Chapter 11 Class Notes

Section 11.1 Monosaccharides Are the Simplest Carbohydrates

• carbohydrates = carbon-based molecules high in hydroxyl groups

  • empirical formula: (CH2O)n

  • can have additional groups or modifications

  • better described as polyhydroxy aldehydes and ketones (and their derivatives)

• Monosaccharides are aldehydes or ketones that contain two or more hydroxyl groups.

• The smallest monosaccharides are composed of three carbons.

• Monosaccharides exist in many isomeric forms.

Monosaccharides

• monosaccharides = carbohydrates that are three to seven carbons in length

• also called simple sugars

• most common monosaccharides

  • 4-7 in length

  • D or L sugars

    • based on glyceraldehyde

Monosaccharide Nomenclature

• Nomenclature is based on carbon-chain length:

  • three carbons: trioses

  • four carbons: tetroses

  • five carbons: pentoses

  • six carbons: hexoses

    • fructose is a hexose

  • seven carbons: heptoses

• Nomenclature is also based on the identity of the most oxidized group:

  • keto group: ketose

  • aldehyde group: aldose

Isomers

• constitutional isomers

  • molecules with identical

  • molecular formulas that differ in how the atoms are ordered

  • different arrangement

• stereoisomers

  • molecules that differ in spatial arrangement but not bonding order

  • have either D or L configuration

  • can be enantiomers (mirror images of each other) or diastereoisomers (not mirror images of each other)

  • number possible = 2n where n is the number of asymmetric carbon atoms

Fischer projections have the most oxidized group on top

Isomeric Forms of Carbohydrates

Common Monosaccharides

• epimers = sugars that are diastereoisomers differing in configuration only at a single asymmetric center

Most Monosaccharides Exist as Interchanging Cyclic Forms

• An aldehyde can react with with an alcohol to form a hemiacetal

• A ketone can react with an alcohol to form a hemiketal

come from top: OH down

come from bottom: OH up

CH2OH and OH opposite → alpha

CH2OH and OH same → beta

fructose has 2 different CH2OH groups

Pyranose Formation

• called pyranose because of similarity to pyran

Furanose Formation

• called furanose because of similarity to furan

Anomers of Glucose

• anomer = a diastereoisomeric form of sugars that forms when a cyclic hemiacetal is formed and an additional asymmetric center is created (look at ring chains)

• In glucose, C-1 (the anomeric carbon atom) becomes an asymmetric center, forming two ring structures:

– α-D-glucopyranose (hydroxyl group attached to C-1 is on the opposite side of the ring as C-6)

– β-D-glucopyranose hydroxyl group attached to C-1 is on the same side of the ring as C-6)

D-Fructose Rapidly Interchanges Between Four Distinct Ring Structures

• C-2 is the anomeric carbon atom.

• The pyranose form predominates in solution due to reduced steric hindrances.

• The furanose form predominates in fructose derivatives.

The Most Common Monosaccharides Exist Primarily in Their Ring Forms

Pyranose and Furanose Rings Can Assume Different Conformations

• Pyranose rings are not planar because of the tetrahedral geometry of its saturated carbon atoms.

• They can adopt two types of conformation: boat and chair.

• In chair form, substituents on the carbon ring atoms can be axial (nearly perpendicular) or equatorial (nearly parallel).

• Axial substituents sterically hinder each other if on the same side of the ring.

Chair and Boat Forms of β-D-Glucose

• The chair form predominates because all axial positions are occupied by hydrogens.

• The boat form is disfavored because it is sterically hindered.

Envelope Conformations of Furanose Rings

• Furanose rings are not planar and commonly adopt a conformation called the envelope form.

• In the ribose moiety of most biomolecules, there are two common confirmations:

– C-2-endo (C-2 is out of the plane on the same side as C-5)

– C-3-endo (C-3 is out of the plane on the same side as C-5)

D-Glucose Is an Important Fuel for Most Organisms

• blood sugar = D-glucose circulating in the blood

– only fuel used by the brain in non-starvation conditions

– only fuel used by red blood cells

• potential reasons why D-glucose an important fuel:

– glucose is formed from formaldehyde under prebiotic conditions and may have been available as a fuel source for primitive biochemical systems

– glucose is relatively inert

– the most stable ring structure is β-D-glucopyranose

Solutions of Glucose

• The two anomeric forms (α and β) are in an equilibrium that passes through the open-chain form.

• There is a roughly 2:1 ratio of β-to-α anomer conformations for D-glucose in an equilibrium solution.

D-Glucose Is a Reducing Sugar and Reacts Nonenzymatically with Hemoglobin

• In its linear form, glucose can react with oxidizing agents.

