Chapter 7 Biochemistry

Important Hexose Derivatives

(Blue, Purple, and Orange) β-D-glucose β-D-glucose 6 -phosphate β-D-glucosamine N-Acetyl-β-D-glucosamine β-D-galactosamine β-D-mannosamine Muramic acid N-Acetylmuramic acid

Three Major Functions of Carbohydrates

1. energy source and energy storage 2. structural component of cell walls and exoskeletons 2. informational molecules in cell-cell signaling

Trehalose

A constituent of hemolymph of insects→ Provides protection from drying Resurrection plant (> 15 yrs) disaccharide formed upon condensation of two glucose via α1 → α1 bond

Glycolipids

A lipid with covalently bound oligosaccharide Parts of plant and animal cell membranes In vertebrates, ganglioside carbohydrate composition determines blood groups In gram-negative bacteria, lipopolysaccharides cover the peptidoglycan layer

Starch

A mixture of two homopolysaccharides of glucose: Amylose and Amylopectin Starch is the main storage polysaccharide in plants Degraded by amylases and α-glucosidase and debranching enzyme

Glycoprotein

A protein with small oligosaccharides attached Carbohydrate attached via its anomeric carbon About half of mammalian proteins are glycoproteins Carbohydrates play role in protein-protein recognition Only some bacteria glycosylate few of their proteins Viral proteins heavily glycosylated; helps evade the immune system

Blood Type

ABO blood group antigens are oligosccharide components of glycoproteins and glycolipids on cell surface (not just red blood cells) Type A cells -A antigen on cell surface, anti-B antibody (Ab) in blood Type B cells-B antigen on cell surface, anti-A Ab in blood Type AB cells - A and B antigens on cell surface, no antibodies in blood Type O - neither A nor B antigen (H antigen) on cell surface, both anti-A and anti-B Ab in blood

Agar and Agarose

Agar is a complex mixture of hetereopolysaccharides containing modified galactose units Agarose is one component of agar Agar solutions form gels that are commonly used in the laboratory as a surface for growing bacteria Agarose solutions form gels that are commonly used in the laboratory for separation DNA by electrophoresis Extra Info: The repeating unit consists of D-galactose (β1→4)-linked to 3,6-anhydro-L-galactose

Tollens' test

Aldehyde can reduce Ag+ to Ag0

Fehling's test

Aldehyde can reduce Cu2+ to Cu+

What do Fehling's and Tollens' tests do?

Allows detection of reducing sugars, such as glucose

What confirmation does amylose form?

An amylose helix = not strong such as an alpha-helix in proteins, but does form as a stable conformation.

Ramachandran Plots of Carbohydrates

Carbohydrates can bend around much more than amino acids...angles all over the Ramachandran plots.

Cellulose

Cellulose is a unbranched homopolysaccharide of glucose Glucose monomers form (β1 → 4) linked chains Hydrogen bonds form between adjacent monomers Additional H-bonds between chains Structure is now tough and water-insoluble Most abundant polysaccharide in nature

Why the animal storage polysaccharide is much more highly branched?

Chemical degradation proceeds through non-reducing end catalzyed by enzymes (alpha-amylase), the more non-reducing ends the faster is the degradation which is critical for adaption to fast changes in energy demands (eg motion and resting in animals)

Chitin

Chitin is a linear homopolysaccharide of N-acetylglucosamine that form (β1→ 4)-linked chains Forms extended fibers that are similar to those of cellulose Hard, insoluble, cannot be digested by vertebrates Structure is tough but flexible, and water-insoluble Found in cell walls in mushrooms, and in exoskeletons of insects, spiders, crabs, and other arthropods

Carbohydrate formula

Cn(H2O)n

Pentoses and hexoses readily undergo intramolecular _____________________.

Cyclization

Lysozyme

Damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid (NAM) and N-acetyl-D-glucosamine (NAG) residues in a peptidoglycan Cleavage of the cell wall leads to the lysis of bacteria X-ray structures of lysozyme with bound substrate analogs show that the C-1 carbon is located between Glu 35 and Asp 52 residues Lysozyme is an antibacterial enzyme

Dextrans

Dextrans are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrans are mixtures of polymers of glucose units linked by (α1 → 4) or (α1 → 6) or (α1 → 3) glycosidic bonds. Low molecular weight

Chiral carbohydrates are usually drawn in __________________ _________________.

