Carbohydrates Energy Cycle of Life Monosaccharides

Carbohydrates

Energy Cycle of Life

Monosaccharides (Monomeric Sugars) • Formula: (CH 2 O)n (with n ≥ 3) – – – Trioses: n=3 Tetroses: n=4 Pentoses: n=5 Hexoses: n=6 Heptoses: n=7 Octoses: n=8 • Sugars contain one aldehyde or ketone carbonyl and at least two alcohols. • Pentoses and hexoses can be linear or cyclic (cyclic or ring form favored under physiological conditions)

Trioses: The Simplest Monosaccharides Aldehyde-containing sugar = aldose Ketone-containing sugar = ketose

Aldose-Ketose Interconversion via Enediol Intermediate Note: Isomerization of dihydroxyacetone phosphate to glyceraldehyde-3 -phosphate during glycolysis is catalyzed by triose phosphate isomerase, a “perfect enzyme. ”

The Enantiomers of Glyceraldehdye Most naturally occurring sugars are D-sugars.

RS Nomenclature System (Cahn, Prelog, Ingold System)

Some Abundant Monosaccharides

Stereochemistry of Aldotetroses <- Enantiomers -> <- Diastereomers ->

The Two Enantiomers of the Ketotetrose Erythrulose

Stereochemical Relationships of the DAldoses

Stereochemical Relationships of the DKetoses

Cyclization of Hexoses Pentoses can cyclize also (e. g. , ribofuranose).

Pyranoses vs. Furanoses

Pyranose Ring Sugars in Chair and Boat Conformations Chair conformation more stable

Conformations of Chair Form of a Pyranose

The Two Chair Conformations of b-DGlucopyranose Conformation on the left more stable since relatively bulky OH and CH 2 OH groups all occupy equatorial positions (in the conformation on the right they occupy more crowded axial positions).

Conformational Isomers of Furanoses

a and b Anomers of Glucose a anomer b anomer Anomers: different configurations at C 1

The Four Most Common Hexoses

Abundance of Different Forms of Some Monosaccharides

Terminology Describing Structure of Sugars

Derivatives of the Monosaccharides

Modified Sugars

Acid and Lactone Derivatives Oxidation of a monosaccharide yields the corresponding acid and lactone derivatives.

Alditols Reduction of the carbonyl group on a sugar yields the corresponding alditol.

Amino Sugars Often found in complex oligoand polysaccharides.

Glycosides and the Glycosidic Bond

Two Toxic Glycosides Produced by Plants Inhibits Na+/K+ ATP-dependent pump Yields hydrogen cyanide (HCN) on hydrolysis.

Oligosaccharides

Structures of Some Important Disaccharides Reducing sugar ide Non-reducing sugars H A reducing sugar has a free anomeric carbon not in glycosidic bond (free hemiacetal or hemiketal).

Reducing sugars

Occurrence of Some Dissaccharides

Formation of the Glycosidic Bond

Formation of Lactose in vivo

Polysaccharides • Storage polysaccharides • Structural polysaccharides • Glycosaminoglycans

Amylose: A Linear Glucose Polymer (Glucan) A major storage form of glucose in plants. Polymer of a-D-glucopyranose with a(1 ->4) glycosidic linkages

Amylopectin: A Branched Glucan Another storage form of glucose in plants. a(1 ->4) and a(1 ->6) glycosidic linkages R = reducing end N = nonreducing end

Glycogen: The Major Storage Form of Glucose in Animals • • Also a branched glucan with a(1 ->4) and a(1 ->6) glycosidic linkages, like amylopectin. Has more frequent a(1 ->6) branches. Glycogen granules in a liver cell

Organization of Plant Cell Walls

Cellulose: A Structural Polysaccharide in Plants Polymer of b-Dglucopyranose with b(1 ->4) glycosidic linkages

Chitin: A Structural Polysaccharide in Exoskeletons of Many Arthropods and Mollusks Polymer of N-acetyl-b-Dglucosamine (Glc. NAc) with b(1 ->4) glycosidic linkages

Repeating Structures of Some Glycosaminoglycans constitute the carbohydrate component of proteoglycans, conjugates of glycosaminoglycans and proteins, with structural functions in vertebrate cartilage and connective tissue.