• example: linear glucose reacts with Cu2+ yielding Cu+ and gluconic acid

Reducing Sugars

• Fehling's solution = solutions of Cu2+ that test for the presence of sugars that adopt an open structure

Cu2+ is easy to see

• reducing sugars = sugars that react with oxidizing agents (any sugar than can get to an open chain)

reducing agen is what is getting oxidized

– all monosaccharides that can adopt linear structures in solution

• non-reducing sugars = sugars that do not react with oxidizing agents

Glycation of Sugars

• glycation = nonenzymatic addition of a carbohydrate to another molecule

– can be benign or detrimental

• example: Reducing sugars nonspecifically react with free amino groups on proteins (often Lys or Arg) to form a stable covalent bond.

• D-glucose has a low tendency to glycate proteins unless concentrations of sugar and protein are very high for long periods of time.

Advanced Glycation End Products (AGEs)

• advanced glycation end products (AGEs) = products resulting from cross-linking following the primary modification

– implicated in aging, arteriosclerosis, diabetes, and other pathological conditions

Assessing Treatments for Diabetes Mellitus by Monitoring A1C Levels

• D-glucose reacts with hemoglobin to form glycated hemoglobin (hemoglobin A1c, A1C).

– has no effect on O2 binding

• In nondiabetic individuals, <6% of the hemoglobin is glycated.

• In patients with uncontrolled diabetes, almost 10% of the hemoglobin is glycated.

can be used as a makrer to look for diabetes

Monosaccharides Are Joined to Alcohols and Amines Through Glycosidic Linkages

• Monosaccharide biochemical properties can be modified by reactions with:

– alcohols.

– amines.

– phosphates → keep sugar in cell

• modifications increase biochemical versatility

– can serve as signal molecules

– can facilitate metabolism

sugars always modified

Glycosidic Linkages

• O-glycosidic linkage = covalent linkage formed between the anomeric carbon atom of a carbohydrate and the oxygen atom of an alcohol

• N-glycosidic linkage = covalent linkage formed between the anomeric carbon atom of a carbohydrate and the nitrogen atom of an amine

Monosaccharides Can Be Modified by the Addition of Substituents Other Than Hydroxyl Groups

1 has added methyl

2 and 3 have added amid

used in siganl

not always at anomeric position

2 and 3 most important

Phosphorylated Sugars Are Key Intermediates in Metabolism

• phosphorylation = addition of phosphoryl groups

– common modification of sugars in metabolic reactions

• purposes of phosphorylation:

– makes sugars anionic to prevent crossing the lipid-bilayer membranes and interacting with transporters of the unmodified sugar

– blocks the formation of alternative ring conformation

– creates reaction intermediates that more readily undergo metabolism

Several Intermediates in the Breakdown of Glucose Are Phosphorylated Sugars

Section 11.2 Monosaccharides Are Linked to Form Complex Carbohydrates

• oligosaccharides = sugars that contain two or more monosaccharides linked by O-glycosidic bonds

– have directionality defined by their reducing and nonreducing ends

• reducing end = has a free anomeric carbon atom that can form the open-chain form

• nonreducing end = has an anomeric carbon in a glycosidic linkage that cannot covert to the open-chain form

Maltose Is a Disaccharide of D-Glucose

• α-1,4-glycosidic linkage = glycosidic linkage between the α-anomeric form of C-1 on one sugar and the hydroxyl oxygen atom on C-4 of the adjacent sugar

Sucrose, Lactose, and Maltose Are the Common Disaccharides

• disaccharide = two sugars joined by an O-glycosidic linkage

• Cleavage products of disaccharides can be processed to provide energy in the form of ATP.

The Disaccharide Sucrose

• sucrose = disaccharide of sugar cane or sugar beets that consists of glucose linked to fructose

– the anomeric carbon of glucose is linked to the anomeric carbon of fructose

– the configuration is α for glucose and β for fructose

– not a reducing sugar

– can be cleaved by sucrase (invertase)

The Disaccharide Lactose

• lactose = disaccharide of milk that consists of a galactose linked to a glucose

– linked by a β-1,4-glycosidic linkage.

– can be hydrolyzed by lactase in human beings and by β-galactosidase in bacteria

The Disaccharide Maltose

• maltose = disaccharide resulting from the hydrolysis of large oligosaccharides that consists of two linked glucose molecules

– joined by an α-1,4-glycosidic linkage

– can be hydrolyzed to glucose by maltase (α-glucosidase)

Maltase Inhibitors Can Help to Maintain Blood Glucose Homeostasis

• After a meal, starch and glycogen are degraded by α-amylase.

