Carbohydrates

CHEM464 Carbohydrates Carbohydrates • General formula Cn(H2O)n (carbon-hydrates) • Also known as saccharides or sugars • They serve as energy stores or fuel • They are components of DNA and RNA • They are the primary component of cell walls of bacteria and plants • They are linked to proteins and lipids to form key intermediates and regulators of physiological processes Monosaccharides • They are polyhydroxy aldehydes (aldose) or ketones (ketose). • Carbon chain from 3 to 9 with general name of triose, hexose etc or aldohexose, ketohexose etc. • Fischer projection formulas are written as linear C chains with the aldehyde numbered as C1 in aldoses and ketone as C2 in ketoses. • Each monosaccharide (except dihydroxyacetone) has at least one chiral carbon. The highest numbered C is generally achiral • The orientation of the –OH around the chiral carbons determines the identity of the sugar. 1 CHEM464 Carbohydrates Stereoisomers • The (n-1) C determines the enantiomeric form of the sugar. If the hydroxyl on Cn-1 is to the left, it is the L-isoform; if the n-1 hydroxyl is to the right, it is the D isoform. Enantiomers are mirror-images of each other. All sugars in biomolecules are in the D-form • All of the aldoses that posses the same number of C atoms are called as diastereomers. They are not mirror images of each other. • Usually there is n-2 chiral carbons in hexoaldoses. And there are 2(n-2) possible hexoaldose stereoisomers (enantiomers + diastereoisomers) • Two sugars that differ in the orientation of only one hydroxyl are called as epimers of each other. Eg: glucose and galactose Haworth Projections • Alcohols react with aldehydes to form hemiacetals and with ketones to form hemiketals • Most long-chain aldoses and ketoses form intramolecular hemiacetals / hemiketals to form 5-(furanose) or 6- (pyranose) membered ring structures. • The ring-form depictions of monosaccharide structures are called Haworth projections. When converting Fischer to Haworth formulas follow the rule: right down • Ring formation results in C1 becoming achiral. The α-anomer is when –OH is below and β-anomer is when –OH is above the plane of the ring • Complete name of sugar includes: anomer, enantiomer, sugar identity and ring type. Eg: α-D-Glucopyranose 2 CHEM464 Carbohydrates Reactions of monosaccharides • Monosaccharides have a free carbonyl group which is prone to oxidation. Thus, monosaccharides reduce other compounds. Any sugar with a ‘free’ carbonyl C is called a reducing sugar • A acetal can react with an alcohol or an amine to form an adduct with the removal of H2O. The resulting bond with an alcohol results in an O-glycosidic linkage. Reaction with an amine results in a N-glycosidic bond • O-glycosidic linkages can join one monosaccharide to another to form polymers. These are called as disaccharides, oligosaccharides or polysaccharides. • Polysaccharides may be linear or branched. The repeating unit may be a monosaccharide or a disaccharide. Structures to Memorize *Glyceraldehyde *Ribose *Glucose *Galactose *Dihydroxyacetone *Fructose *Lactose: galactose β(1Æ4)glucose *Sucrose: glucose α,β(1Æ2)fructose (non-reducing sugar) *Amylose: glucose α(1Æ4) glucose *Starch / Glycogen: glucose α(1Æ4) glucose; branches by α (1Æ6) linkages *Cellulose: glucose β(1Æ 4) glucose 3 CHEM464 Carbohydrates Important Modified Monosaccharides • Glucosamine/Galactosamine: -OH on C2 is replaced by –NH2 • The amines are usually esterified with methanol to form Nacetylglucosamine and N-acetylgalactosamine • Glucuronic / galactouronic acid: C6 is oxidized to –COOH. • These modified monosaccharides are abundantly found in mucopolysaccharides and glycosaminoglycans • Neuraminose is a 9 C ketose. In sialic acid (Nacetylneuraminic acid) C1 is oxidized to COOH and C4 has a N-acetyl group. Sialic acid is the terminal sugar of oligosaccharides found on glycoproteins • Sulfated polysaccharides: The –OH on C4 or C6 may be sulfated (-OH to -OSO3-) on several glycosaminoglycans. The degree of sulfation is variable Complex Carbohydrates • Disaccharides and polysaccharides formed by O-glycosidic linkages between monosaccharides are called complex carbohydrates • In sucrose, the carbonyl carbons of both glucose and fructose participate in