Glycoconjugates

Functions of Oligosaccharides in Glycoconjugates • • • Structural May affect protein stability or activity Cell markers and cell surface recognition factors Immunochemical markers (e. g. , ABO blood group antigens) Cell-substratum and cell-cell adhesion Sperm/egg recognition Cell growth signals Tagging for transport to lysosomes, tagging for cleavage/destruction Protein folding

Bacterial Cell Walls

Bacterial Cell Walls: Peptidoglycans

Peptidoglycan Structure of Gram-Positive Bacteria

Structure of Lipoteichoic Acid A membrane-anchored glycolipid that protrudes through the cell wall, helps to anchor peptidoglycan to membrane, as well as playing a role in adhesion and cell-cell interactions. It is also a major antigenic determinant in the immune response against Gram-positive bacteria. Ends in diacylglycerol moiety anchored in membrane.

Bacterial Cell Walls: Gram-Negative Bacteria NAM = N-acetylmuramic acid NAG = N-acetylglucosamine

Glycoconjugates Associated with Animal Cells

Some Glycoproteins Most extracellular, secreted proteins, as well as extracellular domains of membrane-bound proteins, are glycosylated. In addition, some proteins resident in the endoplasmic reticulum and Golgi apparatus are glycosylated.

Glycoprotein Bonds: N- and O-Linked Oligo- and Polysaccharides N-linked sugars Attached in endoplasmic reticulum on specific asparagine residues. Attached in Golgi apparatus on specific threonine or serine residues. O-linked sugars Most commonly: b-galactosyl-(1 ->3)-a-Nacetylgalactosamine.

N-Linked Oligosaccharides • 14 -residue oligosaccharide (9 mannose, 3 glucose, 2 Nacetylglucosamine (Glc. NAc)) attached cotranslationally in ER to asparagine residues in sequence Asn-X-Ser or Asn-X-Thr (where X is any amino acid except Pro or Asp). • Some sugars removed in ER and Golgi by glucosidases and mannosidases. • All final N-linked oligosaccharides have common pentasaccharide core: Mana(1 ->6) Manb(1 ->4)Glc. NAcb 1 ->Protein Mana(1 ->3) • Additional sugars (variable) attached to core, such as Glc. NAc, galactose, fucose and sialic acid, by glycosyltransferases in Golgi. • A single protein can be both N-glycosylated and O-glycosylated.

Attachment and Processing of N-Linked Oligosaccharides

Proteoglycans are glycosaminoglycanprotein conjugates that are one of the components of the matrix around cells in skin and connective tissue. Proteoglycans are highly hydrated and, in conjunction with collagen, help give these tissues their resilience and resistance to shear forces. Aggrecan: the major proteoglycan in cartilage.

Structure of Aggrecan

Syndecan: A Transmembrane Proteoglycan

Example of a Glycolipid Conjugate: The ABO Blood Group Antigens No additional sugar here = Type O (non-antigenic oligosaccharide) Individual makes antibodies against A and B antigen. + N-Acetylgalactosamine = Type A antigen Both A and B antigens = Type AB Individual makes antibodies against B antigen. Individual makes neither anti -A nor anti-B antibodies. + Galactose = Type B antigen Individual makes antibodies against A antigen.

Structural Dynamics of Oligosaccharides Attached to Proteins

Cell Surface Recognition Factors

Lectins: Carbohydrate-Binding Proteins Lectins are carbohydratebinding proteins often found on the surface of cells, providing one mechanism of cell-cell recognition through binding of lectin on one cell to carbohydrate on other cell.

Sequencing Oligosaccharides

Cleavage of an Oligosaccharide by Glycosidases