• Oligosaccharides generated by α-amylase are further digested by maltase.

• Acarbose (Precose) and miglitol (Glyset) are competitive inhibitors of

maltase.

Glycogen and Starch Are Storage Forms of Glucose

• Free glucose cannot be stored because high concentrations will disturb the cell's osmotic balance.

• polysaccharides (glycans) = large polymeric oligosaccharides formed by the linkage of multiple monosaccharides

– plays roles in energy storage and structural integrity

• homopolymer = polymer in which all the monosaccharide units are the same

Glycogen

• glycogen = large, branched polymer of glucose residues

– most common homopolymer in animal cells

– storage form of glucose

– most glucose units are linked by α-1,4-glycosidic linkages

– branches are formed by α-1,6-glycosidic linkages

– hydrolyzed by α-amylase

• Branching increases the surface area to allow better access for enzymes to rapidly breakdown glycogen.

Starch

• starch = homopolymer that serves as the nutritional reservoir in plants

– two forms: amylose and amylopectin

• amylose = unbranched type of starch composed of glucose residues in α-1,4 linkage

• amylopectin = branched type of starch with ~1 α-1,6 linkage per 30 α-1,4 linkages

– identical structure to glycogen but with a lower degree of branching

• Amylose and amylopectin are hydrolyzed by α-amylase.

Cellulose Is the Main Structural Polysaccharide of Plants

• cellulose = unbranched polymer of glucose residues joined by β-1,4 linkages

– serves a structural role instead of a nutritional role

• The β configuration allows cellulose to form long, straight chains that interact with one other through hydrogen bonds

– yields a rigid, supportive structure

• The α linkages of starch and glycogen form compact hollow cylinders suitable for accessible storage.

Glycosidic Linkages Determine Polysaccharide StructureInsoluble and Soluble Fiber Are an Important Part of the Diet

• Mammals cannot digest cellulose because they lack cellulases, but plant fibers are still important in the mammalian diet.

• Insoluble fibers increase the rate at which digestion products pass through the large intestine.

– softens stools and makes them easier to pass

• Soluble fibers (e.g., pectin or polygalacturonic acid) slow the movement of food through the gastrointestinal tract.

– facilitates absorption of nutrients from the diet

Chitin Is the Main Structural Polysaccharide of Fungi and Arthropods

• chitin = homopolymer of β-1,4 linked N-acetylglucosamine

– found in fungal cell walls and exoskeletons and shells of arthropods

– Fibers are often crosslinked and composited with minerals and proteins to increase rigidity and strength.

Chitin Can Be Processed to a Molecule with a Variety of Uses

• Cellulose is a major constituent of paper, bioadhesives, and clothes.

• Chitin could be recovered from the shellfishing industry by processing the shells into the more versatile chitosan through microbial/enzymatic processes.

• Chitosan can be used as:

– a carrier to assist in drug delivery.

– a component of cosmetic and food products.

– a surgical dressing.

Section 11.3 Carbohydrates Can Be Linked to Proteins to Form Glycoproteins

• glycoprotein = a carbohydrate group covalently attached to a protein

– makes up 50% of the human proteome

• glycosylation increases the complexity of the proteome

– glycoforms = different glycosylated forms

– may occur when a protein has several potential glycosylation sites

Three Classes of Glycoproteins

• glycoproteins = predominantly proteins

– play a variety of roles, including cell adhesion

• proteoglycans = predominantly carbohydrates and the protein component is conjugated to a glycosaminoglycan

– function as structural components and lubricants

• mucins (mucoproteins) = predominantly carbohydrates and the protein components is extensively glycosylated at Ser or Thr residues, usually by N-acetylgalactosamine

– key component of mucus

– function as lubricants

Carbohydrates Can Be Linked to Proteins Through N-Linked or O-Linked

• N-linkage = links the sugars in glycoproteins to the amide nitrogen atom in the side chain of Asn

– Asn must be part of an Asn-X-Ser or Asn-X-Thr sequence, where X is any residue except proline

• O-linkage = links the sugars in glycoproteins to the oxygen atom in the side chain of Ser or Thr

N-Linked Oligosaccharides Have a Common Core

• N-linked polysaccharides have a common pentasaccharide core that consists of three mannoses and two N-acetylglucosamine residues.

