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Biochemistry: Carbohydrates & Glycobiology

Carbohydrate Roles, Structure, Stereochemistry, Glycosidic Bonds

Carbohydrates are found in:

polymers store in muscle and liver cells

Bacterial cell walls (PG)

Attached to specific proteins found in cellular membranes

3 different roles of carbs:

nutrition/energy-glycogen & starch (amylopectin)

structure polymers- cellulose, PG, chitin

informational molecules-DNA, RNA, & glycoproteins

Carbohydrates participate in covalently link bonds & are primarily hydrophilic molecules

Typically six carbons in a typical monosaccharide, having five chiral centers ( five hydroxyl groups)

There is 2^n stereoisomers for a sugar with n chiral centers.

An epimers are monosaccharides which differ in configuration around a single carbon

Look at the furthest chiral carbon from the carbonyl and use its OH group, if OH group is on the right it is assigned a D but if its on the left it is assigned an L when determining L or D configuration

An anomeric carbon is the carbon atom in a sugar at the new stereo center formed when a sugar cyclizes to form a hemiacetal/hemiacetal, this is the carbonyl carbon of aldehydes and ketones

Mutarotation is the change is specific rotation of a pyronose or furanose sugar or glycoside accompanying the equilibration of its alpha or beta anomeric forms

Pyronose=six membered ring compounds

Furanoses= five membered ring

By "reducing" end of a di-, oligo- or polysaccharide we mean end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) commonly known as the reducing end

By "non-reducing" end of a di-, oligo-, or polysaccharide we mean beginning of chain and not an anomeric carbon

3 Major Size Classes

1.)monosaccharides (contain 1 mono. unit, individual carbohydrate subunits ex. glucose, galactose, and fructose, serve as monomers for all higher order structures)

2.)oligosaccharides (contain 2 -20 mono. units covalently bonded, components of glycolipids, glycoproteins, & lactose)

3.)polysaccharides (>20 mono. units up to thousands of monosaccharides covalently linked, many long polymers, ex. starch, glycogen,cellulose, etc.)

Numbering System: required for a point of reference, start at the end nearest to the carbonyl carbon & number from there

3C=tri, 4C=tetra, 5C=pent, 6C=hex, 7C=hept

1% Fisher Projection & 99% Haworth Perspective

Cyclized forms of monosaccharide favored 99%

Glycosidic Bond

-covalent bond that joins monosaccharides (abbreviation & reducing and non-reducing end)

-Hemiacetal vs. Hemiketal

Naming Glycosidic bonds

1.) Identity sugar closer to non-reducing end

2.) describe the configuration of the anomeric carbon (non-reducing end sugar) in the glycosidic bond

3.) identity carbons joined in the bond

4.) identity sugar nearer to reducing end

Polysaccharides

a.k.a. glycans that are medium to high molecular weight polymers, that differ according to the identity of their:

-identity of monosaccharides monomers (homopolysaccharides vs. heterosaccharides)

-molecular weight (# of monosaccharides/polymer)

-differ in the degree of branching (linear vs. branch)

-chain length can differ

-identity of glycosidic bond (ex. alpha 1-4 or beta 1-4)

Homopolysaccharides=contain only a single monomeric species (all monomers are identical)

Heteropolysaccharide= cotain two or more different kinds (all monomers are different)

Some of the common sugars are Maltose, Lactose, and Sucrose

Advantage of a cell storing glucose as a polymer vs. individual molecules-

Advantage of having a highly branched structure of molecules like glycogen or amylopectin-

Polymer

Type (hetero. vs. homo)

Bonds

Role/ Significance

Amylose (starch)

Homopolysaccharide

α1-4 linear

Storage (energy) in plants

Amylopectin (starch)

Homopolysaccharide

α1-4 linear & α1-6 branch

Storage (energy) in plants

Glycogen

Homopolysaccharide

α1-4 linear & α1-6 branch

Storage (energy) in animals

Cellulose

Homopolysaccharide

β1-4 linear

Plant cell wall (structure & rigidity)

Chitin

Homopolysaccharide

β1-4 linear

Exoskeleton of arthropods and fungi (structure & rigidity)

Peptidoglycan

Heteropolysaccharide

β1-4 linear

Bacteria cell wall (structure & rigidity; osmotic entry)

Bacterial cell walls differ from plant cell walls in that bacteria cell wall is made up of PG and plant cell wall is made up of cellulose

Peptidoglycan= found in cell walls of bacteria/microbes, structural/rigid component , and prevents swelling/maintains osmotic pressure, also target for lysozyme enzymes in destroying bacteria

Cellulose characteristics:

fibrous

water resistant

relatively insoluble

indigestible at least by humans and mammals because they lack Beta 1-4 amylose

Glycoaminosglycans

constitute a family of polymers often covalently linked to proteins

are polymers of alternating uronic acid and amino sugar (N-acetyglucoasmine or N-acetylgulactoseamine)

high negative density charge that comes from the carboxyl & sulfate groups COO- & OSO3-

have a repeating heteropolysaccharide structure & allows binding to lots of water

Allows Gel formation ; lubrication/viscosity (protect in between joints to prevent pain & pressure), Adhesiveness/sticky helps held cells together ex. eyeballs, tensile strength /cushion (connective tissue)

Good for viscosity, adhesiveness, and tensial strength to Extracellular Matrix (ECM)

*include chondroitin sulfate, dermatansulfate, and keratin sulfate

Glycoconjugates

1.) Carbohydrates in the form of poly- or oligo- covalently linked to peptides, proteins, or lipids

Proteoglycans

Glycoproteins

Glycolipids & lipopolysaccharides

2.) Variety of physiological roles

cell-cell recognition, adhesion or migration

blood clotting

immune response

wound healing

Glycosglation- why attach carbohydrates?

molecular diversity (greater sub-level control)

molecular solubility (carbohydrates are polar molecules)

molecular structure (protein)

molecular protection (proteases-enzymes that cleaves peptides & proteins)

Lectins- are proteins that bind carbohydrates with high specificity & with modern to high affinity

Roles

cell-cell recognition

signaling

adhesion process

intro-cellular targeting of newly synthesized proteins

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