the O-glycosidic linkage, so it is a non-reducing sugar. • Lactose formed from glucose and galacotose is the disaccharide of milk. • Polysaccharides contain more than 2 monomeric sugars • Homopolysaccharides are repeats of the same monomer, heteropolysaccharides contain a disaccharide repeating unit 4 CHEM464 Carbohydrates Polysaccharides • Storage polysaccharides: Starch and glycogen are polymers of glucose in α (1Æ4) linkages with α(1Æ6) branches. Starch is a mixture of amylose (linear) and amylopectin (branched). Glycogen is highly branched • Structural polysaccharides: Cellulose makes up the cell wall of plant cells. It is a polymer of glucose in β(1Æ4) linkages. Other eg: chitin, pectin. • Mucopolysaccharides/glycosaminoglycans: Very viscous heteropolysaccharides found in extracellular cavities and connective tissue. Hyaluronic acid and chondroitin sulfate serve as lubricants for joints. Heparin is an anticoagulant. • Glycosaminoglycans attach to proteins to form proteoglycans. Carbohydrates, rather than protein are the main component Dietary Carbohydrates • Digestion begins in the mouth: Saliva contains amylase which is specific for α (1Æ4) glycosidic linkages. • Complex carbohydrates are hydrolysed to their component monosaccharides in the small intestine by specific glycosidases on the epithelial cell lining. • Deficiency of glycosidases results in reduced capacity to digest certain carbohydrates. Instead, gut bacteria metabolize the sugars liberating CO2 and H2 that cause gas and bloating. Eg: Lactose-intolerant people have lost the ability to synthesize lactase. One can rectify the problem by consuming glycosidase in the form of pills before a meal. • Mammals lack cellulase and cannot digest vegetable fiber 5 CHEM464 Carbohydrates Oligosaccharide Assembly • Oligosaccharides are synthesized by the action of enzymes called glycosyl transferases. • Many different glycosyl transferases, each specific for formation of a specific O-glycosidic linkage • The monomeric sugar has to be activated in the form of a sugar nucleotide prior to attachment to the growing oligosaccharide • Specificity of glycosyl transferases is demonstrated by human ABO blood groups. The basic O-type oligosaccharide antigen on RBC membranes is substituted with galNac by the action of N-acetylgalactosamine transferease to form A antigen. In individuals with the B antigen, galactosyl transferase adds on galactose to the oligosaccharide. The two enzymes differ by only 4 aa are absent in O-type individuals due to premature termination of polypeptide synthesis Glycoproteins • Glycoproteins are proteins with covalently attached oligosaccharide groups • Most cell surface proteins as well as secreted proteins are glycoproteins with the oligosaccharide component essential for protein activity and function. • Carbohydrates are linked to proteins by O-glycosidic linkages with ser or thr. They also form N-glycosidic linkages with Asn. Asn has to be in the seq NXS or NXT. No specific motif requirement for O-linked Ser / Thr. • All N-liked oligosaccharides have a pentasaccharide core that grows to various oligosaccharide patterns. Two most common ones are the high-mannose type and complex type. The first sugar in O-linked oligosaccharides is usually glcNac. 6 CHEM464 Carbohydrates Protein Glycosylation • Specific glycosyltransferases are responsible for the glycosylation of proteins. Monomers are activated in the form of UDP- or GDP-sugar. • O-linked glycosylation: Takes place in the golgi apparatus by the serial addition of monosaccharide units to a completed polypeptide chain • N-linked glycosylation: Starts co-translationally in the ER lumen and processing continues in the golgi complex. • A 14-unit oligosaccharide complex is first assembled on dolichol phosphate, a lipid present in the ER membrane N-linked glycosylation (continued) • The 14-unit oligosaccharide is transferred from dolichol phosphate to an acceptor Asn on the nascent protein in the ER • The oligosaccharide is trimmed before the protein leaves the ER • As the proteins traverse the different compartments of the golgi, their oligosaccharides are further processed to their final version. • Oligosaccharide composition is important in protein targeting and protein folding 7