Chapter 11 - Carbohydrates and Glycoproteins

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 the simplest carbohydrates

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

• The smallest ones are composed of three carbons

• Exist in many isomeric forms

• Carbohydrates that are three to seven carbons in length

• Also called simple sugars

Are joined to alcohols and amines through glycosidic linkages

Monosaccharide biochemical properties can be modified by reactions with:

• alcohols.

• amines.

• phosphates.

Monosaccharide Nomenclature

Based on carbon-chain length:

• three carbons: trioses

• Four carbons: tetroses

• Five carbons: pentoses

• Six carbons: hexoses

• Seven carbons: heptoses

Also based on the identity of the most oxidized group:

• keto group: ketone

• Aldehyde group: aldose

Constitutional isomers

molecules with identical molecular formulas that differ in how the atoms are ordered

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

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 an alcohol to form a hemiacetal

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

Pyranose Formation

Called pyranose because of similarity to pyran

Furanose Formation

Called furanose because of similarity to furan

Anomer

a diastereoisomeric form of sugars that forms when a cyclic hemiacetal is formed and an additional asymmetric center is created

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

D-Glucose

Is an important fuel for most organisms

• 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

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 structures 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.

Fehling’s solution

Solutions of Cu2+ that test for the presence of sugars that adopt an open structure

Reducing sugars

Sugars that react with oxidizing agents

• all monosaccharides that can adopt linear structures in solution

Non-reducing sugars

Sugars that do not react with oxidizing agents

Glycation of Sugars

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 glycation proteins unless concentrations of sugar and protein are very high for long periods of time

Advanced glycation end products (AGEs)

Products resulting from cross-linking following the primary modification

• implication in aging, arteriosclerosis, diabetes, and other pathological

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

modifications increase biochemical versatility

• can serve as signal molecules

• can facilitate metabolism

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

• linked to form complex carbohydrates

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

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

Sucrose

Disaccharide of sugar can 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)

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

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 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

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

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

• 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 structure

Insoluble 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

• 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.

• can be processed to a molecule with a variety of uses

• Could be recovered from the shell fishing 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

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

• Proteoglycans

• Mucins (mucoproteins)

Glycoproteins

Predominantly proteins

• plays 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 expect 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 at 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 and protein kinases

• May be involved in cross talk

GlcNAcylation sites

Are also potential phosphorylation sites

Improper regulation of O-GlcNAc transferase

Has been linked to:

• insulin resistance

• Diabetes

• Cancer

• Neurological pathologies

Proteoglycans

Have important structural roles

• are up to 95% glycosaminoglycan by weight

• Resembles a polysaccharide more than a protein

• Function as lubricants and structural components in connective tissue

• Mediate adhesion of cells to extracellular matrix

• Bind factors that regulate cell proliferation

• important components of cartilage

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

• made of repeating units

The inability to degrade glycosaminoglycans

Causes diseases marked by skeletal deformities and reduced life expectancies

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

• cushions compressive forces

The Proteoglycan from cartilage

Has an enormous and complex structure

Water

Is bound to the glycosaminoglycans to cushion compressive forces

• is squeezed from the glycosaminoglycan under pressure

• 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

• 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