The Glycoprotein Erythropoietin Is a Vital Hormone

• erythropoietin (EPO) = a glycoprotein secreted by the kidneys into the blood serum to stimulate production of red blood cells

– cloned recombinant form has improved treatment for anemia, but has been abused by some endurance athletes

– glycosylation enhances the stability of the protein in the blood

Oligosaccharides Attached to Erythropoietin

• N-glycosylated at three Asn residues

• O-glycosylated a Ser residue

• 40% carbohydrate by weight

Glycosylation Functions in Nutrient Sensing

• GlcNAcylation = the post-translational, covalent

attachment of a single N-acetylglucosamine (GlcNAc) to Ser or Thr residues of proteins

– catalyzed by O-GlcNAc transferase

– occurs when nutrients are abundant

– reversible

O-GlcNAc Transferase

• GlcNAcylation sites are also potential phosphorylation sites.

– O-GlcNAc transferase and protein kinases may be involved in cross talk.

• Improper regulation of O-GlcNAc transferase has been linked to:

– insulin resistance.

– diabetes.

– cancer.

– neurological pathologies.

Proteoglycans Have Important Structural Roles

• Proteoglycans are up to 95% glycosaminoglycan by weight)

– resembles a polysaccharide more than a protein

• Proteoglycans:

– function as lubricants and structural components in connective tissue.

– mediate adhesion of cells to extracellular matrix.

– bind factors that regular cell proliferation.

Glycosaminoglycans

• glycosaminoglycans = composed of repeating units of disaccharides containing a derivative of an amino sugar

– amino sugar derivative is either glucosamine or galactosamine

– at least one of the two sugars in the unit has a negatively charged carboxylate or sulfate group

• The inability to degrade glycosaminoglycans causes diseases marked by skeletal deformities and reduced life expectancies.

Glycosaminoglycans Are Made of Repeating Units

Proteoglycans Are Important Components of Cartilage

• Cartilage contains the protein collagen protein and the proteoglycan aggrecan.

• aggrecan = large molecule with three globular domains

– site of glycosaminoglycan (keratan sulfate and chondroitin sulfate) attachment is in the extended region between G2 and G3

– G1 noncovalently binds to a central polymer of hyaluronate

The Proteoglycan From Cartilage Has an Enormous and Complex Structure

Aggrecan Cushions Compressive Forces

• Water is bound to the glycosaminoglycans to cushion compressive forces.

– Water is squeezed from the glycosaminoglycan under pressure.

– Water rebinds when pressure is released.

• osteoarthritis = form of arthritis that results when water is lost from proteoglycan with aging

Mucins Are Glycoprotein Components of Mucus

• tandem repeats (VNTR) region = region of the protein backbone of mucins that is rich in O-glycosylated Ser and Thr residues

• Core carbohydrate structures are conjugated to the protein component of mucin.

Functions of Mucins

• Mucins:

– adhere to epithelial cells and act as a protective barrier.

– hydrate the underlying cells.

– play roles in fertilization, the immune response, and cell adhesion.

• Overexpression occurs in bronchitis, cystic fibrosis, and adenocarcinomas.

Protein Glycosylation Takes Place in the Lumen of the Endoplasmic Reticulum and in the Golgi Complex

• Endoplasmic reticulum (ER) and Golgi complex are organelles that play central roles in protein trafficking.

• N-linked glycosylation begins in the ER and continues in the Golgi complex.

• O-linked glycosylation occurs only in the Golgi complex.

Dolichol Phosphate

• dolichol phosphate = specialized lipid molecule located in the ER membrane

– contains about 20 isoprene (C5) units.

– location where large oligosaccharides destined for attachment to the Asp residues are assembled

– the terminal phosphate is the site of attachment

The Golgi Complex Is a Sorting Center

• Golgi complex = a stack of flattened membranous sacs

• proteins proceed to lysosomes, secretory granules, or the plasma membrane

– based on signals encoded within their amino acid sequences and three-dimensional structures

Specific Enzymes Are Responsible for Oligosaccharide Assembly

• glycosyltransferases = catalyze the formation of glycosidic linkages

• Activated sugar nucleotides are the most common carbohydrate donor for glycosyltransferases.

Blood Groups Are Based on Protein Glycosylation Patterns

• Blood groups are designated by the presence of one of the three different carbohydrates (A, B, or O) attached to glycoproteins and glycolipids on the surfaces of red blood cells.

• All blood groups have a core O antigen.

Specific Glycosyltransferases Add the Extra Monosaccharide to the O Antigen

• A and B antigens have one extra monosaccharide through an α-1,3 linkage to a galactose moiety of the O antigen

– added by specific glycosyltransferases

• type A transferase = adds N-acetylgalactosamin to form the A antigen

• type B transferase = adds galactose to form the B antigen

The A, B, and O Oligosaccharide Antigens Share a Common Core Structure

Blood Type Phenotypes Result from the Enzymes Present

• Individuals with the:

– O blood type lack both enzymes.

