Glycosidic Bonds, Disaccharides, and Polysaccharides
Nomenclature
(-syl)-(linkage)-(-ose)
Proteoglycans Functions
-Formation of extracellular matrix -Interact with a variety of receptors -Important component of connective tissue and cartilage
Homopolysaccharides Functional Roles
-Fuel storage: Starch and glycogen -Structural: Cellulose and chitin
Why are glycogen and starch so enormous?
-Osmolarity (concenration of glycogen in cell 0.01uM would equal 0.4M as glucose monomers -cause large osmotic imbalance -Provides free-E to draw glucose from extracellular space (about 5mM glucose in blood) -Low effective glucose concentration inside cells creates concentration gradient to draw blood glucose inward
Extracellular Matrix Main Components
-Proteoglycan aggregates -Collagen fibers -Elastin (a fibrous protein)
Glycolipid
A lipid with covalently bound oligosaccharide -outer-leaflet of eukaryotes and bacterial cell membranes -rich in neurons (binding site for lectins and myelin formation) -RBC (determines blood groups)
Abundance
About half of mammalian proteins are glycoproteins -viral proteins are heavily glycosylated, helps evade the immune system -glycoproteins are rare in bacteria
Additional H-bonds between chains
Allow structure to be tough and water-insoluble
Branching
Allows for rapid degradation and synthesis -enzymatic processing occurs at the many non-reducing ends
Glycogen
BRANCHED homopolysaccharide of glucose -functions as the main storage polysaccharide in animals -glucose monomers from alpha 1-->4 linked chains -branched-points with alpha 1-->6 linkers every 8-12 residues -molecular weight reaches several millions
Amylopectin
Branched like glycogen byt the branch-points with alpha 1-->6 linkers occur every 24-30 residues -nonreducing ends and reducing ends
Glycoproteins (glycoconjugates)
Carbohydrate attached via its anomeric carbon -added by glycosyltransferases -removed by glycoside hydrolases
Lectins
Carbohydrate binding proteins that translate the 'sugar code'
Mallard Reaction
Complex reaction between reducing sugar and amino acids -gives brown color to cooked foods and flavor profile
Proteoglycans
Covalently linked to sulfated glucoseaminoglycans (GAGs) (heteropolysaccharide) -40 types both as secreted (extracellular and membrane proteins)
HIV Vaccine Design Strategy
Development of neutralizing antibodies targeted at conserved 'holes' in the glycan shield -conserved holes
Maltose
Disaccharide of two glucose molecules via 1-->4 glycosidic bond -still reducing sugar because the cyclic hemiacetal end is in equilibrium with open chain form
Glycogen and starch
Enormous storage molecules glycogen >1M residues starch >200M residues
Heteropolysaccharides Functional Roles
Extracellular support
Metabolism of Glycogen & Starch
Glycogen and starch often form granules in cells -contain enzymes that synthesize and degrade these polymers
O-linked glycoprotein
Glycosidic bond to hydroxyl of Ser/Thr
Diversity of polysaccharides
Have a variety of functional roles, and come in a diverse variety of structures (more subunits and types of linkages than proteins).
Glycogen and amylopectin
Have one reducing end but many nonreducing ends (due to branching)
Viral Coat Proteins
Heavily glycosylated -provides mechanisms to "shield" and evade immune response (i.e. glycan shield of HIV) -conserved hole
The sugar code
In additional to fuel and structural roles, polysaccharides have important roles as information carries (rich complexity of structure) Examples: Binding sites for extracellular signals i.e. growth factors Cell-to-cell communication i.e. immune response Cell-cell recognition, adhesion, migration Transport signal to the plasma membrane, or for degradation THESE ROLES are fulfilled by attaching sugars to proteins/lipids.... glycoconjugates
The glycosidic bond (an acetal) between monomers
Is less reactive than the hemiacetal at the second monomer.
Monomer with anomeric carbon involved in the glycosidic linkage
Is nonreducing
Gram-negative bacteria
Lipopolysaccharides -important for antibody recognition
Extracellular matrix (ECM)
Material outside of the cell -strength, elasticity and physical barrier in tissues -connection and communication to interior of cell
Starch
Mixture of two homopolysaccharides of glucose -main storage polysaccharide in plants -amylose and amylopectin
Reducing end (can still open up to form aldehyde)
Monomer with the hemiacetal (still has alpha and beta-anomeric forms)
N-linked glycoprotein
N-glycosyl bond to amide nitrogen of Asn
Polysaccharides
Natural carbohydrates are usually found as polymers -do not have a defined molecular weight -homopolysaccharides -heteropolysaccharides AND -linear -branched
Polymers of approximately 20 or more, with Mws greater than 20kDa
No template to control synthesis (in contrast to proteins) -polymer length is determined by the fidelity of the enzyme
1<--->1 linkage of two pyranoses
Nonreducing
Syndecans
Protein has a single transmembrane domain
Glypicans
Protein is anchored to a lipid membrane
Glycoprotein (glycoconjugate)
Protein with small oligosaccharide attached -typically smaller glycans -found on outside of plasma membrane, organelles, and in blood -form of reversible-covalent post translational modification -recognized by carbohydrate binding proteins (lectins) -Lectins, decipher and translate the 'sugar code'
1-->4 linkage of two pyranoses
Reducing
Cellulase
Secretion by fungi, bacteria and protozoa that allows them to use wood as source of glucose
Beta 1 -->4 linkage
Supports extended structure
Metabolism of cellulose
The fibrous structure and water-insolubility make cellulose a difficult substrate to process -most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze beta 1-->4 linkages (ruminants i.e. sheep and cattle and termites live symbiotically with microorganisms that produce cellulase)
Non-reducing Disaccharides
Two sugar molecules can also be joined via a glycosidic bond between two anomeric carbons -product has two acetal groups and no hemiacetals -there are no reducing ends, this is a nonreducing sugar -prevents formation of longer chains
Glycosidic Bond
Two sugar molecules conjoined between an anomeric carbon and a hydroxyl carbon.
Cellulose
Unbranched homopolysaccharide of glucose -most abundant polysaccharide in nature -glucose monomers form beta 1-->4 linked chains -hydrogen bonds form between adjacent monomers
Amylose
Unbranched polymer of alpha 1-->4 linked residues -nonreducing end and reducing end
1<-->2 linkage of pyranose and furanose
nonreducing