Fischer projections (Horizontal bonds are pointing toward you; vertical bonds are projecting away from you)

furanoses

Five-membered oxygen-containing rings

Describe analysis of glycoconjugates with acid hydrolysis

Following methyl iodide: acid hydrolysis yields monosaccharides methylated at every -OH except those in glycosidic bonds

Carbohydrates and structure, function and recognition

Glycoforms - variant species of a glycoprotein Oligosaccharides help define protein structure Usually attached at sequences that form surface loops or turns -May play structural roles -May help stabilize protein conformation -Biological recognition -Lectins - proteins that bind carbohydrates

Granules

Glycogen and starch often form granules in cells Granules contain enzymes that synthesize and degrade these polymers Glycogen and amylopectin have one reducing end but many nonreducing ends Enzymatic processing occurs simultaneously in many nonreducing ends

Heparan sulfate

Heparan sulfate is heparin-like polysaccharide but attached to proteins Highest negative charge density biomolecules Prevent blood clotting by activating protease inhibitor antithrombin Binding to various cells regulates development and formation of blood vessels Can also bind to viruses and bacteria and decrease their virulence

Some Types of glycosaminoglycans

Hyaluronate (hyaluronic acid) Chondroitin 4-sulfate Keratan sulfate Heparin

Describe α configuration

If the hydroxyl group is on the opposite side (trans) of the ring as the CH2OH moiety

Describe β configuration

If the hydroxyl group is on the same side (cis) of the ring as the CH2OH moiety

Carbohydrate enantiomers

In sugars that contain many chiral centers, only the one that is most distant from the carbonyl carbon is designated as D (right) or L (left) D and L isomers of a sugar are enantiomers Most hexoses in living organisms are D stereoisomers

Why not store glucose in monomeric form?

It would facilitate rapid degradation when needed but would highly increase osmotic pressure (depends on the number of molecules and not the size of molecules - water would move into the cell)

Glycosaminoglycans

Linear polymers of repeating disaccharide units One monomer is either N-acetyl-glucosamine or N-acetyl-galactosamine They are negatively charged by either Uronic acids (C6 oxidation) or/and Sulfate esters Forms meshwork with fibrous proteins to form extracellular matrix Connective tissue Lubrication of joints

Peptidoglycan

Major component bacterial cell wall Covalently linked polysaccharide and polypeptide chains - heteroglycan chains linked to peptides Composed of alternating GlcNAc (NAG) and N-acetylmuramic acid (NAM) (β1→4) linkages Sugar units linked to D-amino acid tetrapeptide

Lectins

Many sugars have a more polar and a less polar side The more polar side hydrogen-bonds with the lectin, while the less polar undergoes hydrophobic interactions with nonpolar amino acid residues. Biological recognition: Lectins - frequently appear on cell surface Lectins are specific -Many viruses, bacteria and eukaryotic parasites invade target tissues by first binding cell surface carbohydrates P-selectin: recognizes glycoprotein on leukocytes and brings to site of inflammation

ECM (Extra Cellular Matrix)

Material outside the cell Strength, elasticity, and physical barrier in tissues Main components: -Proteoglycan aggregates -Collagen fibers -Elastin (a fibrous protein)

Anomeric carbon

Name of the former carbonyl carbon that becomes a new chiral center

Oligosaccharide microrray

Oligosaccharides with different/distinct moieties are put in wells of a dish, a probe with fluorescently labeled glycan-specific binding protein. Scan wells for fluorescence.

Heteropolysaccharides

Peptidoglycan Agar/agarose Glycosaminoglycans

How are carbohydrates produced?

Produced from CO2 and H2O via photosynthesis in plants

Describe analysis of glycoconjugates with methyl iodide.

Reaction of sugar with methyl iodide and silver oxide will convert the hydroxides to methyl ethers The methylated sugar is a stable base. Only sugars with free -OH groups (not in a glycosidic bond) will be methylated

How do cows and termites eat grass/wood if they can't metabolize cellulose ?

