycosidic bonds

Many common sugars exist in _____ forms

cyclic

Two types of diastereoisomers

epimers and anomers

In mucins, the protein component is

extensively glycosylated to serine and threonine residues beginning with N-acetylgalactosamine. A region of the protein backbone rich in serines and threonines, called variable number of tandem repeats (VNTR), is the site of glycosylation.

Proteoglycans are key components of the

extracellular matrix

As a reducing sugar, glucose can react with hemoglobin,

forming glycosylated hemoglobin (hemoglobin A1c), which is fully functional.

Fructose forms both the pyranose form, which predominates when

fructose is free in solution, and a furanose form, commonly seen in fructose derivatives.

Monosaccharides exist in many _______ forms

isomeric

Diastereoisomers

isomers that are NOT mirror images

Anomers

isomers that differ at a new asymmetric carbon formed on ring closure; OH below=alpha OH above=beta

When glucosinolates are hydrolyzed,

isothiocyanate is produced, which generates a sharp taste that discourages further eating by the insect. Certain glucosinolates may help prevent cancer in humans

Lactase and other enzymes that hydrolyze carbohydrates are present on

microvilli that project from the outer face of the plasma membrane of intestinal epithelial cells

Mucins are glycoprotein components of

mucus

Enantiomers

nonsuperimposable mirror images

A solution of glucose contains

one-third α anomer, two-thirds β anomer, and about 1% open chain.

Mucopolysaccahridoses, such as Hurler disease, are

pathological conditions that result from the inability to degrade proteoglycans.

Carbohydrates also form ester linkages to

phosphates

Cartilage is composed, in part, of the

proteoglycan aggrecan and cartilage.

What important components of cartilage?

proteoglycans

Sugars that react with oxidizing agents are called

reducing sugars, whereas those that do not are called nonreducing sugars.

The glycosaminoglycan component of aggrecan cushions joints by

releasing water on impact, and then rebinding water.

Glycosaminoglycans are composed of

repeating units of a disaccharide, one of which is a derivative of an amino sugar and one of which carries a negative charge, either as a carboxylate or sulfate.

Although mammals CANNOT digest cellulose and other plant fibers,

soluble fibers, such as polygalacturonic acid, aid in digestion

Glycan-binding proteins bind to

specific oligosaccharides on the cell surface.

In plants, glucose is stored as ______ of which there are two forms:

starch; amylose and amylopectin

Proteoglycans, composed of polysaccharides and protein have important

structural role

Glysosylation: ______ attached

sugars

Modified monosaccharides: Carbohydrates can be modified by

the addition of substituents other than hydroxyl groups. Such modified carbohydrates are often expressed on cell surfaces.

For glucose: Because the two anomeric forms are in equilibrium that passes through an open chain form,

the free open chain form reacts with oxidizing agents.

In all classes of glycoproteins, carbohydrates are attached to

the nitrogen atom in the side chain of asparagine (N-linkage) or to the oxygen atom of the side chain of serine or threonine (O-linkage).

The numbers are the standard designations for

the positions of the carbon atoms (e.g., the number 2 identifies the carbon atom in the second position).

What are stereoisomers?

they are isomers where atoms are connected in the same order but differ in spatial arrangement

What are constitutional isomers?

they differ in the order of attachment of atoms

The smallest monosaccharides are composed of _______ carbons.

three

Erythropoietin has oligosaccharides linked to

three asparagine residues and one serine residue.

Most glucose units in glycogen are lined by

α-1, 4-glycosidic bonds, with branches formed by α-1, 6-glycosidic bonds every 10 glucose units.

_______________ adopts the chair conformation because

β-d-glucopyranose ;the axial positions are occupied by hydrogens, reducing steric hindrance.