– AB blood type express both enzymes.

– A blood type express only type A transferase.

– B blood type express only type B transferase.

• have important implications for blood transfusions.

– If an antigen not normally present is introduced, the immune system recognizes it as foreign.

The Cholera Toxin

• cholera = disease caused by a toxin from Vibrio cholerae

• Individuals with blood type O are ~8 times more likely to have severe disease.

– The O antigen binds more tightly to the toxin than other blood type antigens.

Errors in Glycosylation Can Result in Pathological Conditions

• congenital disorders of glycosylation = pathological conditions resulting from improper modification of proteins by carbohydrates and their derivatives

– examples: certain types of muscular dystrophy are linked to improper glycosylation and I-cell disease

I-Cell Disease

• lysosomes = organelles that degrade and recycle damaged cellular components or endocytosed material

• I-cell disease = a lysosomal storage disease that causes severe psychomotor impairment and skeletal deformities

– affected lysosomes contain undigested glycosaminoglycans and glycolipids

– active enzymes responsible for degradation are synthesized

– enzymes lack appropriate glycosylation and are exported instead of being sequestered in lysosomes

• A mannose 6-phosphate residue of the N-oligosaccharide directs the enzymes from the Golgi complex to lysosomes.

• In I-cell disease, the mannose lacks a phosphate because patients are deficient in the N-acetylglucosamine phosphotransferase.

Biochemists Use Several Techniques to Analyze Oligosaccharide Components of Glycoproteins

• Oligosaccharides can be detached from the protein using enzymes that cleave oligosaccharides at specific linkages.

• Mass of an oligosaccharide can be determined using MALDI-TOF or other mass spectrometric techniques.

– Many possible oligosaccharide structures have the same mass.

Determining Oligosaccharide Structure and Points of Attachment

• Structure can be determined by combining additional cleavage of the oligosaccharide with mass spectrometry.

• Points of attachment can be determined by applying proteases to glycoproteins and performing chromatography.

– Fragments attached to oligosaccharides have chromatographic properties that with protease treatment.

– followed by mass spectrometry or direct peptide sequencing

• Oligosaccharides can be sequenced by using enzymes that cleave specific glycosidic bonds.

• MALDI-TOF is used to identify the released sugars.

Oligosaccharides Can Be Characterized by Mass Spectrometry

Section 11.4 Lectins Are Specific Carbohydrate-Binding Proteins

• glycan-binding proteins = bind to specific carbohydrate structures on neighboring cell surfaces

• lectins = class of glycan-binding proteins

– example: the mannose 6-phosphate receptor that binds and directs lysosomal enzymes to the lysosome

Lectins Promote Interactions Between Cells and Within Cells

• Lectins:

– function to facilitate cell–cell contact.

– usually contains 2+ carbohydrate-binding sites.

– are linked to carbohydrates by a number of weak noncovalent interactions.

Lectins are Organized into Two Large Classes

• C-type (calcium-requiring) lectins = found in animals

– function in receptor-mediated endocytosis and cell–cell recognition

• L-type lectins = rich in seeds of leguminous plants

– serve as potential toxins to herbivorous insects

– come act as chaperones in the eukaryotic ER

C-Type Lectins Use Calcium Ions to Bind Carbohydrates

• Ca2+ on lectin acts as a bridge between lectin and the sugar.

• Two Glu residues in lectin bind to Ca2+ and the sugar.

• Other hydrogen bonds form between lectin side chains and the carbohydrate.

Selectins

• selectins = members of C-type lectins

– bind immune-system cells to sites of injury in the inflammatory response

– play a role in recruiting leukocytes to inflammation sites

• different forms of selectins:

– L form = bind to carbohydrates on lymph-node vessels

– E form = bind to carbohydrates on endothelium

– P form = bind to carbohydrates on activated blood platelets

Influenza Virus Binds to Sialic Acid Residues

• hemagglutinin = influenza virus lectin protein that binds to carbohydrates sialic acid residues linked to galactose residues on cell-surface glycoproteins

– the virus is engulfed after binding

• The virus replicates inside the cell and viral particles bud off from the cell.

Neuraminidase Cleaves Oligosaccharide Chains

• Assembled viral particles are attached to sialic acid residues of the cell membrane by hemagglutinin.

• neuraminidase (sialidase) = influenza virus protein that cleaves the glycosidic linkages between sialic acid and the rest of the glycoprotein

– frees the virus to infect new cells

– inhibitors of neuraminidase (Tamiflu and Relenza) are

important anti-influenza agents

Influenza Virus Uses a Lectin for Specific Cell Binding