Ruminants and termites live symbiotically with microorganisms that produces cellulase

pyranoses

Six-membered oxygen-containing rings

Interaction of cells and ECM

Some integral membrane proteins are proteoglycans (Syndecans) Other integral membrane proteins are receptors for extracellular proteoglycans (Integrins) These proteins link cellular cytoskeleton to the ECM and transmit signals into the cell to regulate: -cell growth -cell mobility -apoptosis -wound healing

Homopolysaccharides

Starch Glycogen Cellulose Chitin Dextrans

Proteoglycans

Sulfated glucoseaminoglycans attached to a large rod-shaped protein in cell membrane Different glycosaminoglycans are linked to the core protein Syndecans: protein has a single transmembrane domain Glypicans: protein is anchored to a lipid membrane Interact with a variety of receptors from neighboring cells and regulate cell growth

Proteoglycan Aggregates

Supramolecular assemblies of many core proteins bound to a single molecule of hyaluronan. Hold lots of water (1000x its weight); provides lubrication Very low friction material: "slippery" Fibrous matrix proteins interwoven forming a cross-linked mesh Covers joint surfaces: articular cartilage -Reduced friction -Load balancing

Lactose

The disaccharide formed upon condensation of a galactose and glucose molecules via 1 → 4 bond is

Sucrose

The disaccharide formed upon condensation of a glucose and fructose molecule via α1 → β2 bond

Maltose

The disaccharide formed upon condensation of two glucose molecules via 1 → 4 bond

Cellulose metabolism

The fibrous structure and water-insolubility make cellulose a difficult substrate to act on Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (β1→4) linkages

In a glycosidic bond between monomers, what is the difference in reactivity between the first monomer and second monomer?

The glycosidic bond (an acetal) between monomers is less reactive than the hemiacetal at the second monomer Second monomer, with the hemiacetal, is reducing Anomeric carbon involved in the glycosidic linkage is nonreducing

The position of this group determines if the anomer is α or β

The hydroxyl group formed from the former carbonyl oxygen

The anomeric carbon is usually drawn on which side?

The right

There is an equilibrium between which two forms?

The ring forms exist in equilibrium with the open-chain forms

hemiacetals

aldehydes are attacked by alcohols,

Epimers

are two sugars that differ only in the configuration around one carbon atom

What do glycosidic bonds occur between?

between an anomeric carbon and a hydroxyl carbon

What are glycoproteins and proteoglycans?

carbohydrates covalently linked with proteins

Ketose

contains a ketone functionality

Aldose

contains an aldehyde functionality

Aldehyde and ketone carbons are _________________________ Alcohol oxygen atom is __________________________

electrophilic ; nucleophilic

Sugar molecules can be joined by what type of bond?

glycosidic bond

Polysaccharides can be ...

homopolysaccharides or heteropolysaccharides linear or branched (you can have a branched or linear heteropolysaccharide or branched or linear homopolysaccharides)

Penicillin

interferes with the formation of the amino acid bridge (transpeptidation). a suicide-inhibitor of tranpeptidase

Mannose

is a C2 epimer of glucose

Galactose

is a C4 epimer of glucose

Glycogen

is a branched homopolysaccharide of glucose Glucose monomers form (α1 → 4) linked chains Branch-points with (α1 → 6) linkers every 8-12 residues Functions as the main storage polysaccharide in animals Degraded by glycogen phosphorylase and glycogen debranching enzyme

Amylose

is an unbranched polymer of (α1→ 4) linked residues

Amylopectin

is branched like glycogen but the branch-points with (α1 →6) linkers occur every 24-30 residues

Fructose

is the ketose form of glucose

Ribose

is the standard five-carbon sugar

Glucose

is the standard six-carbon sugar

hemiketals

ketones are attacked by alcohols,

What is an important property of the product of a glycosidic bond that has two acetal groups and no hemiacetals?

no reducing ends (any modification to hydroxyl group of anomeric carbon = locked in cyclized form)

Diastereomers

stereoisomers that are not mirror images Diastereomers have different physical properties