Class Notes (806,856)
Canada (492,490)
Biochemistry (382)
BCH210H1 (352)
Brunt (3)
Lecture 11

Lecture 11 Carbohydrates.docx

28 Pages
Unlock Document

University of Toronto St. George

Lecture 11 Carbohydrates Monosaccharides – (CH O)2 n  n >3, n = 9, usually 5 or 6 max  One C is C=O and each remaining carbon bears a –OH group Oligosaccharides – 2-20 monosaccharides Polysaccharides – >20 monosaccharides  Water is eliminated during polymerization of glycan structures Glycoconjugate – carbohydrates linked covalently to a peptide (peptidoglycan), protein (proteoglycan), or lipid (glycolipid) Monosaccharides  Most are chiral compounds  Water-soluble  White crystalline  Aldoses – polyhydroxyl aldehydes with most oxidized carbon designated C-1 and drawn at the top of a Fischer projection  Ketoses – polyhydroxyl ketones with most oxidized carbon at C-2  Sugars have D confirmation when chiral carbon with highest number (i.e. chiral C most distant from C=O) is same as that of C-2 D-glyceraldehyde – i.e. the –OH group attached to this carbon atom is on the right side of Fischer projection  Mostly D enantiomers that are synthesized in living cells  Epimers – sugar molecules that differ in conformation at only one of several chiral centers o E.g. D-mannose and D-galactose are epimers of D-glucose, but not each other  Ketoses have one fewer chiral carbons than the aldose with the same empirical formula  D-Erythrulose has one chiral carbon – two stereoisomers Cyclic monosaccharide structures and anomeric forms  Glucose (aldose) can cyclize to form cyclic hemiacetal – pyranose  Fructose (ketose) can cyclize to form cyclic hemiketal – furanose Haworth projections  Cyclization – resembles reaction of an alcohol with an aldehyde/ketone  Cyclic forms possess anomeric carbons – the most oxidized carbon attached to two oxygens  Anomeric carbon is chiral  For D-sugars, alpha configuration has OH down, beta configuration has OH up  For L-sugars, reverse  Anomeric forms (alpha/beta) are in rapid equilibrium  In Haworth projections, lower edges of ring (thick lines) project in front of plane of the paper and upper edges project behind Cyclization of D-glucose to form glucopyranose  Fischer projection rotation of the bond between C-4 and C-5 brings the C-5 hydroxyl group close to the C-1 aldehyde group  Reaction of the C-5 hydroxyl group with one side of C-1 gives α-D-glucopyranose – reaction with the other side gives β-D-glucopyranose  Linear D-fructose undergoes an intramolecular reaction to form cyclic hemiketal  D-glucose can cyclize in two ways forming furanose or pyranose – in monosaccharides there are two or more hydroxyl groups that can react with an aldehyde or ketone at the other end of the molecule to form hemiacetal or hemiketal  E.g. Glucose – if C-5 OH reacts a six member ring is formed; if C-4 OH reacts, a five carbon ring is formed  E.g. D-ribose (5 carbon) readily forms 5 or 6 member rings Chair and Boat Configurations  Chair and boat configurations are the favored structures of pyranose rings  For each pyranose there are two distinct chair conformations and 6 distinct boats  Chair conformation minimizes steric repulsion of ring substituents and are generally more stable than boats  Equatorial – coplanar with the ring  Axial – parallel to an axis drawn through the ring  First chair more stable since bulky ring substituents are equatorial  Axial substituents sterically hinder each other Conformation of β-D-ribofuranose – variety of conformations Monosaccharide Derivatives  Sugar phosphates – monosaccharides converted into phosphate esters like ATP o Metabolicaly important sugar phosphates o Sugar esters  Deoxy sugars – constituents of DNA, etc.  Amino sugars – contain an amino group in place of a hydroxyl group in the parent monosaccharide (occasionally acetylated)  Sugar alcohols – mild reduction of sugars  Sugar acids (reducing sugars) – carboxylic acids derived from aldoses o Sugars with free anomeric carbons – will reduce oxidizing agents, such as peroxide, ferricyanide, and some metals (Cu and Ag) o These redox reactions convert the sugar to sugar acid o Glucose is a reducing sugar – so these reactions are the basis for diagnostic tests for blood sugar Disaccharides  Glycosidc bond – acetal linkage in which anomeric carbon of the sugar is condensed with an alcohol/amine/thiol o Primary structural linkage or joining monosaccharides o Reducing sugar – end containing free anomeric carbon atom  Other end is nonreducing end  HOH – configuration can be either α/β  Sucrose – distinguished as a sugar made by plants with s glycosidic bond linking two anomeric carbons, therefore neither residue is free to equilibrate between α/β anomers  Maltose – starch  Cellobiose – cellulose Polysaccharides  Homopolysaccharides and heteropolysaccharides  Starch and glycogen are storage molecules  Chitin and cellulose are structural molecules  Cell surface polysaccharides are recognition molecules Starch and Glycogen  D-glucose is made by all species  Starch – most common storage in plants and fungi; homoglycan o Mixture of amylose (10-30%) and amylopectin (70-90%) o Amylose – unbranched D-glucose residues (up to appox. 1000) connected with α(1-4)glycosidic linkages, one reducing end o Amylopectin – branched every 12-30 residues via α(1-6)glycosidic linkages  Glycogen – most common storage in animals  Bacteria have both types  The starch phosphorylase reaction cleaves glucose residues from amylase producing α-D-glucose-1-phosphate Cellulose and Chitin  Cellulose – structural polysaccharide of the cell wall o Linear polymer of glucose joined by β-(1-4) linkages (cellobiose) o Vary in size to up to 15 000 glucose residues  Chitin – structural homoglycan found in exoskeletons of insects and cell walls of many fungi and algae o Linear polymer of N-acetylglucosamine joined by β (1-4) linkages o Adjacent strands of chitin form H bonds resulting in a linear fibril of great
More Less

Related notes for BCH210H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.